Title of Invention: Hairpin peptides with a novel structural motif and methods relating thereto Abstract: The invention is directed to a model system for structure-activity relationship analysis of peptide or protein molecules involved in important biological processes. Provided by the invention are combinatorial peptide libraries comprising peptides with a novel “tryptophan zipper” scaffold (trpzip) that forms stable β-hairpin structure in solution. Methods of selecting and using such scaffold are provided herein, which are useful for mimicking native protein structures and interactions and designing therapeutic agents. Thus, the invention has profound utility for biological studies and drug development. Detailed Description: Part general_description_paragraphs: Part FIELD OF THE INVENTION: The present invention relates in general to protein chemistry, and more specifically to the identification and characterization of a novel small peptide motif with stable conformation, as well as to libraries of conformationally-constrained peptides and methods of generating and screening such libraries for biological and pharmaceutical uses. Part BACKGROUND OF THE INVENTION: Structure-Activity Relationship (SAR) studies provide valuable insights for understanding intermolecular interactions between bioactive molecules. In their natural states, bioactive molecules often adopt unique, conformationally-constrained structures in order to recognize and bind to their binding partners, to form a molecular complex therewith, and in turn to elicit specific activities. In particular, protein-protein interactions are crucial events involved in most biological and pathological processes, and are therefore logical targets for drug design. Important protein-protein interactions occur between such binding partners as enzyme-substrate, ligand-receptor, and antigen-antibody complexes. One of the revolutionary advances in drug discovery is the development of combinatorial libraries. Combinatorial libraries are collections of different molecules, such as peptides, that can be made synthetically or recombinantly. Member peptides in a combinatorial peptide library include amino acids incorporated randomly into certain or all positions of their sequences. Such libraries have been generated and used in various ways to screen for peptide candidates which bind effectively to target molecules and to identify such sequences. Many methods for generating peptide libraries have been developed and described. For example, members of the peptide library can be created by split-synthesis performed on a solid support such as polystyrene or polyacrylamide resin, as described by Lam et al. (1991) Nature 354:82 and PCT publication WO 92/00091. The method disclosed by U.S. Pat. No. 4,833,092 involves the synthesis of peptides in a methodical and predetermined fashion, so that the placement of each library member peptide gives information concerning the synthetic structure of that peptide. Phage display of peptide libraries has become a powerful tool for rapidly screening and identifying novel ligands of virtually any protein target. Of particular interests are display methods using filamentous bacteriophages. U.S. Pat. No. 5,821,047. This method allows the preparation of libraries as large as 1010-1012 unique peptide members, many orders of magnitude larger than libraries that may be prepared synthetically. In addition to large library sizes, advantages of phage display include ease of library construction (Kunkel mutagenesis), coupling of the binding entity (displayed peptide) to a unique identifier (its DNA sequence), a selection protocol for amplifying rare binding clones in a pool, and the high fidelity of biosynthesis (compared to synthetic methods). Furthermore, rapid and inexpensive selection protocols are available for identifying those library members that bind to a target of interest. However, only natural peptides composed of L-amino acids may be displayed on phage, so the problem of defining three-dimensional structure-activity relationships is more difficult than it might be for a constrained peptidomimetic containing non-naturally occurring amino acids or nonpeptide components. One possible solution to this problem is to use the structural constraints of a folded protein to present small variable peptide segments. Considerable effort has been devoted to introducing structural constraints into combinatorial peptide libraries so that the member peptides represent more closely their native states. Several protein scaffolds capable of presenting a sequence of interest in a conformationally-restricted fashion have been identified, including minibody structures (Bianchi et al. (1994) J Mol Biol 236:649-659), β sheets, coiled-coil stem structures (Myszka & Chaiken (1994) Biochem 33:2363-2372), zinc-finger domains, cysteine-linked (disulfide) structures, transglutaminase linked structures, cyclic peptides, helical barrels or bundles, leucine zipper motifs (Martin et al. (1994) EMBO J 13:5303-5309), etc. A number of identified scaffolds have been used in the construction of combinatorial peptide libraries with structural constraints. U.S. Pat. No. 5,824,483 describes a synthetic peptide library containing peptides featuring α-helical conformation and thus capable of forming coiled-coil dimers with each other. McBride et al. (1996) J Mol Biol 259:819-827 describe a synthetic library of cyclic peptides mimicking the anti-tryptic loop region of an identified proteinase inhibitor. WO 00/20574 and U.S. Pat. No. 6,180,343B1 describe fusion constructs using scaffold proteins such as green fluorescent protein (GFP). Several small protein domains have also been proposed as peptide display scaffolds. Nygren & Uhlen (1997) Curr. Opin. Struct. Biol. 7:463-469; Vita et al. (1998) Biopolymers 47:93-100; Vita et al. (1999) Proc. Natl. Acad. Sci. USA 96:13091-13096; Smith et al. (1998) J. Mol. Biol. 277:317-332; Gururaja et al. (2000) Chem. & Biol. 7:515-527; Christmann et al. (1999) Protein Engng. 