Q-Band Resonance Raman Enhancement of Fe−CO Vibrations in Ferrous Chlorin Complexes:  Possible Monitor of Axial Ligands in d Cytochromes

Abstract

Resonance Raman (rRaman) spectroscopy has been used extensively in the studies of the heme chemistry of carbon monoxy adducts. In porphyrin systems, the axial ligand vibrational modes of the CO, ν(Fe−CO) and ν(CO), are enhanced with Soret excitation via an A-term (Franck−Condon) mechanism, but are not expected with Q excitation (B-term or vibronic mechanism). For the first time, these modes have been obtained with Q y as well as Soret excitation in rRaman spectra of CO complexes of ferrous chlorins. The enhancement with Q y excitation arises from an A-term mechanism of Raman scattering for these chlorins owing to their reduced molecular symmetry. Thus, in iron chlorins or other heme systems with reduced molecular symmetry, axial ligand vibrational modes may be enhanced with Q y excitation if they are observed with Soret excitation in the corresponding iron porphyrins. These findings show rRaman spectroscopy to be exceptionally valuable in the study of chlorin chromophores with Q y enhancement using red or orange-red excitation. Furthermore, the method appears to be selective for chlorin cofactors in proteins containing multiple heme centers such as cytochrome bd oxidase (see, for example, Sun; et al. Biochemistry 1995, 35, 2403−2412). It has been known that ν(Fe−CO) and ν(CO) frequencies of CO complexes of iron porphyrins and heme proteins exhibit linear correlations, falling into distinct sets for complexes possessing the same fifth ligand (for example, Ray; et al. J. Am. Chem. Soc. 1994, 116, 162−176). In this work, we have found that ν(Fe−CO) and ν(CO) of iron-chlorin−CO complexes also respond to the nature of the opposite axial ligand and follow the same correlations derived from porphyrin systems. Thus, the reduction of one of the pyrrole rings of porphyrins has little effect on ν(Fe−CO) and ν(CO) frequencies, and their correlation behavior may perhaps be used to ascertain the identity of the proximal ligand of the chlorin in a protein system of unknown coordination, as in cytochrome bd oxidase.

Replacement of the Proximal Histidine Iron Ligand by a Cysteine or Tyrosine Converts Heme Oxygenase to an Oxidase

“…The ν Fe-CO is observed at 529 cm -1 (downshifts 5 cm -1 with 13 CO) and the ν CO at 1962 cm -1 (downshifts 45 cm -1 with 13 CO). These frequencies are identical to those observed in the H25A hHO-1 mutant ( 21) and are characteristic of a heme-CO complex with a very weak (e.g., H 2 O) or no proximal ligand (46,47). Our RR data show that the cysteinate or tyrosinate ligation observed in the ferric states of the mutated enzymes is lost upon reduction, resulting in a heme configuration identical to that in the H25A hHO-1 mutant in which there is no proximal ligand to the heme iron.…”

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

“…With the exception of some low-wavenumber E u modes we obtained a 1 : 1 correspondence for all in-plane macrocycle modes of CuOMC and CuOEP. Taken together, these results substantiate the earlier notion of Sun et al 27 that in spite of the localization effects caused by the pyrrole reduction, the normal modes of porphyrins and chlorins are to a major extent comparable to each other.…”

“…36 In principle, the results of our calculations suggest that the normal mode patterns are indeed significantly changed by pyrrole reduction in accordance with results of Boldt et al 18 and Procyk et al 19 However, in spite of significant localization and mixing effects, the normal mode pattern resulting from these studies seem still to be comparable to porphyrins modes, because nearly the same local coordinates determine the eigenvectors. 27 This notion is corroborated by the fact that some of the high-wavenumber modes of metallochlorins and metalloporphyrins show a similar dependence on core size 37 and out-of-plane distortions. 35 Therefore, the question arises of whether it is possible to obtain sufficient evidence for correlations between chlorin and porphyrin modes from their resonance Raman spectra.…”

“…Loehr and coworkers, although recognizing localization for some modes, argue that the eigenvectors of the most relevant structure sensitive modes such as 3 and 10 can still be related to hydroporphyrin modes. 26,27 Recently, Ceccarelli et al 28 reported a normal coordinate analysis of bacteriochlorophyll a, which for the first time is based on a force field obtained from DFT ab initio calculations. Their results seem to be more in agreement with the those from Bocian's group.…”

Polarized Resonance Raman Spectroscopy Reveals Two Different Conformers of Metallo(II)octamethylchlorins in CS2

“…By comparing the normal modes of Fe(II)OEC and Fe(II)OEP, Sun et al came to the conclusion that pyrrole reduction has only a limited influence on the characteristic properties of the classical marker modes ν 2 , ν 3 , ν 4 , ν 11 , and ν 19 . They found, for instance, that the frequency shifts of Fe(II)OEC isotopomers are in the range of those found for corresponding iron porphyrins.…”