Proton transfer and polarizability of hydrogen bonds formed between cysteine and lysine residues

(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.