12:797-806. Among the identified protein scaffolds, β-turns (hairpins) have been implicated as an important site for molecular recognition in many biologically active peptides. Smith & Pease (1980) CRC Crit Rev Biochem 8:315-300. Thus, peptides containing conformationally-constrained β-turns are particularly desirable. The great majority of the identified β-turn bearing peptides are cyclopeptides which have been generated by the cyclization of a peptide similar to a sequence in the natural substrate. Milner-White (1989) Trends Pharmacol Sci 10:70-74. These cyclopeptides, however, may still retain significant flexibility. For this reason, many studies have attempted to introduce rigid, nonpeptide compounds which mimic the β-turn. Peptides with such nonpeptide β-turn mimic provide useful leads for drug discovery. Ball & Alewood (1990) J Mol Recog 3:55-64; WO 94/03494. The structural mechanisms by which β-turns are stabilized, and specific strand registers are selected, continue to be the subject of considerable interest. Several examples have been reported of disulfide-constrained peptides intended to mimic protein hairpins or as de novo designed hairpins. In many cases the design includes D-cysteines at one or both ends, as it was initially thought that disulfide bond geometry was not compatible with the cross-strand geometry of hairpins. However, there are some examples that do use L-cys. Evidence for structure is lacking in most studies of disulfide-cyclized peptides. Examples listed here are those whose structures have been experimentally determined, or that use no unusual amino acids and have potency close to a larger, hairpin-containing natural protein in a biological assay. The structure of a hexapeptide (Boc-CL-Aib-AVC-NMe) was determined crystallographically, revealing a type II′ turn and β-sheet geometry. Karle et al. J. Am. Chem. Soc. (1988) 110:1958-1963. An octapeptide with the same cysteine spacing was studied by NMR, and has a similar structure with a turn centered on Pro-Gly. Walse et al. (1996) J. Comput.-Aided Mol. Des. 10:11-22. Peptides of the form Ac-CXPGXC-NMe (SEQ ID. NO. 11) were evaluated by measurement of disulfide exchange equilibria, which indicated turn preferences between peptides of as much as 1 kcal/mol. Milburn et al. (1987) J. Am. Chem. Soc. 109:4486-4496. An eleven-residue cyclic peptide, CGVSRQGKPYC, (SEQ ID NO. 12) based on the gene 5 protein from M13 is stably structured in aqueous solution, as demonstrated by NMR analysis. The cyclic peptide adopts a structure that is quite similar to the corresponding protein loop. The authors claim that well-defined β-hairpin structure had not been previously reported for any unprotected disulfide-constrained cycle. Rietman et al. (1996) Eur. J. Biochem. 238:706-713. This peptide has a Val-Pro pair at the nonhydrogen-bonded sites nearest to the cysteines. Disulfide-cyclized peptides from the hairpin region of a rabbit defensin have antibacterial activity exceeding (about 5 to 10-fold) that of the linear analogs. Circular dichroism spectroscopy indicates some non-random structure in phosphate buffer. The more potent peptide (CAGFMRIRGRIHPLCMRR) (SEQ ID NO: 13 ) has a Gly-Pro pair at the nonhydrogen-bonded sites nearest to the cysteines. Thennarasu & Nagaraj (1999) Biochem. Biophys. Res. Commun. 254:281-283. Several peptides from the loops of domain 1 of human CD4 have been studied in Zhang et al. (1996) Nature Biotechnology 14:472-475; Zhang et al. (1997) Nature Biotechnology 15:150-154. In addition to a disulfide constraint, the authors have added exocyclic aromatic amino acids to the peptide termini. No evidence for structure is given, but one cyclic peptide was reported to antagonize both normal CD4 interactions and those involved in CD4-mediated cell entry by HIV. Few examples exist of small peptides that form a stable tertiary structure without assistance from disulfide bonds or metal ions. Most natural peptides encompassing hairpins are mainly devoid of structure in water or form aggregates. Ramirez-Alvarado et al. (1997) Protein Sci. 6:162-147. A hairpin peptide derived from the B1 domain (the 41-56 residue fragment) of protein G (GB 1) has been reported to form a well-populated hairpin (about 50%) in water. Blanco et al.(1994) Nat. Struct. Biol. 1:584-590. The GB1 hairpin has four threonine residues at hydrogen-bonded sites in the strands, including one Thr-Thr cross-strand pair. This is generally believed to be an unfavorable pairing. In addition, there are Trp-Val and Tyr-Phe pairs at adjacent nonhydrogen-bonded sites that might interact to form a small hydrophobic core. Analysis of hairpin sequences in crystal structures has allowed the de novo design of a series of β-hairpin peptides based on the BH8 peptide. Ramirez-Alvarado et al. (1996) Nat. Struct. Biol. 3:604-612. The target structure was a type I′ turn flanked by three-residue strands. Arg-Gly sequences were added to the ends to improve solubility. One peptide was partially folded into a hairpin conformation (about 30%) as determined by NMR. The importance of inter-strand side chain-side chain interactions was indicated by replacement of certain strand residues with alanine. None of the alanine-substituted peptides showed any tendency to form a hairpin. The same authors reported a second series of experiments in which position i+1 of the turn was varied. Ramirez-Alvarado et al. (1997) J. Mol. Biol. 273:898-912. No peptide was more structured than the original sequence with Asn in the turn. A review describing this work suggested that adding Glu-Lys pairs to the termini of the model peptide may help to stabilize the hairpin. Ramirez-Alvarado et al. (1999) Bioorg. Med. Chem. 7:93-103. A peptide comprising the N-terminal 17 residues of the globular protein ubiquitin has been shown to form a native-like hairpin in both aqueous methanol and water, albeit at low apparent population. Zerella et al. (1999) Protein Sci. 8:1320-1331. A recent study, Zerella et al. (2000) Protein Sci. 9:2142-2150, focused on the contributions to the stability of the isolated peptides by residues within the turn region. The data indicated that in a peptide where Thr at position 9 was replaced by Asp, U(1-17)T9D, the native conformation was stabilized significantly over that of the wild type sequence. The estimated population of the folded hairpin was only 64%. Moreover, as the authors noted, the structure of the folded state of U(1-17)T9D may be more dynamic than indicated by the final ensemble. The reason for the greater stability upon substitution of the turn residue remains uncertain. It is an object of the present invention to provide a simple model system for displaying small peptides with stable hairpin structure and methods of using such a model system in constructing and screening constrained peptide libraries useful in biological and therapeutic applications. Part SUMMARY OF THE INVENTION: The present invention is based on the surprising identification of a novel structure motif, the tryptophan zipper (trpzip), that enables the stabilization of hairpin structures in very short peptides. Some of the trpzip peptides showing stable tertiary structures have a minimum length of 10-12 amino acids. Therefore, in one aspect, the invention provides a minimal peptide scaffold having the newly identified stable trpzip motif, comprising a presented turn sequence