INTERPRETATION OF RESONANCE RAMAN SPECTRA OF BIOLOGICAL MOLECULES

Abstract

In the past few years there has been an exponential increase in the USe of resonance Raman (RR) spectroscopy for the study of biological molecules (see 1-4). In the RR process it is possible to use laser excitation within a given electronic level and to achieve an enormous enhancement of the Raman transitions coupled to that elec­ tronic level. This allows selective enhancement of the vibrations of different chromo­ phores and probing of their local environment in biologically interesting concentrations. The RR vibrational spectra of certain substrates can be obtained even when they are inside the protein active site. These spectra can provide direct information about the interactions between the substrate and its active site. RR spectroscopy can be used for studying zero-order questions in biological systems. That is, many important biological problems can be solved by locating the region of a given change during the biological process, or by determining whether or not there has been any change in some active site. In some cases these questions can be answered by RR techniques even in vivo. On the other hand, RR spectro­ scopy can be used for a more detailed study involving analysis of the nature of the changes in the active site. This type of study is the main subject of this review. The experimental aspects of the RR spectroscopy (reviewed in 2, 4) are not covered; I give only a brief account of the use of RR as a probe for zero-order questions. The main attention is devoted to interpreting RR results and their use as a potential tool for detailed analysis of biological systems. The first section covers the theoretical background of the RR effect. The main attraction is given to a direct vibronic approach for calculating the RR spectra in conjugated molecules. Semiclassical approaches, reviewed elsewhere (1), are not considered here. In the second section RR studies of the visual pigments are dis­ cussed. In these systems the RR spectra contain very detailed information about the