Hydrogen exchange and protein hydration: the deuteron spin relaxation dispersions of bovine pancreatic trypsin inhibitor and ubiquitin.

Water deuteron (2H) spin relaxation was used to study hydrogen exchange, hydration, and protein dynamics in aqueous solutions of the globular proteins bovine pancreatic trypsin inhibitor (BPTI) and ubiquitin. The frequency dispersion of the longitudinal 2H relaxation rate was measured in the pD range 2 to 11 at 27 degrees C. In contrast to the previously reported water 17O relaxation dispersion from the same samples, the 2H dispersion depends strongly on pD. This pD dependence is due to labile protein deuterons in acidic side-chains and surface peptide groups, which exchange rapidly with water deuterons. The pD dependence of the 2H relaxation in BPTI solutions could be quantitatively accounted for in terms of known pK values and hydrogen exchange rate constants. For ubiquitin, labile protein deuterons contribute importantly to the 2H relaxation dispersion even in the neutral pD range. The 2H relaxation data also provided information about the orientational order and internal motion of OD and ND bonds in side-chains and surface peptides. A comparison of the water contribution to the 2H dispersion with the 17O dispersion indicates that one of the four internal water molecules of BPTI, presumably the deeply buried water molecule W122, exchanges more slowly (10(-6) to 10(-4) second) than the other three (10(-8) to 10(-6) second).