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- ') are provided for Raman fundamentals of protoporphyrin IX in ferrocytochrome c between 17000 and 22500 cm-', 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-' (Azs), at 1363 cm-' (Als) and at 1621 cm-' (B,#) 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 timedependent 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. INTRODUCGONResonance Raman scattering has been used extensively to study vibronic coupling in the excited states of porphyrin molecules. In this class of molecule, the lowest singlet excited states are located in the visible region and are therefore easily accessible to the measurement of excitation profiles (EP) and depolarization ratio dispersion (DPD) by tunable dye lasers. Under D,, symmetry, as is approximately the case for octaethylporphyrin for instance, the depolarization ratio (DPR) of the A', , B,, , A,, and B,, Raman fundamentals is independent of the exciting laser frequency.' A,, modes exhibit a value of 1/8, B1, and B2, modes a value of 3 14 and A', modes a value of infinity.Experimental results on porphyrins, which are subject to symmetry-lowering perturbations, show, however, strong dispersion of their depolarization ratio with exciting wavelength. Symmetry-lowering perturbations are either asymmetric side-chains on the porphyrin ring skeleton' or interactions of the porphyrin molecule with the protein matrix in which it is Consequently, intense theoretical work has been carried out to elucidate Raman EPs and depolarization ratios in pure D,, symmetry' and in environments of low symmetry.1,6-8 This work has led to a good understanding of vibronic couplings that determine the features exhibited by the Raman data, in both the weak and the strong coupling limits.Zgierski and Pawlikowski' have given a systematic treatment of the influence of asymmetric perturbations of electronic and vibronic origin on the EPs and DPD of metalloporphyrins and metallophthalocyanines by exact diagonalization of the corresponding energy matrix, which contains both the electronic and the vib-* Author to whom correspondence should be addressed. ronic Hamiltonian in D, , symmetry and also the symmetry-lowering environmental perturbations. The matrix is diagonalized in a basis set of products of harmonic oscillator functions and the I Qxo), I Q y o ) , I B x o ...

Polarized resonance Raman dispersion spectroscopy on metalporphyrins

“…The occurrence of the former was earlier derived from the DPD of n 19 , but these studies only focused on pure electronic distortions, while Refs. [30,66] show that vibronic distortions are more relevant for understanding the experimental DPDs. Moreover, the exceptionally strong DPD of n 4 for heme proteins (cf.…”

“…Though theoretically sound, this approach yielded only a qualitative description of the data. In response to Zgierski's criticism Bobinger et al [66] reported much more detailed REPs and DPDs of four ferrocytochrome c Raman lines, namely, n 4 (A 1g ), n 19 and n 21 (both A 2g ), and n 10 (B 1g ), which covered the entire resonance region of Q 0 and Q v and additionally the preresonant region of the B-band. While the authors employed the theoretical approach of Schweitzer- Fig.…”

The importance of vibronic perturbations in ferrocytochrome c spectra: A reevaluation of spectral properties based on low-temperature optical absorption, resonance Raman, and molecular-dynamics simulations

“…18 For the vibrational ground state of the Q state, second-order Rayleigh-Schrödinger perturbation theory yields…”

“…17 Schweitzer-Stenner and co-workers performed several resonance Raman dispersion studies to obtain quantum-mechanical parameters which reflect electronic and vibronic perturbations of the cytochrome macrocycle. 18,19 A new and particularly important aspect was added to the debate on heme c distortions when Shelnutt and co-workers described comparatively large out-of-plane deformations ͑ruffling and waving͒, which were found to be mostly conserved for cytochrome c species for which the amino acid residues between the cysteine linkages to the heme group is homologous. 20,21 More recently, Manas et al attempted to explore the influence of heme-protein interactions on the optical properties of the heme group in ferrocytochrome c. 3 They measured the Q-band spectra of horse heart ͑hhc͒, beef, porcine, tuna ͑thc͒, chicken, and yeast cytochrome c ͑yc͒ at cryogenic temperature which allows a better separation of overlapping bands due to the absence of thermal broadening, and reported splittings of different magnitudes for different cytochromes.…”

Non‐planar heme deformations and excited state displacements in horseradish peroxidase detected by Raman spectroscopy at Soret excitation

“…In a recent study of resting HRP C, 5 we modeled the depolarization ratio dispersion of several structuresensitive ip modes of the respective heme group by utilizing coupling values obtained from the earlier analysis of Ni(II) porphin 17 and Ni(II) octaethyltetraphenylporphin, 16 and heme proteins such as ferrocytochrome. 18 In the present study, we focused on Raman scattering by oop modes at Soret excitation, for which the intensity arises mainly from BB intra-state coupling. 13 This comprises Franck-Condon coupling for all modes with an A 1g -type Raman tensor and Jahn-Teller coupling for modes with B 1g and B 2g tensors.…”