Showing papers on "Porphyrin published in 1978"

Oxygen binding to cobalt porphyrins

Abstract: The thermodynamic constants of oxygen binding to cobalt "picket fence" porphyrin complexes, meso-tetra(a,o,a,o- o-pivalamidophenyl)porphyrinatocobalt(II)- 1 -methylimidazole and 1,2-dimethylimidazole, are reported. In contrast to pre- viously studied cobalt porphyrins, these complexes bind oxygen with the same affinity as cobalt substituted myoglobin and he- moglobin. Solvation effects are discussed as the source of this difference. The use of sterically hindered axial bases as models of T state hemoglobin is discussed. In studies of myoglobin (Mb) and hemoglobin (Hb), the replacement of the neutral iron porphyrin prosthetic group with different metalloporphyrins has proved to be a useful tech- nique.2 Artificial hemoglobins containing ~inc,~.~ manga- ne~e,~-* ~opper,~ and nickel9 have been reconstituted, and their properties compared with those of the native iron proteins. These artificial systems, however, are incapable of reversible oxygenation. In contrast, cobalt substituted hemoglobin and myoglobin (CoHb and CoMb)l0 are functional,I1-l3 although their oxygen affinities are 10-100 times less than those of native Hb and Mb. CoHb exhibits cooperativity in oxygen binding, though to a lesser degree than Hb. The extent of this cooperativity is conveniently expressed as AG;, , the free energy difference between the intrinsic binding of the first and the fourth 02 to Hb.13b,c For CoHb, AG:l is roughly one-third that of Hb under comparable condition^.'^^ Because of the different stereo- chemical and electronic factors involved in binding oxygen to a cobalt porphyrin, the observation of cooperativity in CoHb has been variously used either to que~tion~9~~J~ or s~pport'~~,'~ the elegant proposal of Perutz concerning the molecular mechanism of cooperativity in natural Hb.16 At the heart of this proposal is the assumption, based on earlier ideas of Hoard" and Williams,18 that the high-spin iron in the unli- gated, low 02 affinity form of Hb (T state) lies out of the porphyrin plane, and that on binding 02, the iron becomes low spin and moves into the plane. The resulting motion of the proximal imidazole (0.6 A) then causes conformational changes in the protein which produce a higher 02 affinity quaternary form of the protein (R state). In the deoxy form of coboglobin, cobalt is low spin rather than high spin and the best estimates from simple cobalt model systems indicate that the proximal imidazole in CoHb moves -0.4 A upon oxygen- ation'4,'9,20 as opposed to the 0.6 8, for Hb. The resulting motion of the proximal histidine upon binding 02 will therefore only be two-thirds as great. This seems to be qualitatively consistent with the lowered AGil of CoHb.

Comparative studies of porphyrin production in Propionibacterium acnes and Propionibacterium granulosum.

Abstract: Porphyrin production by Propionibacterium acnes and that by Propionibacterium granulosum were compared. Porphyrin synthesized by both organisms was identified as coproporphyrin III on the bases of absorption and fluorescence spectra and behavior on paper chromatography and thin-layer chromatography. Quantitative, rather than qualitative, differences in production were found between these organisms. In general, P. granulosum produced significantly greater amounts (P less than 0.001) of porphyrin than did P. acnes. delta-Aminolevulinic acid synthetase appeared to be the rate-limiting enzyme of the heme biosynthetic pathway in both organisms. The increased porphyrin production in P. granulosum is apparently associated with increased delta-aminolevulinic acid synthetase activity.

Proton nuclear magnetic resonance characterization of heme disorder in hemoproteins.

Abstract: A proton NMR method is described for determining the orientation of a porphyrin within the heme pocket of a hemoprotein. The pattern of the hyperfine-shifted heme methyl resonances in low-spin ferric model compounds is demonstrated to characteristically reflect the position of a localized low-symmetry perturbation on the pi system. The specific assignments via deuteration of the two interconvertible sets of methyl resonances observed for deuteroporphyrin-reconstituted sperm whale metmyoglobin cyanide lead to the conclusion that the low-symmetry perturbations on the heme due to the apo-protein contacts differ for the two protein components by a 180 degrees rotation about the alpha-gamma meso axis. Hence the heme in the reconstituted myoglobin is "disordered" in solution, and the altered functional properties of the reconstituted protein cannot be simply attributed to the local effect of the heme substituent. This NMR technique has applicability for determining the relative heme orientation in related hemoproteins, and may clarify the origin of doubling of heme resonances observed in several native hemoproteins.

