Nikolai V. Shokhirev

The effect of axial ligand plane orientation on the contact and pseudocontact shifts of low-spin ferriheme proteins

Abstract The effect of axial ligand nodal plane orientation on the contact and pseudocontact shifts of a symmetrical low-spin octamethylferriheme center has been calculated as a function of the angle of the axial ligand. Simple Hückel techniques have been used to estimate the contact contribution, and values obtained from model hemes, together with counter-rotation of the g-tensor, have been used to estimate the pseudocontact contribution, for the eight β-pyrrole methyl and four meso-H positions. It is found that the maximum and minimum contact shifts occur when the axial ligand is aligned at an angle of ±15° to the meso-H axes of the heme, rather than when the axial ligand plane lies along the porphyrin nitrogens, as assumed previously by some investigators. For systems having one planar axial ligand or two ligands in parallel planes, the contact and pseudocontact contributions at the meso-H positions are comparable in size (at least on the basis of simple Hückel estimates), while the contact contribution clearly dominates the isotropic shifts of the heme methyls. Allowing for the substituent effect of the 2,4-vinyls of protohemin, or the 2,4-thioethers of hemin c, as well as the average diamagnetic shifts of the heme methyls and meso-H, plots of the predicted shifts as a function of axial ligand nodal plane orientation have been constructed for hemin b- and c-containing proteins. Excellent agreement in the order of shifts, and reasonable agreement in the sizes of the observed shifts, is observed in the majority of the ferriheme proteins for which the methyl and meso-H resonances have been assigned and proton shifts reported. Plots have also been constructed for hemin c-containing proteins having the two axial ligand nodal planes oriented at relative angles of 40°, 70°, and 80°. Excellent agreement in the order of shifts, and reasonable agreement in the magnitudes of the observed shifts, is observed in all cases of bacterial cytochromes which do not fit the plots that assume the ligands are in parallel planes, except one – the cytochrome c-552 of Nitrosomonas europae. Except for this case, where the order of the predicted methyl shifts at any angle of the axial ligands disagrees with the observed, the reasons can usually be attributed to a large dihedral angle between two axial ligand nodal planes, to strong H-bonding interactions involving His and/or CN– ligands, or to off-axis binding of one (or both) axial ligand(s). Ruffling of the porphyrin ring may also contribute to the contact shift in as yet undefined ways. Hence, despite the simplicity of the calculations, the agreement with observed data is highly satisfying and the concept of the importance of axial ligand plane orientation on the observed proton shifts of heme proteins is fully confirmed.

119Sn CIDNP: calculations and experiment

Abstract 119 Sn CIDNP effects observed in the photolysis of 2-methylpropanoyltripropylstannane, Pr 3 SnC(O)CHMe 2 , are in good agreement with those calculated in the frames of the diffusion model of radical pair (RP) theory. Simulations performed for arbitrary magnetic field strengths have shown that mainly S-T 0 transitions in RPs where the Zeeman term is comparable with the hyperfine interaction are responsible for CIDNP formation in the fields of modern NMR spectrometers (several tesla). CIDNP in lower magnetic fields result from S-T +,− transitions in all RPs involving tin-centered radicals. In high fields these transitions lead to the appearance of net polarization in the RPs where the Zeeman term is well below the hyperfine interaction.

Simple analytical approach to the description of flash-CIDNP formation for ion radicals

Abstract The simple analytical expressions for the stationary bimolecular rate constants of recombination of charged radicals in external magnetic field were obtained for the case of high magnetic field, stepwise exchange integral and contact recombination. A comparison of the analytical and the precise numerical calculations shows good accuracy of the proposed approach. Also discussed is the non-trivial influence of solvent polarity and viscosity on the rate constants via singlet—triplet conversion efficiency.

EPR studies of weakly coupled oxomolybdenum (V) and low-spin iron (III) porphyrin centers

Novel compounds containing twoS=1/2 coupled spin centers (Mo(V) and low spin Fe(III) have been investigated in detail by X- and Q-band EPR spectroscopy, spectral simulation and molecular modelling calculations. For one system with a Mo−Fe distance of ≈9.4 Å the dominant dipolar coupling allows distinction among structures that are consistent with molecular modelling calculations. For the second system with a Mo−Fe of ≈ 7.9 Å the exchange interaction is dominant (0.5 <J < 3.0 GHz). These coupled systems are preliminary benchmarks for using EPR to investigate the Mo−Fe interaction in sulfite oxidase.