A proton nuclear magnetic resonance study of human serum albumin in the neutral pH region.

By means of 500 MHz 1H-NMR spectroscopy detailed information has been obtained about the proton titration behaviour of the histidine residues of human serum albumin under various experimental conditions. The approximate number of His C2 protons resolved varied from 11 at pH* 9.0 to 16 at pH* 5.5. The latter is the same as the number of histidine residues reported to be present in the molecule. The symbol pH* stands for the meter readings uncorrected for the deuterium isotope effect. Whereas some of the NMR signals remained visible over the entire pH* range examined (5-9), others, having pH*-dependent resonance intensities, were observed in part of the pH* range only. The latter effect is indicative of an alteration in the histidine environment in the course of pH* titration. It is thought that the so-called N-B conformational transition, occurring in the neutral pH region, is responsible for this alteration. The spectral effects observed upon the addition of Ca2+ can be interpreted in terms of a downward pK* shift for a number of histidine residues and a concomitant downward shift in the midpoint pH* of the N-B transition. This corroborates the results of previous investigations on this matter.