flanked by two opposite strands with a defined backbone hydrogen-bonding pattern, each strand comprising at least two Trp residues at non-hydrogen-bonded positions. The four Trp residues from the two strands form two Trp-Trp pairs that constitute a cross-strand zipper-like motif with great structural stability. Significantly, the trpzip motif does not require any disulfide bonds. In one aspect, the presented turn sequence comprises at least 4 amino acids. In another aspect, the presented turn sequence comprises at least 6 amino acids. In addition to the four Trp residues, the two flanking strands comprise other amino acids, preferably naturally occurring L-form amino acids. In one preferred embodiment, the peptide scaffold has a minimum length of 10 amino acids, with 4 amino acids as the presented turn sequence and 3 amino acids each for the flanking strands. In other preferred embodiments, additional residues are included in the strand region and/or the turn region of the scaffold. As such, some preferred peptide scaffolds comprise 12, 14, 16, 18 or 20 amino acids. More preferably, the scaffold is no more than 20 amino acids in length. The invention also encompasses libraries of structurally-constrained peptides, each peptide having the trpzip scaffold as described above, wherein the presented turn sequence consists of random amino acids. Methods of constructing such libraries are also contemplated. The subject libraries can be used for selecting novel peptides capable of binding to identified target molecules. Accordingly, the invention provides methods of identifying peptides capable of binding to a bioactive target molecule, comprising the steps of: a) providing a library of peptides comprising the novel trpzip scaffold; b) contacting the library with the target molecule; c) selecting from the library peptides capable of forming a noncovalent complex with the binding partner; and d) optionally isolating the peptides selected in step c). The selected peptides are useful per se as diagnostics or therapeutics (e.g., agonists or antagonists) used in treatment of biological organisms. Compositions and methods of the invention may also be useful in analyzing the structure-activity relationship of proteins of interest, thereby providing information for rational drug design. Part DETAILED DESCRIPTION OF THE INVENTION: The design of peptides that have well-defined tertiary structures tests our understanding of the principles governing the folding of larger proteins. Short peptides with significant hairpin structure have recently emerged as β-sheet model systems. In a separate study, the inventors of the present invention have discovered that in a disulfide-cyclized β-hairpin peptide, tryptophan was much more stabilizing in a non-hydrogen-bonded (NHB) strand position than other amino acids. Cochran et al. (2001) J. Am. Chem. Soc. 123:625-632. Paired, cross-strand NHB residues in the Cys-cyclized hairpin made roughly independent contributions to stability; thus, a single tryptophan-tryptophan cross-strand pair was shown to be highly stabilizing (and the best NHB residue pair identified). Cochran et al. WO 00/77194; Russell and Cochran (2000) J. Am. Chem. Soc. 122:12600-12601. The present invention provides a novel structural motif, the tryptophan zipper (trpzip), that greatly stabilizes the β-hairpin conformation in short peptides, without any disulfide bonds. As shown in the Examples, peptides having 12 or 16 amino acids in length with different turn sequences are monomeric and fold cooperatively in water. Surprisingly, the folding free energies of the trpzip peptides exceed substantially those of all previously reported β-hairpins and even those of some larger designed proteins. NMR structures of some of the exemplary trpzip peptides revealed exceptionally well-defined β-hairpin conformations stabilized by cross-strand pairs of indole rings. The peptides of the present invention are the smallest peptides to adopt an unique tertiary fold without requiring metal binding, unusual amino acids, or disulfide crosslinks. Therefore, the present invention provides a novel peptide scaffold for β-turn display. By “scaffold”, “peptide scaffold” or “protein scaffold” is meant an amino acid framework useful for presenting a peptide of interest, in a way that the peptide of interest is accessible to other molecules. Preferably, the peptide scaffold has stable, defined tertiary structure, such that the presented peptide adopts a constrained conformation for display. The term “β-turn” or “β-hairpin”, as used herein, refers to an antiparallel β-sheet structure comprising a turn region flanked by two opposite strands with defined backbone hydrogen-bonding pattern. There are several types of hairpins depending on the types of turn, including for example, types I, I′, II, and II′. A “presented turn sequence” refers to the central subset region of a β-turn that forms the actual turn structure. As used herein, the term represents a segment with variable amino acid residues that is to be presented in a combinatorial library display. The segment is used to present randomized residues in searching for sequences exhibiting binding affinities to other target molecules of interest. For example, the presented turn sequences can be sequences capable of serving as substrates or inhibitors, being recognized by antibodies, binding to receptors or ligands, or being useful in column affinity chromatography. Using well known methods such as those further described below, such presented sequences can be identified and isolated for further studies and uses. The term “tryptophan zipper” or “trpzip” refers to a “zipper-like” peptide motif characterized by four tryptophan residues capable of forming two Trp-Trp cross-strand pairs and stabilizing a β-hairpin tertiary structure. The Trp residues within a trpzip are located at non-hydrogen-bonded positions of the opposite strands. “A defined backbone hydrogen-bonding pattern” as used herein refers to a tertiary structure with defined conformation that is formed and stabilized by interstrand hydrogen bonding participated by amide and/or carboxyl moieties of individual strand residues. “Non-hydrogen-bonded positions” or “NHB positions” as used herein refers to strand positions within the hairpin scaffold that do not contribute to and participate in the hydrogen-bonding pattern. See Sibanda et al. (1989) J. Mol. Biol. 229:759-777 for further description of the hydrogen-bonding patterns of β-hairpins and their nomenclature. The scaffold of the present invention comprises at least two Trp—Trp NHB cross-strand pairs. The combination of at least two Trp-Trp NHB cross-strand pairs greatly stabilizes β-hairpin