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synopsis(L-Cys),, (L-LYS),, and (L-G~u), were studied by ir spectroscopy in terms of their degree of deprotonation or protonation. It is shown that structurally symmetrical, easily polarizable SH-S-+ -S--HS, N+H-N =N.--H+N, and OH---0-* -0-HO hydrogen bonds are formed between the side chains. The different wave number distributions of the ir continua caused by these hydrogen bonds show that the barrier in the double-minimum proton potential decreases in the series of these hydrogen bonds. The stability of these hydrogen bonds against hydration increases in this series. The OH-0-+ -O.-HO bonds are not broken by small amounts of water. With (L-CYS), the formation of easily polarizable hydrogen bonds and a fl-structure-coil transition are strongly interdependent. As a result of this coupling effect, the fi-structure-coil transition becomes cooperative. With (L-G~u),, the formation of the polarizable hydrogen bonds and the observed conformational change are independent processes. The (L-Glu), conformation changes from a-helix to coil only if more than 80% of the residues are deprotonated. Finally, on the basis of the various types of easily polarizable hydrogen bonds, charge shifts in active centers of enzymes and the proton-conducting mechanism through hydrophobic regions of biological membranes are discussed.

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(L‐Cys)n + N‐base systems and (L‐Cys)n + (L‐Lys)n systems were studied by ir spectroscopy. It is shown that in the water‐free systems, SH ⃛N ⇌ S− ⃛H+N hydrogen bonds are formed. With the (L‐Cys)n + N‐base systems, both proton‐limiting structures in the SH ⃛N ⇌ S− ⃛H+N bonds have equal weight when the pKa of the protonated N‐base is 2 pKa units larger than that of (L‐Cys)n. The same is true with the water‐free (L‐Cys)n + (L‐Lys)n system. Thus, with regard to the type of proton potentials present, these hydrogen bonds are proton‐transfer hydrogen bonds showing very large proton polarizabilities. This is confirmed by the occurrence of continua in the ir spectra. Small amounts of water open these hydrogen bonds and increase the transfer of the proton to (L‐Lys)n. In the (L‐Lys)n + N‐base systems, with increasing proton transfer the backbone of (L‐Cys)n changes from antiparallel β‐structure to coil. In (L‐Cys)n + (L‐Lys)n, the conformation is determined by the (L‐Lys)n conformation and changes depending on the chain length of (L‐Lys)n. Finally, the reactivity increase in the active center of fatty acid synthetase, which should be caused by the shift of a proton, is discussed on the basis of the great proton polarizability of the cysteine–lysine hydrogen bonds.