LOW-TEMPERATURE OPTICAL SPECTROSCOPY OF LOW-SPIN FERRIC HEMEPROTEINS

We report the Soret absorption spectra (500-350 nm) of the cyanomet de rivatives of human hemoglobin and horse myoglobin, in the temperature range 300-20 K and in two different solvents (65% v/v glycerol-water o r 65% v/v ethylene glycol-water). In order to obtain information on st ereodynamic properties of active site of the two hemeproteins, we perf orm an analysis of the band profiles within the framework of electron- vibrations coupling. This approach enables us to single out the variou s contributions to the spectral bandwidth, such as those arising from non-radiative decay of the excited electronic state (homogeneous broad ening) and from the coupling of the electronic transition i) with high frequency modes (that determines the vibronic structure of the band) and ii) with a ''bath'' of low frequency modes (that is responsible fo r the temperature dependence of the experimental spectral. We discuss the relevant parameters and their temperature dependence and compare t hem with the ones already reported for other derivatives of the same h emeproteins in the same solvents. In particular, non-harmonic contribu tions to soft modes are found, for cyanomet derivatives, to be larger than those observed for liganded carbonmonoxy but smaller than those o bserved for unliganded deoxy derivatives. The reported data enable us to obtain information on the dependence of stereodynamic properties of the heme pocket upon iron oxidation state, dimensions of the exogenou s ligand and composition of the external matrix.