Vibrational analysis of Ni(II)‐ and Cu(II)‐octamethylchlorin by polarized resonance Raman and Fourier transform infrared spectroscopy

Abstract

We measured the polarized resonance Raman spectra of Cu(II)-2,2,7,8,12,13,17,18-octamethylchlorin in CS 2 at various excitation wavenumbers in a spectral region covering the Q y , Q x and B x optical absorption bands. Additionally, we measured the FTIR-Raman spectrum of the highly overcrowded spectral region between 1300 and 1450 cm −1 . The spectral decomposition was carried out by a self-consistent global fit to all spectra obtained. The thus identified Raman and IR lines were assigned by comparison with the resonance Raman spectra of Cu(II)-octaethylporphyrin, by utilizing their depolarization ratio dispersions and by a normal mode analysis. The latter was based on a modified transferable molecular mechanics force field of Ni (II)-octaethylporphyrin [E. Unger, M. Beck, R.J. Lipski, W. Dreybrodt, C.J. Medforth, K.M. Smith and R. Schweitzer-Stenner, J. Phys. Chem. B 103, 10229 (1999)]. A comparison of normal mode patterns obtained for Cu(II)-octamethylchlorin and Cu(II)-octaethylporphyrin revealed that some modes are significantly distorted by the reduction of the pyrrole ring, in accordance with results which Boldt et al. reported earlier for Ni(II)-octaethylchlorin [N.J. Boldt, F.J. Donohoe, R.R. Birge and D.F. Bocian, J. Am. Chem. Soc. 109, 2284]. In contrast to conclusions drawn from this study, however, the results of our vibrational analysis and several further lines of evidence suggest that the normal modes of corresponding chlorines and porphyrins are still comparable, because they display contributions from the same local coordinates. Thus, the classical normal mode classification developed for metalloporphyrins is also applicable to metallochlorins. Finally, we performed a preliminary analysis of the absorption spectrum and the resonance excitation profiles and depolarization ratio dispersions of some Raman lines. The results show that the electronic properties of Cu(II)-octamethylchlorin can still be described in terms of Gouterman's four orbital model [M. Gouterman, J. Chem. Phys. 30, 1139Phys. 30, (1959]. In regions of the Q bands, Raman scattering of A 1 modes is determined by interferences between Franck-Condon coupling and interstate Herzberg-Teller coupling between Q x .Q y / and B x .B y / states. The B 2 modes are resonance enhanced by Herzberg-Teller coupling between Q x and Q y and between Q x .Q y / and B y .B x /. Franck-Condon coupling of A 1 modes with large contributions from C a C m stretching vibrations is comparatively strong for Q x . This is interpreted as reflecting the expansion of the chlorin macrocycle by an electronic transition into this excited state.

Using spectroscopic tools to probe porphyrin deformation and porphyrin-protein interactions

“…Generally, values of the unmixing factor lie between 0.05 and 0.15 [50][51][52]. The situation changes dramatically when the symmetry of the heme chromophore is substantially perturbed as it is the case in hydroporphyrins like metallochlorins and chlorophylls [53,54]. In this case, the original MO-transitions are no longer accidentally degenerate and as a consequence the unmixing factor becomes very large.…”

“…The total set of macrocycle modes contains of course more than one representative for each symmetry, which seem to suggest that it might be difficult to unambiguously decompose an observed heme deformation into individual contributions from a set 21 oop-and 45 ip-normal coordinates. These numbers would even increase for porphyrins with bulky substituents, since macrocycle and substituent modes generally mix [54,106]. However, the situation is simpler owing to the fact that the energy, which has to be invested for distorting the heme along a distinct coordinate, is proportional to the square of the frequency of the respective normal mode [79,104].…”

“…polarized resonance Raman dispersion spectroscopy. In its most advanced form this involves the measurement and global analysis of resonance excitation profiles and depolarization ratio (DPR) dispersions over a broad range of excitation wavenumbers [51,52,54,69,117]. It is well established that the Raman cross section of porphyrin modes can vary by orders of magnitude over the spectroscopic range defined by the B and Q-bands of the absorption spectrum.…”

Out‐of‐plane deformations of the heme group in different ferrocytochrome c proteins probed by resonance Raman spectroscopy

“…by an excited state displacement along the normal coordinate of the vibration with respect to the respective equilibrium position in the deformed ground state. 39,43,44 When the heme is perturbed into a lowered symmetry, substituent modes and macrocycle modes of the same symmetry vibrationally mix, especially when their vibrational frequencies are in close proximity. 39 Raman bands from oop deformations such as doming (A 2u ), ruffling (B 1u ) and waving (E g ) have been identified in the low-wavenumber region of the Raman spectra, as well as some bands of ip modes and of substituents, deformations, (ruffling), 21 and 22 (waving), which correspond to the dominant SNCDs found for the heme groups of the three investigated cytochrome c molecules.…”

Short‐time photodecay dynamics of meso‐tetra(p‐hydroxyphenyl)porphine in the condensed phase explored via resonance Raman spectroscopy and density functional theory calculation

“…Egawa et al also reported that the A 2 modes should be considered as the best means for elucidating the vibronic character of the absorption bands of porphyrins. [15,34] B y -band THPP photodissociation dynamics and comparison with previous work on porphyrin…”