Intramolecular Vibrational Excitation of Unfolding Reactions in Zn~(II)-Substituted and Metal-Free Cytochromes c:Activation Enthalpies from Integrated Fluorescence Stokes Shift and Line Shape Excitation Profiles

摘要

We have employed continuous-wave fluorescence spectroscopy to observe the light-induced formation of partially unfolded states of Zn~(II)-substituted and metal-free(or free-base)cytochrome c(ZnCytc and fbCytc,respectively).In these experiments,the intrinsic porphyrin chromophore provides a vibrational excitation to the protein structure via intramolecular vibrational redistribution of the excess vibronic energy above the first excited singlet state.As the excitation light source is tuned,the fluorescence spectrum of both systems exhibits steplike transitions of the integrated Stokes shift,vibronic structure,and line width that mark apparent activation enthalpy barriers for structural transitions of the protein from the native state to a set of at least three partially unfolded states.The vibronic structure of the ZnCytc spectrum reports the exchange of the Zn~(II)ion's native H18 and M80 axial ligands with non-native ligands as the excitation wavenumber is scanned through the three barriers.The metal ion's axial ligands contribute substantially to the stability of ZnCytc;the activation enthalpies for the corresponding transitions in fbCytc are one-third of those in ZnCytc.A comparison of the present results from ZnCytc with those obtained previously with picosecond time-resolved methods[Lampa-Pastirk and Beck,J.Phys.Chem.B 2006,110,22971-22974]indicates that the vibrationally excited protein structure propagates along an unfolding pathway from the native state that specifically populates the three states in order of their activation enthalpies.The excitation-wavenumber profile of the fluorescence line width is markedly inconsistent with a Maxwell-Boltzmann distribution over the three states.These results contrast with the general expectation of the protein-folding funnel hypothesis that a distribution of intermediate structures should result from the diffusive propagation of a nonequilibrium protein structure.