Investigation of electron distribution and hyperfine properties of hemin by first-principles Hartree-Fock self-consistent field procedure

As part of a program for first-principles quantitative study of hyperfine properties of large biological molecules, we have applied the Self-Consistent Field Unrestricted Hartree-Fock procedure to investigate the electronic structure of hemin, the associated charge and unpaired spin population distributions and the hyperfine constants of⁵⁷Fe,¹⁴N,¹³C,¹H and³⁵Cl nuclei. The results of the present work indicate significantly more localized charge and spin distributions than were found earlier by the semi-empirical Self-Consistent Charge Extended Hückel procedure. The theoretical values of the⁵⁷Fe,¹⁴N and methene proton hyperfine constants, the only nuclei for which experimental data are available, are found to be -34.99, 11.89 and -1.50 MHz in satisfactory agreement with the experimental values of -26.47, 7.55 and -1.014 MHz, respectively. The contact and dipolar contributions to the hyperfine constants and their breakdowns into direct and exchange polarization contributions are analyzed. Possible sources that could bridge the remaining gap between experiment and theory will be analyzed.