Influence of Copper(II) on Proton Nuclear Relaxation Rates of Poly-L-histidine

Proton NMR chemical shifts and nuclear relaxation rates have been measured for the C2 and C4 protons of poly-L-histidine (PLH) as a function of pH. The NMR titration curves of chemical shift indicate a pKa value of 6.1 for the imidazole groups. The C4 proton is relaxed more efficiently than the C2 proton at all values of pH due to its proximity to the protons. The relaxation rates are not affected appreciably by changes in pH. In the presence of Cu(II) the relaxation rates show a pronounced maximum at ca. pH 3.5. Above this pH value, temperature dependence studies indicate that slow exchange is occurring between Cu(II) and PLH. On the basis of this and other evidence, we conclude that the maximum in the relaxation rates appears to represent the pH of most rapid exchange of Cu(II) ions between the ligand sites.