Highly resolved depolarization dispersion and excitation profiles of Raman fundamentals of protoporphyrin IX in a cytochrome c matrix

Abstract

New experimental data for depolarization dispersion (DPD) and excitation profiles (EP) with high spectral resolution (20 cm⁻¹) are provided for Raman fundamentals of protoporphyrin IX in ferrocytochrome c between 17000 and 22 500 cm⁻¹, covering the resonant region of the Q-bands and the pre-resonant region of the B-bands. Both DPD and EPs of the lines at 1312 and 1585 cm⁻¹ (A_2g), at 1363 cm⁻¹ (A_1g) and at 1621 cm⁻¹ (B_1g) exhibit sharp structures. These result from multi-mode contributions of vibrations which give intensity to the vibronic sideband of the Q-absorption. On the basis of these highly resolved new data we have shown that the Raman tensor can be formulated in terms of Loudon's strictly time-dependent perturbation formulation to third order augmented by a fifth-order term representing the multi-mode effects. From the EPs, which were measured in absolute units, we obtained values of vibronic coupling parameters for the Raman fundamentals which are consistent with the optical absorption spectra and show that vibronic coupling is weak for this type of molecule.