Iron-carbonyl bond geometries of carboxymyoglobin and carboxyhemoglobin in solution determined by picosecond time-resolved infrared spectroscopy.

The iron-carbonyl geometries in carboxymyoglobin (MbCO) and carboxyhemoglobin (HbCO) in ambient temperature solution have been investigated using picosecond time-resolved infrared spectroscopy. Polarized infrared and visible beams were used to monitor the change in infrared absorbance of the bound CO stretch bands on photodissociation of the ligand. The ratio of the change in absorbance for perpendicular and parallel relative polarizations of the photolysis and infrared probe beams is directly related to the angle between the ligand bond axis and the normal to the heme plane. Ratios, and hence the angles, have been obtained for the configurations giving rise to the principal CO stretch infrared absorption bands of HbCO and MbCO: 18 degrees for the 1951 cm-1 band of HbCO; 20 degrees and 35 degrees, respectively, for the 1944 cm-1 and 1933 cm-1 bands of MbCO. Structures consistent with x-ray diffraction and the picosecond experiments reported here are proposed for MbCO and HbCO in which the Fe-C bond tilts to the heme normal and the Fe-C-O angle differs significantly from 180 degrees.