structures. As further disclosed in the Examples, several trpzip variants having different turn sequences are highly water-soluble, well-structured, and monomeric. High-resolution NMR structures of the peptides show the two cross-strand Trp pairs interdigitating in a zipper-like motif on the surface of the folded peptide. This arrangement of the indole side chains confers unusual spectroscopic properties on the folded molecules, and folding can therefore be monitored readily by changes in circular dichroism (CD) signal. The stabilities of the tryptophan zippers are significantly higher than those reported for other small β-structures. Indeed, on a per-residue basis, the tryptophan zippers have stabilities comparable to much larger native protein domains. The scaffolds of the invention are non-cysteine constrained. In other words, the scaffolds do not require the involvement of disulfide bridges between strands in order to maintain the stability of the tertiary structures. As such, trpzip peptides of the invention are particularly useful in applications where the disulfide bond formation is either undesirable or unfavorable. For example, the trpzip scaffold can be used for the intracellular display of peptides. The trpzip peptides of the invention are among the smallest peptides to adopt an unique and stable tertiary fold without requiring metal binding, unusual amino acids, or disulfide bridges. Previous studies have suggested that the minimal size of a stable protein domain without cysteine bridges is approximately 50 amino acids. Nygren and Uhlen (1997) Curr. Opin. Struct. Biol. 7:463-469; Privalov and Gill (1988) Adv. Protein Chem. 39:191-243. Because of their small size, unusual stability, and very favorable spectroscopic properties, the trpzip scaffolds of the invention provide a useful and simple system for the study and display of β-turns. The invention provides a library of trpzip peptides for turn display. Preferably, the presented turn sequence consists of random amino acids. Randomization of the turn sequences can be achieved by using methods and techniques well known in the art. Generally, at least 2, preferably at least 4, more preferably at least 6, even more preferably at least 10 amino acid positions need to be randomized. In a preferred embodiment, the random peptide sequence is provided by oligonucleotide synthesis using randomized codon assignments. It should be realized, however, that in a library system encoded by random nucleotides, codons encoding stop signals (i.e., TAA, TGA and TAG) may be undesirably introduced into the structure. For example, in a synthesis with NNN as the random region, there is a 3/64 chance that the codon will be a stop codon. Thus, in a region of 10 residues, there is a likelihood that 46.7% of the peptides will prematurely terminate. In order to alleviate this problem, random residues can be encoded, for example, as NNK or NNS instead, where K=T or G; and S=C or G. This allows for encoding of all potential amino acid (changing their relative representation slightly), yet preventing the encoding of two stop residues TAA and TGA. In a preferred embodiment, the peptide library is “fully randomized,” meaning there are no sequence preferences or fixed residues at any position within the turn region. In another preferred embodiment, the library is randomized with bias. That is, some positions within the region are either held constant, or selected from a limited number of possibilities. For example, in a preferred embodiment, the residues are randomized within a defined category, such as of hydrophobic residues, hydrophilic residues, aliphatic residues, unbranched residues, branched residues, or aromatic residues, etc. In a preferred embodiment, the random residues are biased to β-turn formation. In addition to random residues in the turn regions, the invention also encompasses amino acid variations at strand positions of the scaffold, other than those occupied by the core Trp residues. For example, variations can occur at NHB strand sites and/or hydrogen-bonded strand sites. A position and its cross-strand pairing partner can have the same or different residues. Many methods for generating peptide libraries are known in the art and can be used to generate the libraries of the invention. In one embodiment, members of the peptide library can be created by split-synthesis performed on a solid support such as polystyrene or polyacrylamide resin, as described by Lam et al. (1991) Nature 354:82 and PCT publication WO 92/00091. In another embodiment, the trpzip scaffold of the invention can be used in constructing and displaying intracellular peptide libraries. A preferred method of generating the library of the present invention is phage display. Bacteriophage (phage) display is a known technique by which variant polypeptides are displayed as fusion proteins to the coat protein on the surface of bacteriophage particles (Scott, J. K. and Smith, G. P. (1990) Science 249: 386). A “phagemid” is a plasmid vector having a bacterial origin of replication, e.g., ColE1, and a copy of an intergenic region of a bacteriophage. The phagemid may be based on any known bacteriophage, including filamentous bacteriophage. The plasmid will also generally contain a selectable marker for antibiotic resistance. Segments of DNA cloned into these vectors can be propagated as plasmids. When cells harboring these vectors are provided with all genes necessary for the production of phage particles, the mode of replication of the plasmid changes to rolling circle replication to generate copies of one strand of the plasmid DNA and package phage particles. The phagemid may form infectious or non-infectious phage particles. This term includes phagemids which contain a phage coat protein gene or fragment thereof linked to a heterologous peptide gene as a gene fusion such that the heterologous peptide is displayed on the surface of the phage particle. The term “coat protein” means a protein, at least a portion of which is present on the surface of the virus particle. From a functional perspective, a coat protein is any protein which associates with a virus particle during the viral assembly process in a host cell, and remains associated with the assembled virus until it infects another host cell. The coat protein may be the major coat protein or may be a minor coat protein. A “major” coat protein is a coat protein which is present in the viral coat at 10 copies of the protein or more. A major coat protein may be present in tens, hundreds or even thousands of copies per virion. A “fusion protein” is a polypeptide having two portions covalently linked together, where each of the portions is a polypeptide