Resonance Raman characterization of iron-chlorin complexes in various spin, oxidation, and ligation states. 2. Low-frequency skeletal vibrations and iron ligand stretching modes

Resonance Raman and absorption infrared with density functional theory studies of Fe(III)/Fe(II) and Ni(II) complexes of modified 21-oxaporphyrins

This work presents a complete vibrational analysis of iron [ Fe (II) and Fe (III)] and nickel [ Ni (II)] complexes with 5,10,15,20-tetraphenyl-21-oxaporphyrin [OTPPH] and 5,20-bis(p-tolyl)-10,15-diphenyl-21-oxaporphyrin [ODTDPPH]. In these porphyrins, a furan ring replaces one of the pyrrole rings. The six-coordinate (OTPP) FeIIICl2 and (ODTDPP) FeIIICl2 as well as the five-coordinate (OTPP) FeIICl and (OTPP) NiIICl complexes were investigated using experimental and theoretical methods. The experimental part of this work involved Fourier-transform absorption infrared (FT-IR), resonance Raman (RR), and electron absorption (UV-vis) measurements for all of the investigated complexes. In the theoretical section, optimized geometries and vibrational frequencies for model compounds are provided. The theoretical calculations were performed at the B3LYP level with the LANL2DZ basis set. Good agreement was achieved between the experimental and theoretical vibrational spectra. In addition, charge distributions (GAPT) and geometrical aromaticity indices (Bird's I5 and HOMA) were calculated and discussed.

Resonance Raman studies of Escherichia coli cytochrome bd oxidase. Selective enhancement of the three heme chromophores of the "as-isolated" enzyme and characterization of the cyanide adduct

Cytochrome bd oxidase is a terminal bacterial oxidase containing three cofactors: a low-spin heme (b558), a high-spin heme (b595), and a chlorin d. The center of dioxygen reduction has been proposed to be at a dinuclear b595/d site, whereas b558 is mainly involved in transferring electrons from ubiquinone. One of the unique functional features of this enzyme is its resistance to high concentrations of cyanide (Ki in the millimolar range). With the appropriate selection of laser lines, the ligation and spin states of the b558, b595, and d hemes can be probed selectively by resonance Raman (rR) spectroscopy. Wavelengths between 400 and 500 nm predominantly excite the rR spectra of the b558 and b595 chromophores. Spectra obtained within this interval show a mixed population of spin and ligation states arising from b558 and b595, with the former more strongly enhanced at higher energy. Red excitation wavelengths (590-650 nm) generate rR spectra characteristic of chlorins, indicating the selective enhancement of the d heme. These rR results reveal that cytochrome bd oxidase "as isolated" contains the b558 heme in a six-coordinate low-spin ferric state, the b595 heme in a five-coordinate high-spin (5cHS) ferric state, and the d heme in a mixture of oxygenated (FeIIO2 <--> FeIIIO2-; d650) and ferryl-oxo (FeIV = O; d680) states. However, the rR spectra of these two chlorin species indicate that they are both in the 5cHS state, suggesting that the d heme is lacking a strongly coordinated sixth ligand.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytochrome d axial ligand of the bd-type terminal quinol oxidase from Escherichia coli

Using various spectroscopic techniques, we studied the structure of the dioxygen reduction site of the bd-type terminal quinol oxidase in the aerobic respiratory chain of Escherichia coli. Resonance Raman and FT-IR spectroscopies identified the v(Fe(2+)-CO) and v(C-O) stretching frequencies at 471 and 1980.7 cm-1, respectively, at the cytochrome d center of the dithionite-reduced CO-bound enzyme. The CO ligation in the cytochrome bd complex is considerably different from those of the heme-copper terminal oxidases. Anaerobic addition of NO to the air-oxidized enzyme caused an exchange of cytochrome d-bound dioxygen with NO leading to an appearance of cytochrome d-NO EPR signal. But there is no superhyperfine structure originating from the cytochrome d proximal 14N ligand in the central resonance of the NO EPR signal. These results suggest that cytochrome d axial ligand of the cytochrome bd complex is likely a histidine residue in an anomalous condition or other than a histidine residue and, therefore, the molecular structure around the dioxygen-binding site is different from that of the heme-copper terminal oxidases.