Transfer of excitation energy between porphyrin centers of a covalently-linked dimer*

Abstract: — Three covalently-linked porphyrin hybrid dimers were synthesized, each containing a metallotetraarylporphyrin [Zn(II), Cu(II), or Ni(II)], and a free base tetraarylporphyrin. Transfer of singlet excitation energy from the metalloporphyrin center to the free base porphyrin center was determined by measuring fluorescence properties. The Zn hybrid dimer displayed excellent intramolecular transfer of energy ( 85%) from the excited singlet state of the Zn(II) chromophore to the free base chromophore. No evidence for such transfer of the excited singlet state energy was found in the Ni(II) or Cu(II) analogues. From our experimental data, the fluorescence quantum yield of the Zn hybrid dimer was the same as for the free base monomer porphyrin (0.11; Seybold and Gouterman, 1969). Thus, the covalent attachment of another fluorescent porphyrin center effectively doubled the antenna size without decreasing the quantum yield even though the fluorescence quantum yield of the Zn(II) containing monomer was substantially less (0.03, according to Seybold and Gouterman, 1969) than that of the free base porphyrin. The donor-acceptor distance and the rate constant for energy transfer were calculated using the Forster equation. Assuming random orientation, a donor-acceptor distance of 15 A was calculated with an associated rate constant (kci) for energy transfer of 1.9 ± 109 s–1.

Raman spectra of heme a, cytochrome oxidase-ligand complexes, and alkaline denatured oxidase.

Abstract: We report 441.6 nm excitation resonance Raman spectra of oxidized and reduced monomeric heme a-imidazole, cytochrome oxidase-exogenous ligand complexes in various redox states, and alkaline denatured oxidase. These data show that, in reduced oxidase, the cytochrome a3 Raman spectrum has bands at 215, 364, 1230, and 1670 cm-1 not observed in the cytochrome a spectrum. The appearance of these bands in the reduced cytochrome a3 spectrum is due to interactions between the heme a of cytochrome a3 and its protein environment and not to intrinsic properties of heme a. These interactions are pH sensitive and strongly influence the vibrational spectra of both heme a groups. We assign the 1670-cm-1 band to the heme a formyl substituent and propose that the intensity of the 1670 cm-1 is high for reduced cytochrome a3 because the C==O lies in the porphyrin plane and is very weak for oxidized and reduced cytochrome a, oxidized cytochrome a3, and oxidized and reduced heme a-imidazole because the C==O lies out of the plane. We suggest that movement of the C==O in and out of the plane explains the ligand induced spectral shift in the optical absorption spectrum of reduced cytochrome a3. Finally, we confirm the observation of Adar & Yonetani (private communication) that, under laser illumination, resting oxidase is photoreactive.

The direct determination of porphyrin carboxylic acids. High-pressure liquid chromatography with solvent systems containing phase-transfer agents.

Abstract: A novel method for separating porphyrin polycarboxylic acids is described and illustrated by its application to the direct analysis of biological (deep-sea medusae), clinical (urine) and chemical ('haematoporphyrin derivative') samples.

Resonance Raman studies of hepatic microsomal cytochromes P-450: evidence for strong pi basicity of the fifth ligand in the reduced and carbonyl complex forms.

Abstract: Resonance Raman spectra have been measured for cytochromes P-450 purified from liver microsomes of phenobarbital-treated rabbits (PB P-450 and PB P-448) and of 3-methylcholanthrene-treated rabbits (MC P-448). In the reduced state, all three cytochromes P-450 exhibit Raman spectra of ferrous high-spin type but show the so-called "oxidation state marker" (band IV) at unusually low frequencies, indicating extensive delocalization of electrons from the iron dpi orbital to the porphyrin II (eg) orbital and, consequently, the strong pi basicity of the fifth ligand of the heme iron. The reduced CO complexes of the cytochromes P-450 also exhibit band IV at markedly lower frequencies than CO complexes of hemoglobin and myoglobin. These anomalies observed for the reduced form and CO complex disappear upon conversion of the cytochromes to the catalytically inactive form called cytochrome P-420. Oxidized PB P-450 shows a Raman spectrum which is characteristic of typical ferric low-spin heme compounds, whereas those of PB P-448 and MC P-448 are of the ferric high-spin type. PB P-450 is also clearly distinguishable from the two P-448 preparations in the reduced state. The reduced form of cytochrome P-420, produced by laser illumination, exhibits two sets of Raman lines and, therefore, seems to be a mixture of both high- and low-spin species.

Synthesis, Characterization, and Structure of Tri-micron-halogeno- hexacarbonyldirhenate(I) Salts of Monocationic Porphyrin Acids

Abstract: : The metal-carbonyl insertion method has been found useful for the preparation of bimetallic porphyrin complexes of rhodium, rhenium, and technetium. Metal carbonly halides are more reactive reagents than metal carbonyls for the synthesis of bimetallic complexes of porphyrins as well as bimetallic salts of porphyrin acids.