Republic of Germany
Synopsis
We studied films of poly(L-tyrosine) with hydrogen phosphate (residue/phosphate, 1:l) by ir spectroscopy. The influences of the alkali cations (Li+, Na+, K+) and of the degree of hydration were clarified. If Li+ ions are present, the OH-. --OP hydrogen bonds formed in the dried films between the tyrosine OH groups and hydrogen phosphate are asymmetrical.
The formation of hydrogen phosphate-hydrogen phosphate hydrogen bonds is prevented by the presence of the Li+ ions. With an increase in the degree of hydration, the tyrosinephosphate bonds are not broken hut become slightly stronger. Completely different behavior is found if K+ ions are present. In dry films, the OH-. .-OP + 0-. . .HOP hydrogen bonds formed between tyrosine and hydrogen phosphate show large proton polarizability. The tyrosine proton has a noticeable residence time at the acceptor 0 atom of the phosphate. The difference in the behavior of the system with K+ ions when compared to the system with Li+ ions can be explained, since the hydrogen acceptor 0 atom of phosphate ions is more negatively charged due to the weaker influence of the K+ ions. Furthermore, POH-. 0-OP hydrogen bonds between hydrogen phosphate molecules are formed. With an increase in the degree of hydration, the tyrosine-hydrogen phosphate hydrogen bonds are broken, all tyrosine protons are found a t the tyrosine residues, and the -POBgroupings are in a symmetrical environment, indicating that the K+ ions are removed from these groupings. If' the degree of hydration increases further, hydrogen-bonded systems such as hydrogen phosphatewater-hydrogen phosphate are formed that show large proton polarizability due to collective proton motion. When Na+ ions are present, the OH. . .-OP + 0-. . -HOP hydrogen bonds formed in dry films still show proton polarizability, but the residence time of the tyrosine proton a t the phosphate is very short.