Hydrogen-deuterium exchange of the C-2 protons of histidine and histidine peptides and proteins

Kinetic equations previously developed for the H-D exchange of the C-2 protons of an isolated imidazole group or for an imidazole group with an adjacent charged group of pK 6-12, have been analysed in detail. By measurement of the pseudo-first-order rate constant k for exchange in D₂O as a function of pD, it is possible to obtain the second-order rate constant k ₂ and the pK of the histidine residue. A nearby charged group with a pK two or more units greater than that of the histidine residue, causes a double S shaped curve when k is plotted against pD, from which the pK of the nearby charged group may be determined. The size of the second step in the curve is related to the closeness of the charged group to the histidine residue. Kinetic equations have been developed for more complex cases involving (a) two adjacent imidazole rings and one adjacent charged group of pK 6-12 and (b) an imidazole group with two adjacent charged groups of pK 6-12. The equations involve numerous second-order rate constants and equilibrium constants which cannot be evaluated explicity using experimental data obtained for L-histidyl-L-histidine, L-histidyl-L-tyrosine, and L-histidyl-L-lysine. For histidine residues in proteins it is possible to determine their pK and k₂ values for H-D exchange. From the magnitude of these values one can infer the type of environment of the histidine residue. Thus the data for certain histidine residues in lysozyme, ribonuclease, myoglobin, and leghaemoglobin have been rationalised in terms of their known environments obtained from X-ray structural studies.