High‐field orientation effects in the high‐resolution proton NMR spectra of diverse porphyrins

Abstract

Dipolar splittings with a quadratic dependence on static magnetic field strength are reported for selected pairs of nuclei in several porphyrins. These splittings are detectable as a result of a slight molecular alignment caused by the interaction of the static magnetic field with the anisotropic magnetic susceptibilities of the porphyrin rings. The magnitude of the splitting yields an effective value for Ax, the anisotropy of the susceptibility. This value includes association effects which may be estimated therefrom. Dipolar splittings between protons of attached vinyl groups can also be measured, and provide information about the average dihedral angle between the planes of the vinyl group and porphyrin ring.

Orientation of molecules by magnetic field as a new source of information on their structures

“…(6) This expression includes the projection operators of the spin moment on the quantization axis z´ (Î z´i ) and the raising and lowering operators (Î i + , Î j -, Î i -, and Î j + ), which act on the spin states of the nuclei i and j. In the first order approximation one can restrict himself to the first two terms of the Hamiltonian (6).…”

“…The estimates of the J ij 0 and D ij constants and the chemical shifts are then used as parameters of the intial approximation for the next series of calculations of the spectra using the Hamiltonian of oriented spin system (6). The best correspondence between the frequencies of the experimental lines and transitions for all theoretical spec tra is found by varying the values of the chemical shifts (separately for each spectrum) and the dipole dipole cou pling constants.…”

Relationship between heme vinyl conformation and the protein matrix in peroxidases

“…The occurrence of intermediate wavenumbers near to the extreme torsion angles, j j ³ 0, /2 and , and a maximum or a minimum wavenumber at intermediate angles, j j ³ /4 or j j ³ 3 /4, respectively, indicates that a trigonometric function may be an appropriate choice to fit the results. Both the average torsional angle in monovinyl-porphyrin models 41 and the theoretical evaluation of the vinyl torsional potential in protoporphyrins 7 have been obtained by using a squared sinusoidal function as an expectation function. The preferred vinyl orientation of a monovinyl-substituted hemin model in solution determined by NMR dipolar splitting measurements based on an average hsin 2 i expectation function, where  is the dihedral angle, was found to be hi D 52°.…”

“…This conclusion was supported also by the theoretical work by Findsen et al, 7 which showed negligible ground-state conjugation when the vinyl groups are oriented out of the plane of the porphyrin ring by an angle of ¾50°and consistent with the structural studies on 1-methylimidazole Fe(III) porphyrin complex 57 and the NMR experimental data for vinylphylloerythrin. 41 Over the last 15 years, resonance Raman spectra of many heme proteins have shown that the vinyl stretching wavenumber might be sensitive also to the oxidation, coordination and spin states of the heme iron. However, the present analysis of the UV-visible absorption and resonance Raman spectra of many different heme proteins and selected site-directed mutants and their fluoride complexes agree with the finding that minimal conjugation is expected for proteins exhibiting an out-of-plane vinyl orientation corresponding to a torsion angle ³ 50°.…”

Backbone-only restraints for fast determination of the protein fold: The role of paramagnetism-based restraints. Cytochrome b562 as an example

“…We chose for this purpose the oxidized cytochrome b 562 which has already been well characterized. Oxidized cytochrome b 562 contains a low spin iron (III) heme whose relatively small magnetic anisotropy is responsible for partial self orientation [32,33]. Also the four helix bundle fold contributes to the overall magnetic anisotropy [34].…”