DETERMINATION OF THE ELECTRIC FIELD GRADIENT TENSOR IN MYOGLOBIN-O2AND -CO COMPOUNDS

Abstract

The EFG tensors at the Fe atoms in myoglobin-02 and -CO are determined by Mzssbauer ex- periments on single crystals. In the MbOz compound the quadrupole interaction (e&Vzz) is negative and the largest principal axis of the EFG lies nearly in the heme plane, while in the MbCO compound eQVzZ is positive and the axis of the EFG is nearly perpendicular to the heme plane. Mzssbauer spectroscopy has provided the most direct knowledge of the iron-oxygen binding in myo- globin (Mb) and hemoglobin (Hb). Although both the 02 and CO compounds of Mb and Hb are diamagnetic, the quadrupole splittings in the 02-form are quite different from those in the CO-form, and the pre- sence of large asymmetric charge transfer is sug- gested in the 02-form /I/. In order to further dis- cuss the bonding structure of ligands to heme iron, not only the principal values of the electric field gradient (EFG) tensor, but also its orientation is strongly required to be determined for these com- pounds. Single crystals were grown from a solution of sperm whale met%, enriched with 57~e, and vi- sually selected perfect crystals were converted into VELOCITY lrnrn/s1 -3 -2 -1 0 1 2 3,-3 -2 -1 0 1 2 3