having a different property. The property may be a biological property, such as activity in vitro or in vivo. The property may also be a simple chemical or physical property, such as binding to a target molecule, catalysis of a reaction, etc. The two portions may be linked directly by a single peptide bond or through a peptide linker containing one or more amino acid residues. Generally, the two portions and the linker will be in reading frame with each other. In one preferred embodiment, the trpzip peptides are fused to at least a portion of a phage coat protein to form a fusion protein. The fusion protein can be made by expressing a gene fusion encoding the fusion protein using known techniques of phage display such as those described below. The fusion protein may form part of a phage or phagemid particle in which one or more copies of the trpzip peptide are displayed on the surface of the particle. A gene comprising a nucleic acid encoding the trpzip peptide or the fusion protein are within the scope of the invention. In another embodiment, the invention is a method comprising the steps of constructing a library containing a plurality of replicable expression vectors, each expression vector comprising a transcription regulatory element operably linked to a gene fusion encoding a fusion protein, wherein the gene fusion comprises a first gene encoding a trpzip peptide of the invention and a second gene encoding at least a portion of a phage coat protein, where the library comprises a plurality of genes encoding variant trpzip peptide fusion proteins. Variant first genes and libraries thereof encoding variant trpzip peptides are prepared using known mutagenesis techniques described in more detail below. The invention also includes expression vectors comprising the fusion genes noted above, as well as a library of these vectors. The library of vectors may be in the form of a DNA library, a library of virus (phage or phagemid) particles containing the library of fusion genes or in the form of a library of host cells containing a library of the expression vectors or virus particles. The invention also contemplates a method of selecting novel binding peptides capable of binding to a bioactive target molecule. By “binding peptide” as used herein is meant any peptide that binds with a selectable affinity to a target molecule. By “bioactive target molecule” as used herein is meant any molecule exerting any biological activity in vitro or in vivo, for which it is desirable to produce a ligand. Preferably, the target molecule is a protein. More preferably, the target molecules include receptors, hormone ligands, growth factors, antigens, antibodies, enzymes and enzyme substrates. In a preferred embodiment, the method of selecting novel binding peptides comprises the steps of: (a) constructing a library of variant replicable expression vectors comprising a transcription regulatory element operably linked to a gene fusion encoding a fusion protein wherein the gene fusion comprises a first gene encoding the trpzip peptide, and a second gene encoding at least a portion of a phage coat protein, where the variant expression vectors comprise variant first genes; (b) transforming suitable host cells with the vectors; (c) culturing the transformed host cells under conditions suitable for forming recombinant phage or phagemid virus particles containing at least a portion of the expression vector and capable of transforming the host, so that the particles display one or more copies of the fusion protein on the surface of the particle; (d) contacting the particles with a target molecule so that at least a portion of the particles bind to the target molecule; and (e) separating the particles that bind from those that do not. In the method of the invention, the phage coat protein is preferably the gene III or gene VIII coat protein of a filamentous phage such as M13. Further, preferably the culturing of the transformed host cells is under conditions suitable for forming recombinant phage or phagemid particles where the conditions are adjusted so that no more than a minor amount of phage or phagemid particles display one or more copies of the fusion protein on the surface of the particle (monovalent display). The invention also includes a method of introducing structural bias into a phage-displayed library, using steps (a) through (e) described above. The invention further includes a method of selecting beta-hairpin forming peptide structures from a phage-displayed library, using steps (a) through (e) described above where the target is known to bind beta-hairpin peptide structures, preferably a protein target known to so bind. The utility of phage display lies in the fact that large libraries of selectively randomized protein variants (or randomly cloned cDNAs) can be rapidly and efficiently sorted for those sequences that bind to a target molecule with high affinity. Display of peptide (Cwirla et al. (1990) Proc. Natl. Acad. Sci. USA 87:6378) or protein (Lowman et al. (1991) Biochemistry 30:10832; Clackson et al. (1991) Nature 352: 624; Marks et al. (1991), J. Mol. Biol. 222:581; Kang et al. (1991) Proc. Natl. Acad. Sci. USA 88:8363) libraries on phage have been used for screening millions of polypeptides for ones with specific binding properties (Smith (1991) Current Opin. Biotechnol. 2:668). Sorting phage libraries of random mutants requires a strategy for constructing and propagating a large number of variants, a procedure for affinity purification using the target receptor, and a means of evaluating the results of binding enrichments. Typically, variant polypeptides, such as the trpzip compounds of the invention, are fused to a gene III protein, which is displayed at one end of the virion. Alternatively, the variant polypeptides may be fused to the gene VIII protein, which is the major coat protein of the virion. Such polyvalent display libraries are constructed by replacing the phage gene III with a cDNA encoding the foreign sequence fused to the amino terminus of the gene III protein. Monovalent phage display is a process in which a protein or peptide sequence is fused to a portion of a gene III protein and expressed at low levels in the presence of wild-type gene III protein so that particles display mostly wild-type gene III protein and one copy or none of the fusion protein (Bass et al. (1990) Proteins 8:309; Lowman, H. B. and Wells, J. A. (1991) Methods: a Companion to Methods in Enzymology 3:205). Monovalent display has the advantage over polyvalent phage display that progeny phagemid particles retain full infectivity. Avidity effects are reduced so that sorting is on the basis of intrinsic ligand affinity, and