1H-NMR study on the tautomerism of the imidazole ring of histidine residues. II. Microenvironments of histidine-12 and histidine-119 of bovine pancreatic ribonuclease A.

The NMR titration curves of proton chemical shifts were observed for the C2 protons of histidine residues in intact bovine pancreatic RNAase A (EC 3.1.27.5) and carboxyalkylated RNAase A. By comparing the methyl region of NMR spectra, the 250-340 nm region of circular dichoic spectra, and the NMR titration curves of tyrosine ring protons among intact and modified RNAase A, it was ascertained that the carboxyalkylation of histidine residues at position 12 or 119 did not make any appreciable conformational changes to RNAase A. With the pK values determined for intact and modified RNAase A, the microscopic pK values and molar ratios of tautomers were estimated for His-12 and His-119 by means of the procedure described in the preceding paper. The estimated microscopic pK values of tautomers were 6.2 for the N1-H tautomer of His-12, more than 8 for the N3-H tautomer of His-12, 7.0 for the N1-H tautomer of His-119, and 6.4 for the N3-H tautomer of His-119, respectively. These values were interpreted in terms of the microscopic environments surrounding the histidine residues. The microscopic structure estimated in the present study was discussed, comparing it with those from X-ray crystallography and hydrogen-tritium (or hydrogen-deuterium) exchange technique.