Folding of staphylococcal nuclease: magnetic resonance and fluorescence studies of individual residues.

The reversible unfolding and folding of staphylococcal nuclease in the acid transition has been studied by 220 MHz proton magnetic resonance spectroscopy. The values of area, line-width, and chemical shift of each of the imidazole C2 proton resonances of the four histidine residues have been measured in this transition. The change of areas of three histidine resonances and the change of fluorescence of the single tryptophan residue, as a function of pH, appear to follow a single equilibrium. In contrast, a fourth histidine resonance follows a biphasic transition. These findings indicate that local conformational changes can be detected by magnetic resonance spectroscopy in the cooperative transition of the overall structure.