The term molecular recognition refers to the specific interaction between two or more molecules through noncovalent bonding such as hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, π-π interactions, halogen bonding, or resonant interaction effects. In addition to these direct interactions, solvents can play a dominant indirect role in driving molecular recognition in solution. The host and guest involved in molecular recognition exhibit molecular complementarity. Exceptions are molecular containers, including e.g. nanotubes, in which portals essentially control selectivity. Biological systems Molecular recognition plays an important role in biological systems and is observed in between receptor-ligand, antigen-antibody, DNA-protein, sugar-lectin, RNA-ribosome, etc. An important example of molecular recognition is the antibiotic vancomycin that selectively binds with the peptides with terminal D-alanyl-D-alanine in bacterial cells through five hydrogen bonds. The vancomycin is lethal to the bacteria since once it has bound to these particular peptides they are unable to be used to construct the bacteria's cell wall. Synthetic molecular recognition Recent work suggests that molecular recognition elements can be synthetically produced at the nano-scale, circumventing the need for naturally occurring molecular recognition elements for the development of sensing tools for small molecules. Bio-mimetic polymers such as molecular imprinted polymers and peptoids can be used to recognize larger biological targets such as proteins and the conjugation of polymers to synthetic fluorescent nanomaterials can generate synthetic macromolecular structures that serve as synthetic antibodies for optical protein recognition and detection. Supramolecular systems Chemists have demonstrated that many artificial supramolecular systems can be designed that exhibit molecular recognition. One of the earliest examples of such a system are crown ethers which are capable of selectively binding specific cations. However, a number of artificial systems have since been established. Static vs. dynamic Molecular recognition can be subdivided into static molecular recognition and dynamic molecular recognition. Static molecular recognition is likened to the interaction between a key and a keyhole; it is a 1:1 type complexation reaction between a host molecule and a guest molecule to form a host–guest complex. To achieve advanced static molecular recognition, it is necessary to make recognition sites that are specific for guest molecules. In the case of dynamic molecular recognition the binding of the first guest to the first binding site of a host affects the association constant of a second guest with a second binding site. leading to cooperativity of binding. In the case of positive allosteric systems the binding