Metal-ligand vibrations of cyanoferric myeloperoxidase and cyanoferric horseradish peroxidase: evidence for a constrained heme pocket in myeloperoxidase

The low-frequency FeCN vibrations of cyanoferric myeloperoxidase (MPO) and horseradish peroxidase (HRP) have been measured by resonance Raman spectroscopy. The ordering of the frequencies of the predominantly FeC stretching and FeCN bending normal vibrational modes in the two peroxidases differs. These normal mode vibrations are identified by their wavenumber shifts upon isotopic substitution of the cyanide ligand. For MPO, the stretching mode nu 1 (361 cm-1) occurs at a lower frequency than the bending mode delta 2 (454 cm-1). For HRP, the order is reversed as nu 1 (456 cm-1) is at a higher frequency than delta 2 (404 cm-1). Normal coordinate analyses and model complexes have been used to address the origin of this behavior. The nu 1 stretching frequencies in cyanide complexes of iron porphyrin and iron chlorin model compounds are similar to one another and to that of HRP. Thus, the inverted order and altered frequencies of the nu 1 and delta 2 vibrations in MPO, relative to those in HRP and the model compounds, are not inherent to the proposed iron chlorin prosthetic group in MPO but, rather, are attributed to distinct distal environmental effects in the MPO active site. The normal coordinate analyses for MPO and HRP showed that the nu 1 and delta 2 vibrational frequencies are not pure; the potential energy distributions for these modes respond not only to the geometry but also to the force constants of the nu(FeC) and delta(FeCN) internal coordinates.(ABSTRACT TRUNCATED AT 250 WORDS)

Spectroscopic analysis of chlorophyll model complexes: methyl ester ClFe(III)pheophorbides

As models for chlorophyll a (Chl a), methyl ester ClFe(III)pheophorbides (1, pheophorbide a; 2, mesopheophorbide a; and 3, mesopyropheophorbide a) were examined by Fourier transform infrared (FTIR) absorption and resonance Raman (RR) spectroscopy. The infrared (IR) chlorin band above 1600 cm-1, assigned as a Ca-Cm mode (Andersson et al. (1987) J. Am. Chem. Soc. 109, 2908-2916) is shown to be metal-sensitive and responsive to spin state and coordination number for dihydroporphyrins, as well as being diagnostic for the chlorin vs. porphyrin or bacteriochlorin macrocycle. Frequency variations for this metallochlorin IR band thus parallel those of the v10 RR mode of porphyrins in their predictive utility. Qy excitation SERRS spectra of Chl a were compared with Qy excitation RR spectra of 1 and methyl Ni(II)pyropheophorbide a. The data demonstrate that 5-coordinate ClFe(III)pheophorbides are better models for chlorophylls than are ruffled 4-coordinate Ni(II)pheophorbides. Major spectral differences between the three chlorophyll models are associated with the C-9 keto and/or C-10 carbomethoxy vibrational modes. The approx. 1700 cm-1 IR band was formerly assigned solely to v(C = O) of the C-9 keto group. However, this IR feature shifts down to approx. 1685 cm-1 and nearly doubles in intensity when the C-10 carbomethoxy is removed, as for 3. Similar frequency downshifts coupled with intensity increases in the IR are found in the literature on chlorophylls. RR spectra of pheophorbides having the C-10 carbomethoxy group (1 and 2) have bands at both approx. 1700 and approx. 1735 cm-1. However, the C-9 keto v(C = O) mode of pyrophorbins also downshifts to approx. 1685 cm-1, as in the IR spectra. The approx. 1735 cm-1 ester RR mode disappears in the case of pyrophorbins, and is never RR active for nonconjugated esters of porphyrins or chlorins. These data demonstrate an interaction between the C-10 and C-9 carbonyls of phorbins. They also indicate that phorbins tend toward conjugation of the C-10 ester. Biological examples of such conjugation effects have recently been reported, e.g., for the Chl a pi-cation radical (Heald et al. (1988) J. Phys. Chem. 92, 4820-4824). Because the phorbin E ring is the major structural feature distinguishing chlorophylls from non-photosynthetic systems, the participation of the C-10 ester in ring conjugation is suggestive of its biological importance.

Identification and properties of an oxoferryl structure in myeloperoxidase compound II

Myeloperoxidase compound II has been characterized by using optical absorption and resonance Raman spectroscopies. Compared to compounds II in other peroxidases, the electronic and vibrational properties of this intermediate are strongly perturbed due to the unusual active-site iron chromophore that occurs in myeloperoxidase. Despite this difference in prosthetic group, however, other properties of myeloperoxidase compound II are similar to those observed for this intermediate in the more common peroxidases (horseradish peroxidase in particular). Two forms of the myeloperoxidase intermediate species, each with distinct absorption spectra, are recognized as a function of pH. We present evidence consistent with interconversion of these two forms via a heme-linked ionization of a distal amino acid residue with a pKa congruent to 9. From resonance Raman studies of isotopically labeled species at pH 10.7, we identify an iron-oxygen stretching frequency at 782 cm-1, indicating the presence of an oxoferryl (O = FeIV) group in myeloperoxidase compound II. We further conclude that the oxo ligand is not hydrogen bonded above the pKa but possibly exhibits oxygen exchange with the medium at pH values below the pKa due to hydrogen bonding of the oxo ligand to the distal protein group.