Hydrogen exchange in unligated and ligated staphylococcal nuclease

The exchange kinetics of over 70% of the 143 backbone amide hydrogens in staphylococcal nuclease H124L (nuclease H124L), both in its unligated state and in its ternary complex Ca 2+ and thymidine 3',5'-bisphosphate, have been quantified by nitrogen-15 resolved proton nuclear magnetic resonance spectroscopy. Protection factors for the slowly exchanging hydrogens in unligated nuclease H124L at 37 o C and pH * 5.5 were found to vary by over one order of magnitude. This range of protection factors has been interpreted in the framework of global and local structural fluctuations