Proton transfer in and polarizability of hydrogen bonds in proteins. Tyrosine-lysine and glutamic acid-lysine hydrogen bonds — IR investigations

Abstract

The OH ⋯ N ⇌ O⁻ ⋯ H⁺N hydrogen bonds formed between tyrosine and lysine, and between glutamic acid and lysine residues are studied by infrared spectroscopy considering the following systems: (l-lys)_n + phenol, copoly (l-lys, l-tyr)_n, (l-lys)_n + (l-tyr)_n and (l-lys)_n + (l-glu)_n. The phenol-lysine hydrogen bonds are largely symmetrical in the average if the pK_a of the protonated lysine is 2.2 units larger than that of the phenols. In the case of the hydrogen bonds between tyrosine and lysine residues in copoly (l-lys, l-tyr)_n and (l-lys)_n + (l-tyr)_n, the weight of the proton limiting structure OH ⋯ N is 80–90%, and that of the polar O⁻ ⋯ H⁺N structure 10–20%. Double minimum proton potentials occur but the proton is preferentially present at the tyrosine residues. In the (l-lys)_n + (l-glu)_n system, the protons are present at the lysine residues. Thus, these hydrogen bonds have very large dipole moments (about 10 D). With the lysine-phenole hydrogen bonds, hydration shifts the proton transfer equilibrium a little in favour of the polar proton limiting structure O⁻ ⋯ H⁺N. These hydrogen bonds are broken to a large extent, however, when only about 3 water molecules are present per lysine residue. When less water is present, as in the copoly (l-lys, l-tyr)_n and (l-lys)_n + (l-tyr)_n systems, these hydrogen bonds are, however, formed quantitatively. Thus — as discussed in this paper — the tyrosine-lysine hydrogen bonds can participate in proton conducting hydrogen bonded systems — as, for instance, present in bacteriorhodopsin — performing the proton transport through hydrophobic regions of biological membranes.