<title>Polarization sensitive CARS of protein</title>

Abstract

One of the most interesting problems of modern biophysics is the problem of enzyme conformational changes during the catalytic act. There are some bands in the vibrational spectra of proteins that may be sensitive to the conformational state of the macromolecule. The aim of the present study is to reveal the changes in protein vibrational spectra associated with the interaction with the substrate by means of highly sensitive PSCARS spectroscopy and to refer these spectroscopic changes to the possible conformations of the protein molecule. The peculiarities of the experimental method applied are associated mainly with a very high nonresonant background to resonant signal ratio. This makes necessary the polarization suppression of the background, the sufficient quality of which may only be achieved with very high polarization quality of the pumping beams and rather thin samples (about 2 mm). The PSCARS spectra were measured of protein chymotrypsin and its complex with antranilic acid (model of substrate in this system) solutions in water and heavy water. The spectra were obtained within three frequency ranges: 800 - 900 cm^-1, 1180 - 1300 cm^-1, 1580 - 1700 cm^-1. The vibrational bands analyzed were amide I, amide III, and several bands belonging to tyrosine and tryptophan. It was demonstrated that all the bands studied were sensitive to the ligand binding. The fitting procedure was applied to all the PSCARS spectra and the vibrational bands' parameters were determined (positions, bandwidths, amplitudes). Hereafter we present some details on the results obtained for amide I band.