pH-induced structural transitions of bovine serum albumin. Histidine pKa values and unfolding of the N-terminus during the N to F transition.

1. Titration curves have been constructed for all the resolvable resonances of defatted bovine serum albumin [pH meter readings in 2H2O solution (pH*) 2-10], including complete curves for six His residues and partial curves for a further four. Three His residues have unusually low pKa values (less than 5.5). 2. Abrupt discontinuities in the titration curves of nearly all resonances were observed near pH 4.3 (N to F transition), together with a marked loss of chemical shift dispersion at low pH*. No such loss of dispersion occurs in the high pH region previously associated with the N to B transition (approximately pH 8). The NMR data provide strong evidence that the N to F transition is not localised in domain III as had been previously suggested. 3. A localised unfolding of the N-terminal region occurs below pH* 4.5.