Excited State Dynamics and Heterogeneity of Folded and Unfolded States of Cytochromec

Abstract

Cytochrome c is an electron transfer protein whose redox and folding properties have received a great deal of attention. In this study, transient absorption, transient grating, and three pulse photon echo peak shift (3PEPS) measurements have been used to characterize the photophysics of the heme chromophore in the folded protein and in two different unfolded proteins. The data are interpreted in terms of a reactive three level system. A Soret excited state lifetime of 40 to 50 fs is observed, which is not sensitive to the folded state of cytochrome c. An intermediate time scale of 200 to 400 fs is also observed, which may be associated with the Q state lifetime. The longest time scale, assigned to ground state recovery, is approximately 2 to 4 ps. The dynamics are discussed in terms of the folded and redox states of the protein. The 3PEPS data show that the folded protein has little structural heterogeneity; however, the unfolded proteins show a larger degree of structural heterogeneity, the extent of which depends on the unfolding conditions.