Determination of the iron-carbonyl bond geometry of carboxyprotoheme in solution using picosecond infrared-optical photoselection

Abstract

The iron-carbonyl bond geometry in carboxyprotoheme in ambient temperature solution has been investigated. Polarized visible and infrared beams have been used to monitor the change in infrared absorbance of the bound CO stretch bands on photodissociation of the ligand. A theoretical description of the picosecond infrared-optical photoselection method is presented in which the anisotropic change in the infrared absorption may be expressed in terms of either the photoselection anisotropy or the polarization ratio, which may be used to deduce the properties of the FeCO geometry in carboxyprotoheme in terms of 〈 P 2(cos α)〉, where α is the angle between the ligand bond axis and the normal to the heme plane. Various models for the equilibrium distribution of the angle are considered. The results for cos −1 (〈cos 2α〉) 1 2 in the solutions studied lie in the range 17–26° which correspond to geometries significantly different from the linear and perpendicular configuration observed in single-crystal studies of similar compounds. Additionally, geminate recombination kinetics are observed following photodissociation in high viscosity solvents.