phagemid vectors, which simplify DNA manipulations, are used. One preferred phage display system is described in U.S. Pat. No. 5,821,047. A two-step approach may be used to select high affinity ligands from peptide libraries displayed on M13 phage. Low affinity leads are first selected from naive, polyvalent libraries displayed on the major coat protein (protein VIII). The low affinity selectants are subsequently transferred to the gene III minor coat protein and matured to high affinity in a monovalent format. Although most phage display methods have used filamentous phage, other phage display Systems, such as lambda phage, T4 phage and T7 phage display systems are also known and can be used to create a library of the trpzip peptides of the invention. WO 95/34683; U.S. Pat. No. 5,627,024; Ren et al. (1998) Gene 215:439; Zhu (1997) CAN 33:534; Jiang et al. (1997) CAN 128:44380; Ren et al. (1997) CAN 127:215644; Ren (1996) Protein Sci. 5:1833; Efimov et al. (1995) Virus Genes 10: 173; Smith & Scott (1993) Methods in Enzymology 217:228-257; U.S. Pat. No. 5,766,905. Suitable gene III vectors for display of trpzip peptides of the invention include fUSE5 (Scott, J. K., and Smith G. P. (1990) Science 249:386-390); fAFF1 (Cwirla et al. (1990). Proc. Natl. Acad. Sci. U.S.A. 87:6378-6382); fd-CAT1 (McCafferty et al. (1990) Nature (London) 348:552-554); m663 (Fowlkes et al. (1992) Biotechniques 13:422-427); fdtetDOG, pHEN1 (Hoogenboom et al. (1991) Nucleic Acids Res. 19:4133-4137); pComb3 (Gram et al. (1992) Proc. Natl. Acad. Sci. U.S.A. 89:3576-3580); pCANTAB 5E (Pharmacia); and LamdaSurfZap (Hogrefe (1993)Gene 137:85-91). Phage display methods for proteins, peptides and mutated variants thereof, including constructing a family of variant replicable vectors containing a transcription regulatory element operably linked to a gene fusion encoding a fusion polypeptide, transforming suitable host cells, culturing the transformed cells to form phage particles which display the fusion polypeptide on the surface of the phage particle, contacting the recombinant phage particles with a target molecule so that at least a portion of the particle bind to the target, separating the particles which bind from those that do not bind, are known and may be used with the method of the invention. See WO 97/29185; O'Boyle et al. (1997) Virology 236:338-347; Soumillion et al. (1994) Appl. Biochem. Biotech. 47:175-190; O'Neil and Hoess. (1995) Curr. Opin. Struct. Biol. 5:443-449; Makowski (1993) Gene 128:5-11; Dunn (1996) Curr. Opin. Struct. Biol. 7:547-553; Choo and Klug (1995) Curr. Opin. Struct. Biol. 6:431-436; Bradbury & Cattaneo (1995) TINS 18:242-249; Cortese et al., (1995) Curr. Opin. Struct. Biol. 6:73-80; Allen et al. (1995) TIBS 20:509-516; Lindquist & Naderi (1995) FEMS Micro. Rev. 17:33-39; Clarkson & Wells (1994) Tibtech. 12:173-184; Barbas (1993) Curr. Opin. Biol. 4:526-530; McGregor (1996) Mol. Biotech. 6:155-162; Cortese et al. (1996) Curr. Opin. Biol. 4:616-621; McLafferty et at. (1993) Gene 128:29-36. The gene encoding the coat protein of the phage and the gene encoding the desired trpzip peptide portion of the fusion protein of the invention (i.e., the trpzip peptide of the invention fused to at least a portion of a phage coat protein) can be obtained by methods known in the art. The DNA encoding the gene may be chemically synthesized (Merrifield (1963) J. Am. Chem. Soc. 85:2149) and then mutated to prepare a library of variants as described below. To ligate DNA fragments together to form a functional vector containing the gene fusion, the ends of the DNA fragments must be compatible with each other. In some cases, the ends will be directly compatible after endonuclease digestion. However, it may be necessary to first convert the sticky ends commonly produced by endonuclease digestion to blunt ends to make them compatible for ligation. To blunt the ends, the DNA is treated in a suitable buffer for at least 15 minutes at 15° C. with 10 units of the Klenow fragment of DNA polymerase I (Klenow) in the presence of the four deoxynucleotide triphosphates. The DNA is then purified by phenol-chloroform extraction and ethanol precipitation or other DNA purification technique. The cleaved DNA fragments may be size-separated and selected using DNA gel electrophoresis. The DNA may be electrophoresed through either an agarose or a polyacrylamide matrix. The selection of the matrix will depend on the size of the DNA fragments to be separated. After electrophoresis, the DNA is extracted from the matrix by electroelution, or, if low-melting agarose has been used as the matrix, by melting the agarose and extracting the DNA from it, as described in sections 6.30-6.33 of Sambrook et al. The DNA fragments that are to be ligated together (previously digested with the appropriate restriction enzymes such that the ends of each fragment to be ligated are compatible) are put in solution in about equimolar amounts. The solution will also contain ATP, ligase buffer and a ligase such as T4 DNA ligase at about 10 units per 0.5 μg of DNA. If the DNA fragment is to be ligated into a vector, the vector is at first linearized by cutting with the appropriate restriction endonuclease(s). The linearized vector is then treated with alkaline phosphatase or calf intestinal phosphatase. The phosphatasing prevents self-ligation of the vector during the ligation step. After ligation, the vector with the foreign gene now inserted is purified and transformed into a suitable host cell. A preferred transformation method is electroporation. Electroporation may be carried out using methods known in the art. More than one (a plurality) electroporations may be conducted to increase the amount of DNA which is transformed into the host cells. Repeated electroporations are conducted as described in the art. See, for example, Vaughan et al. (1996) Nature Biotechnology 14:309-314. The number of additional electroporations may vary as desired from several (2,3,4, . . . 10) up to tens (10, 20, 30, . . . 100) and even hundreds (100, 200, 300, . . . 