An 15N NMR spin relaxation dispersion study of the folding of a pair of engineered mutants of apocytochrome b562

N-15 relaxation dispersion NMR spectroscopy has been used to study exchange dynamics in a pair of mutants of Rd-apocyt b(562), a redesigned four-helix-bundle protein. An analysis of the relaxation data over a range of temperatures establishes that exchange in both proteins is best modeled as two-state and that it derives from the folding/unfolding transition. These results are in accord with predictions based on the reaction coordinate for the folding of the protein determined from native-state hydrogen exchange data [Chu, R.; Pei, W.; Takei, J.; Bai, Y. Biochemistry 2002, 41, 7998-8003]. The kinetics and thermodynamics of the folding transition have been characterized in detail. Although only a narrow range of temperatures could be examined, it is clear that the folding rate temperature profile is distinctly non-Arrhenius for both mutants, with the folding barrier for at least one of them entropic.