LOW-TEMPERATURE OPTICAL-ABSORPTION SPECTROSCOPY - AN APPROACH TO THE STUDY OF STEREODYNAMIC PROPERTIES OF HEMEPROTEINS

In this short review we show how suitable analysis of the temperature dependence of the optical absorption spectra of metalloproteins can gi ve insight into their stereodynamic properties in the region of the ch romophore. To this end, the theory of coupling between an intense allo wed electronic transition of a chromophore and Franck-Condon active vi brations of the nearby atoms is applied to the Soret band of hemeprote ins to obtain an analytical expression suitable far fitting the spectr al profile at various temperatures. The reported approach enables one to separate the various contributions to the overall bandwidth togethe r with the parameters that characterize the vibrational coupling. The thermal behavior of these quantities gives information on the dynamic properties of the active site and on their dependence upon protein str ucture and ligation state. The Soret band of hemeproteins appears to b e coupled to high frequency vibrational modes of the heme group (as al ready shown by resonance Raman spectroscopy) and to a ''bath'' of low frequency modes most likely deriving from the bulk of the protein. For the deoxy derivatives inhomogeneous broadening arising from conformat ional heterogeneity appears to contribute substantially to the linewid th. The data indicate the onset, at temperatures near 180 K, of large scale anharmonic motions that can be attributed to jumping among diffe rent conformational substates of the protein.