RESONANCE RAMAN-SCATTERING AS A PROBE OF ELECTRON NUCLEAR COUPLING - APPLICATIONS TO HEME-PROTEINS

We report Raman excitation profile studies of metmyoglobin (metMb) and the CO complexes of Mb and cytochrome P450cam and extract the relevant electron-nuclear coupling parameters using a Kramers-Kronig transform technique. A variety of heme vibrational modes are investigated, including the important core-size marker band, nu-3, and the Fe-CO stretching and bending modes. We find that the "red-shift" previously observed for the nu-3 REP of the oxidized P450 complex is not specific to P450. In fact, it appears to be a rather general phenomenon for nu-3 modes in heme proteins. Detailed analysis of the metMb data leads to the surprising result that this polarized, totally symmetric mode has an unusually small Condon coupling and gains its Raman activity through a non-Condon mechanism involving vibrational mixing of the Soret band with other blue-shifted excitations of the heme (e.g., the N band). The important Fe-CO modes are found to be coupled to the Soret resonance primarily through a simple Condon mechanism for both MbCO and P450-CO. A detailed investigation of the Fe-CO doublet structure (A0 and A1 states) in low-pH solution samples of MbCO, as well as in single crystals, shows that the dispersion of the relative intensities of these modes is consistent with small differences in non-Condon coupling. The relative merits of the transform/time correlator and the wave-packet propagation techniques for analysis of finite-temperature, condensed-phase resonance Raman spectra are also briefly discussed.