1000). Repeated electroporations may be desired to increase the size of a combinatorial library, e.g. an antibody library, transformed into the host cells. Preferably, for library construction, the DNA is present at a concentration of 25 micrograms/ml or greater. More preferably, the DNA is present at a concentration of about 30 micrograms/ml or greater, more preferably at a concentration of about 70 micrograms/ml or greater and even more preferably at a concentration of about 100 micrograms/ml or greater even up to several hundreds of micrograms/ml. Generally, the electroporation will utilize DNA concentrations in the range of about 50 to about 500 micrograms/ml. A time constant during electroporation greater than 3.0 milliseconds (ms) results in a high transformation efficiency. The DNA is preferably purified to remove contaminants. The DNA may be purified by any known method, however, a preferred purification method is the use of DNA affinity purification. The purification of DNA, e.g., recombinant plasmid DNA, using DNA binding resins and affinity reagents is well known and any of the known methods can be used in this invention (Vogelstein, B. and Gillespie, D. (1979) Proc. Natl. Acad. Sci. USA 76:615; Callen, W. (1993) Strategies 6:52-53). Commercially available DNA isolation and purification kits are also available from several sources including Stratagene (CLEARCUT Miniprep Kit), and Life Technologies (GLASSMAX DNA Isolation Systems). Non-limiting examples of suitable methods for DNA purification include column chromatography, the use of hydroxylated silical polymers, rehydrated silica gel, boronated silicates, modified glass fiber membranes, fluorinated adsorbents, diatomaceous earth, dialysis, gel polymers and the use of chaotropic compounds with DNA binding reagents, all of which are known and widely used in the art. After purification, the DNA is eluted or otherwise resuspended in water, preferably distilled or deionized water, for use in electroporation at the concentrations of the invention. The use of low salt buffer solutions is also contemplated. Any suitable cells which can be transformed by electroporation may be used as host cells in the method of the present invention. Suitable host cells which can be transformed include gram negative bacterial cells such as E. coli. Suitable E. coli strains include JM101, E. coli K12 strain 294 (ATCC number 31,446), E. coli strain W3110 (ATCC number 27,325), E. coli X1776 (ATCC number 31,537), E. coli XL-1Blue (Stratagene), and E. coli B; however many other strains of E. coli, such as XL1-Blue MRF′, SURE, ABLE C, ABLE K, WM1100, MC1061, HB101, CJ136, MV1190, JS4, JS5, NM522, NM538, and NM539, may be used as well. Cells are made competent using known procedures. Cell concentrations of about 1010 colony forming units (cfu)/mL) of viable living cells and greater are preferably used for electroporation. More preferably, the viable cells are concentrated to about 1×1011 to about 4×1011 cfu/mL. Preferred cells which may be concentrated to this range are the SS320 cells described below. Cells are preferably grown in culture in standard culture broth, optionally for about 6-48 hrs (or to OD600=0.6−0.8) at about 37° C., and then the broth is centrifuged and the supernatant removed (e.g. decanted). Initial purification is preferably by resuspending the cell pellet in a buffer solution (e.g. HEPES pH 7.4) followed by recentrifugation and removal of supernatant. The resulting cell pellet is resuspended in dilute glycerol (e.g. 5−20% v/v) and again centrifuged to form a cell pellet and the supernatant removed. The final cell concentration is obtained by resuspending the cell pellet in water or dilute glycerol to the desired concentration. A particularly preferred recipient cell for the electroporation is a competent E. coli strain containing a phage F′ episome. Any F′ episome which enables phage replication in the strain may be used in the invention. Suitable episomes are available from strains deposited with ATCC or are commercially available (CJ236, CSH18, DH5alphaF′, JM101, JM103, JM105, JM107, JM109, JM110), KS1000, XL1-BLUE, 71-18 and others). Strain SS320 was prepared by mating MC1061 cells with XL1-BLUE cells under conditions sufficient to transfer the fertility episome (F′ plasmid) of XL1-BLUE into the MC1061 cells. In general, mixing cultures of the two cell types and growing the mixture in culture medium for about one hour at 37° C. is sufficient to allow mating and episome transfer to occur. The new resulting E. coli strain has the genotype of MC1061 which carries a streptomycin resistance chromosomal marker and the genotype of the F′ plasmid which confers tetracycline resistance. The progeny of this mating is resistant to both antibiotics and can be selectively grown in the presence of streptomycin and tetracycline. Strain SS320 has been deposited with the American Type Culture Collection (ATCC), 10801 University Boulevard, Manassas, Va., USA on Jun. 18, 1998 and assigned Deposit Accession No. 98795. This deposit of strain SS320 was made under the provisions of the Budapest Treaty on the International Recognition of the Deposit of Microorganisms for the Purpose of Patent Procedure and the Regulations thereunder (Budapest Treaty). This assures maintenance of a viable culture for 30 years from the date of deposit. The organisms will be made available by ATCC under the terms of the Budapest Treaty, and subject to an agreement between Genentech, Inc. and ATCC, which assures permanent and unrestricted availability of the progeny of the cultures to the public upon issuance of the pertinent U.S. patent or upon laying open to the public of any U.S. or foreign patent application, whichever comes first, and assures availability of the progeny to one determined by the U.S. Commissioner of Patents and Trademarks to be entitled thereto according to 35 USC §122 and the Commissioner's rules pursuant thereto (including 37 CFR §1.14 with particular reference to 886 OG 638). The assignee of the present application has agreed that if the cultures on deposit should die or be lost or destroyed when cultivated under suitable conditions, they will be promptly replaced on notification with a viable specimen of the same culture. Availability of the deposited cultures is not to be construed as a license to practice the invention in contravention of the rights granted under the authority of any government in accordance with its patent laws. A useful method for identification of certain residues or regions of the peptide that are preferred locations for mutagenesis is called “alanine scanning mutagenesis” as described by Cunningham and Wells (1989) Science 244:1081-1085. Here, a residue or group of target residues are identified (e.g., charged residues such as arg, asp, his, lys, and glu) and replaced by a neutral or negatively charged amino acid (most preferably alanine or polyalanine) to affect the interaction of the amino acids with target molecule. Those amino acid locations demonstrating functional sensitivity to the substitutions then are refined by introducing further or other variants at, or for, the sites of substitution. Thus, while the site for introducing an amino acid sequence variation is predetermined, the nature of the mutation per se need not be predetermined. For example, to analyze the