Laser-excited Raman spectroscopy of biomolecules. II. Native ribonuclease and α-chymotrypsin

Laser-excited Raman spectra of native ribonuclease and α-chymotrypsin and of the polypeptide poly-l-glutamic acid have been determined in aqueous solution. With the help of previously determined quantitative spectra of the amino acids, a considerable portion of the spectral details has been interpreted. The strong Raman lines due to the aromatic side chains of the amino-acid residues are clearly observed and are not affected by conformations of the proteins. The bond-stretching vibration of the disulfide link shows up strongly in the spectrum of ribonuclease at a measurably different position (516 cm^-1) from its location in lysozyme (509 cm^-1). The intensity ratio of the C-S to S-S lines is an order of magnitude larger than in lysozyme. Both of these observations suggest that the conformations of the C-S-S-C cross-links in the two proteins are significantly different. The spectrum of poly-l-glutamic acid at pH 10 was observed as a possible model for the effects of denaturation. While the spectra of the denatured proteins have not yet been obtained, comparison of the native spectra with that of poly-l-glutamic acid shows for the latter the expected broadening and overlapping of peaks in the amide I and amide III regions which are sharper and resolved into several components in the protein spectra.