OPTICAL DETECTION OF HEME LIGAND CONFIGURATION IN SPERM WHALE MYOGLOBIN

Abstract

The configuration of O 2 and CO bound to the ferrous heme group in sperm whale myoglobin is evaluated on the basis of polarized electronic absorption spectra of single crystals. Only one spectrum is observed for MbO 2 in crystals, and this is in quantitative agreement with the solution spectrum. The coordination geometry of the Fe-O 2 center in MbO 2 is of the bent (Fe-o-O ≡ 136°) type and is identical to that in HbO 2 . Two types of MbCO complexes are distinguishable on the basis of polarized optical spectra of single crystals through the appearance of z-polarized porphyrin-iron charge-transfer transitions at 37000 and 21000 cm −1 . One type of MbCO complex, like HbCO, exhibits no z-polarized intensity and has a near-linear (Fe-C≡O) configuration. The appearance of z-polarized intensity in the other MbCO complex is diagnostic of a decrease in the ligand field splitting of the Fe(II) ion and indicates a change in ligand configuration to a more bent (Fe–C≡ O type. The polarized single crystal absorption spectrum of the bent MbCO complex is not compatible with the spectrum of MbCO in solution.