performance of a mutation at a given site, ala scanning or random mutagenesis is conducted at the target codon or region and the expressed peptides are screened for the desired activity. Oligonucleotide-mediated mutagenesis is a preferred method for preparing the substitution, deletion, and insertion variants of the invention. This technique is well known in the art as described by Zoller et al. (1987) Nucleic Acids Res. 10: 6487-6504. Briefly, a gene encoding a protein fusion or heterologous polypeptide is altered by hybridizing an oligonucleotide encoding the desired mutation to a DNA template, where the template is the single-stranded form of the plasmid containing the unaltered or native DNA sequence of the gene. After hybridization, a DNA polymerase is used to synthesize an entire second complementary strand of the template which will thus incorporate the oligonucleotide primer, and will code for the selected alteration in the gene. Generally, oligonucleotides of at least 25 nucleotides in length are used. An optimal oligonucleotide will have 12 to 15 nucleotides that are completely complementary to the template on either side of the nucleotide(s) coding for the mutation. This ensures that the oligonucleotide will hybridize properly to the single-stranded DNA template molecule. The oligonucleotides are readily synthesized using techniques known in the art such as that described by Crea et al. (1978) Proc. Nat'l. Acad. Sci. USA 75: 5765. The DNA template is generated by those vectors that are derived from the bacteriophage used in the phage display system, e.g. bacteriophage M13 vectors (the commercially available M13mp18 and M13mp19 vectors are suitable), or those vectors that contain a single-stranded phage origin of replication; examples are described by Viera et al. (1987) Meth. Enzymol. 153:3. Thus, the DNA that is to be mutated can be inserted into one of these vectors in order to generate single-stranded template. To alter the native DNA sequence, the oligonucleotide is hybridized to the single stranded template under suitable hybridization conditions. A DNA polymerizing enzyme, usually T7 DNA polymerase or the Klenow fragment of DNA polymerase I, is then added to synthesize the complementary strand of the template using the oligonucleotide as a primer for synthesis. A heteroduplex molecule is thus formed such that one strand of DNA encodes the mutated form of the gene, and the other strand (the original template) encodes the native, unaltered sequence of the gene. This heteroduplex molecule is then transformed into a suitable host cell, usually a prokaryote such as E. Coli JM101. After growing the cells, they are plated onto agarose plates and screened using the oligonucleotide primer radiolabelled with 32-Phosphate to identify the bacterial colonies that contain the mutated DNA. The method described immediately above may be modified such that a homoduplex molecule is created wherein both strands of the plasmid contain the mutation(s). The modifications are as follows: The single-stranded oligonucleotide is annealed to the single-stranded template as described above. A mixture of three deoxyribonucleotides, deoxyriboadenosine (dATP), deoxyriboguanosine (dGTP), and deoxyribothymidine (dTTP), is combined with a modified thio-deoxyribocytosine called dCTP-(aS) (which can be obtained from Amersham). This mixture is added to the template-oligonucleotide complex. Upon addition of DNA polymerase to this mixture, a strand of DNA identical to the template except for the mutated bases is generated. In addition, this new strand of DNA will contain dCTP-(aS) instead of dCTP, which serves to protect it from restriction endonuclease digestion. After the template strand of the double-stranded heteroduplex is nicked with an appropriate restriction enzyme, the template strand can be digested with ExoIII nuclease or another appropriate nuclease past the region that contains the site(s) to be mutagenized. The reaction is then stopped to leave a molecule that is only partially single-stranded. A complete double-stranded DNA homoduplex is then formed using DNA polymerase in the presence of all four deoxyribonucleotide triphosphates, ATP, and DNA ligase. This homoduplex molecule can then be transformed into a suitable host cell such as E. coli JM101, as described above. Mutants with more than one amino acid to be substituted may be generated in one of several ways. If the amino acids are located close together in the polypeptide chain, they may be mutated simultaneously using one oligonucleotide that codes for all of the desired amino acid substitutions. If, however, the amino acids are located some distance from each other (separated by more than about ten amino acids), it is more difficult to generate a single oligonucleotide that encodes all of the desired changes. Instead, other alternative methods may be employed. In the first method, a separate oligonucleotide is generated for each amino acid to be substituted. The oligonucleotides are then annealed to the single-stranded template DNA simultaneously, and the second strand of DNA that is synthesized from the template will encode all of the desired amino acid substitutions. The alternative method involves two or more rounds of mutagenesis to produce the desired mutant. The first round is as described for the single mutants: wild-type DNA is used for the template, an oligonucleotide encoding the first desired amino acid substitution(s) is annealed to this template, and the heteroduplex DNA molecule is then generated. The second round of mutagenesis utilizes the mutated DNA produced in the first round of mutagenesis as the template. Thus, this template already contains one or more mutations. The oligonucleotide encoding the additional desired amino acid substitution(s) is then annealed to this template, and the resulting strand of DNA now encodes mutations from both the first and second rounds of mutagenesis. This resultant DNA can be used as a template in a third round of mutagenesis, and so on. Cassette mutagenesis is also a preferred method for preparing the substitution, deletion, and insertion variants of the invention. The method is based on that described by Wells et al. (1985) Gene 34:315. The starting material is a plasmid (or other vector) containing the gene to be mutated. The codon(s) in the gene to be mutated are identified. There must be a unique restriction endonuclease site on each side of the identified mutation site(s). If no such restriction sites exist, they may be generated using the above-described oligonucleotide-mediated mutagenesis method to introduce them at appropriate locations in the
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