Application of the transform theory to resonance Raman excitation profiles in the Soret region of cytochrome-c

Publisher Website
Google Scholar
Abstract
Recently, the transform method for calculating Raman excitation profiles (REP’s) from the absorption spectrum has been extended to include both the Condon and non‐Condon sources of scattering. In the present work, this theory is applied to REP’s in the Soret region of ferrocytochrome‐c, where data are presented for two totally symmetric modes: 690 cm−1 (at 296 K) and 1362 cm−1 (at 296 and 77 K). The low temperature data provide an important test of the transform method since the theory is usually implemented in a form that applies only at low temperatures. Previously published REP data for the 1372 cm−1 totally symmetric mode of ferricytochrome‐c are also analyzed. It is found that the transform theory is able to reproduce the subtle differences in the shape of the experimental REP’s very accurately for all the variations of temperature, oxidation state, and Raman frequencies mentioned above. These fits are accomplished with one free parameter, the ratio of the non‐Condon to Condon scattering at the amplitude level. Under the assumptions of the analysis, the best fits to the REP data are obtained when this ratio is about −0.13 for the two modes of ferrocycochrome‐c considered, and when it is approximately zero for the 1372 cm−1 mode of ferricytochrome‐c. The Franck–Condon coupling strength (displacement of the excited state potential) can also be deduced from the transform equation once the absolute scattering cross section is known. Values of the dimensionless Franck–Condon displacement parameter from 0.023 to 0.033 are calculated for the various experimental REP’s of cytochrome‐c. The multimode nature of the Soret band (and REP) is clearly demonstrated by the transform method. It is seen how a distinctly asymmetric shape of the REP of the 1362 cm−1 vibrational mode of ferrocytochrome‐c is qualitatively reproduced by a Condon (A term) only transform of the Soret absorption band. In contrast, the A‐term transform of an absorption band built principally of a single FC active mode cannot yield the desired asymmetry. However, it is found that the best transform fit to the observed REP is obtained only after a 13% addition of non‐Condon (B, C‐term) amplitude. It therefore appears that both multimode and non‐Condon effects are needed to explain the data. If the small empirically determined non‐Condon contribution to the REP is arbitrarily assigned entirely to Soret–Q‐band vibronic coupling (in the full adiabatic scheme), that coupling energy is about 500±200 cm−1. Estimates of the same parameter from Q‐band information (REP and β‐band intensity) come to about 0.6 of this value. This discrepancy, if confirmed by more precise work, would indicate a mild breakdown of the commonly used two‐state (Soret–Q‐band) vibronic model. The present transform approach to the non‐Condon component of the REP, though only linear in nuclear displacement, is otherwise general. Within the full adiabatic context, it corresponds to a complete‐basis‐set vibronic model.