N.m.r. analyses of the histidine microenvironments in a human salivary proline‐rich glycoprotein

The pK_a's of the three histidine residues in a proline‐rich glycoprotein from human parotid saliva (PRG) were determined by 360 MHz proton n.m.r. spectroscopy. The addition of calcium (0.64 mm) caused drops in the pK_a's of all three histidines by ∼0.25 units. When imidazole and cyclo(l‐histidine‐l‐proline) were used as model compounds, corresponding concentrations of calcium had no effect on their pK_a's. Also, the model compounds gave absolute pK_a values in good agreement with similar chemical species reported in the literature. Exchange lifetime data and previously reported hydrogen → deuterium exchange experiments suggest that the PRG histidine N^tH protons are not involved in hydrogen‐bonds. Collectively, these data imply that changes in PRG conformation occur upon the addition of calcium.