LK7b and LE7b, 2, 14, 24 which have sequences Ac-LKLKLKL-NH2 and Ac-LELELEL-NH2, respectively. They fold into amphiphilic, antiparallel b-sheets at the air-water interface, making them ideal for chiral SFG studies. The hydrophobic leucine residues (L) point into the air while the positively charged lysine (K) or the negatively charged glutamate (E) residues point into the water. It should be noted that the results presented here apply to other systems, including other proteins and even non-protein systems (e.g., DNA 1 ), because the main result concerns the chiral water response rather than the response of a specific biomolecule. Therefore, the proof of the model that explains the up-down (or down-up) line shape (equation 5 and Figure ) remains valid regardless of the chemical identity of the biomolecules.