Proton magnetic resonance study of high- and low-spin hemin derivatives

Soluble meso and deuteroporphyrin analogs of the malaria pigment hematin anhydride

Two soluble heme analogs of the insoluble malaria pigment hematin anhydride (HA, or β-hematin), [Fe(III)(protoporphyrin)]2, with either mesoporphyrin (MHA) or deuteroporphyrin (DHA) are characterized by elemental analysis, SEM, IR spectroscopy, electronic spectroscopy, paramagnetic 1H NMR spectroscopy and solution magnetic susceptibility. While prior single crystal and X-ray powder diffraction results indicate all three have a common propionate linked dimer motif, there is considerable solid state variation in the conformation. This is associated with enhanced solubility of MHA and DHA. As with HA, DHA undergoes thermally promoted reversible hydration/dehydration in the solid state. Solution 1H NMR studies of DHA suggest a high spin dimeric structure with the porphyrin methyls distributed between two isomers which are also present in the solid state. These soluble iron(III)porphyrin dimers allow for the first direct solution studies by NMR and UV-Vis spectroscopies of these key species. Taken together the results illustrate the importance and utility of varying the substituents on the periphery of the porphyrin for studying heme aggregation and malaria pigment formation.

Spectroscopic and Kinetic Characterization of Peroxidase-Like π-Cation Radical Pinch-Porphyrin-Iron(III) Reaction Intermediate Models of Peroxidase Enzymes

The spectroscopic and kinetic characterization of two intermediates from the H₂O₂ oxidation of three dimethyl ester [(proto), (meso), (deuteroporphyrinato) (picdien)]Fe(III) complexes ([FePPPic], [FeMPPic] and [FeDPPic], respectively) pinch-porphyrin peroxidase enzyme models, with s = 5/2 and 3/2 Fe(III) quantum mixed spin (qms) ground states is described herein. The kinetic study by UV/Vis at λmax = 465 nm showed two different types of kinetics during the oxidation process in the guaiacol test for peroxidases (1-3 + guaiacol + H₂O₂ → oxidation guaiacol products). The first intermediate was observed during the first 24 s of the reaction. When the reaction conditions were changed to higher concentration of pinch-porphyrins and hydrogen peroxide only one type of kinetics was observed. Next, the reaction was performed only between pinch-porphyrins-Fe(III) and H₂O₂, resulting in only two types of kinetics that were developed during the first 0-4 s. After this time a self-oxidation process was observed. Our hypotheses state that the formation of the π-cation radicals, reaction intermediates of the pinch-porphyrin-Fe(III) family with the ligand picdien [N,N'-bis-pyridin-2-ylmethyl-propane-1,3-diamine], occurred with unique kinetics that are different from the overall process and was involved in the oxidation pathway. UV-Vis, ¹H-NMR and ESR spectra confirmed the formation of such intermediates. The results in this paper highlight the link between different spectroscopic techniques that positively depict the kinetic traits of artificial compounds with enzyme-like activity.

New pinch-porphyrin complexes with quantum mixed spin ground state S=, of iron (III) and their catalytic activity as peroxidase

New complexes of the pinch-porphyrin family were obtained from the dimethylester of (proto-, meso-, and deutero-porphyrinato)iron(III) with the ligand [N,N'-bis-pyridin-2-ylmethyl-propane-1,3-diamine] 1-3 and with the ligand [N-pyridin-2-ylmethyl-N'-[3-[(pyridin-2-ylmethyl)-amino]-propyl]-propane-1,3-diamine] 4-6. The UV/VIS studies of 1-6 indicate an increase in the distortion of the ligand field excited state. The 1H NMR spectra of 1-6 at RT and over the range 223-328 K show iron(III)-complexes with quantum mixed spin state (qms) S=5/2, S=3/2. The chemical shifts of the meso protons are consistent with qms state S=3/2, S=5/2, where the S=3/2 spin state is lowest in energy. For methyl-heme the chemical shifts are also consistent with a qms state but now the S=5/2 ground state is lowest in energy. ESR spectra of 1-6 show two different species, B and C, of iron(III) with qms, S=5/2, S=3/2 consistent with the 1H NMR results. Species B with 70% of S=5/2 and species C with 72.5% of S=3/2. The catalytic activity as peroxidase of 1-6 was quantified by guaiacol test; their theoretical maximum rate constants were k(cat) approximately 10(2)-10(3) M(-1) s(-1). A quantitative empirical correlation is found: the higher the 32 spin contribution to the qms state and the higher proportion of this species into the samples, the higher the peroxidase activity. Such a correlation was also obtained for pinch-porphyrins already reported.

Heme methyl hyperfine-shifted nuclear magnetic resonance peaks assigned by selective deuteration as indicators of heme-protein interactions in metmyoglobins

The heme methyl resonances in a variety of high-spin, low-spin and spin-equilibrium forms of sperm whale metmyoglobin have been assigned by reconstituting myoglobin with selectively deuterated hemes. Two patterns for the heme methyl hyperfine shifts are observed, one characteristic of the low-spin state and the other typical of the high-spin state. The two protein forms which can change the position of their spin equilibrium significantly with changing temperature exhibit the pattern of the dominant spin state component at any temperature. The different hyperfine shift patterns for the low-spin and high-spin states are concluded to arise not from different heme-protein contacts in the two spin states, but from characteristic differential sensitivities of the dominant spin transfer mechanisms to the same rhombic perturbation.

Nuclear magnetic resonance study of heme-heme interaction in hemoglobin M Milwaukee: implications concerning the mechanism of cooperative ligand binding in normal hemoglobin

Hemoglobin M Milwaukee (beta 67E11 val leads to Glu) is a naturally occurring valency hybrid containing two permanently oxidized hemes in the beta-chains. In this mutant, the two abnormal beta-chains cannot combine with oxygen, whereas the two alpha-chains are normal and can combine with oxygen cooperatively with a Hill coefficient of approximately 1.3. High-resolution proton nuclear magnetic resonance spectroscopy at 250 MHz has been used to investigate the hyperfine shifted resonances of the abnormal ferric beta-chains of Hb M Milwaukee over the spectral region from -30 to -60 parts per million from water at pD 7 and 30 degrees.

Functional nonequivalence of and hemes in human adult hemoglobin

Nuclear magnetic resonance studies of the contact-shifted spectra of heme protons in deoxyhemoglobin A from human adults show conclusively that oxygen binds to the alpha hemes in preference to the beta hemes. The preferential binding is produced in 10% hemoglobin solution at neutral pH by either a 15-fold molar excess of 2,3-diphosphoglycerate or a 5-fold molar excess of inositol hexaphosphate. Preferential binding is not observable in the absence of the organic phosphates. The results indicate that the oxygenation of hemoglobin may be described by a sequential model, or by a concerted model that allows the alpha hemes to bind ligand first.