STUDY OF THE RIBONUCLEASE S-PEPTIDE S-PROTEIN COMPLEX BY MEANS OF RAMAN DIFFERENCE SPECTROSCOPY/

The isolated S-peptide (enzymatically cleaved 1-20 fragment of the rib onuclease, RNase S) is partly helical in solution. Both S-peptide and its truncated 1-15 analogue pep01 can combine with S-protein (residual part of RNase S without S-peptide), restoring a RNase S' complex. Par t 3-13 of the peptides forms an alpha-helix when within the RNase S' c omplex. To compare the solvated forms of both peptides with their stru ctures when complexed with the S-protein, we have measured the Raman s pectra of these forms by means of difference spectroscopy. The spectra of the peptides in water solutions are characterized by much wider vi brational bands than the spectrum of the peptides within the RNase S' complexes. This shows that the structure of the solvated peptides cons ists of a heterogeneous mixture of interconverting species. Only two m ain bands characteristic for the exposed alpha-helix (in D2O) and for an unordered chain are observed for amide-l regions of the dissolved p eptides spectra. Relative intensities of the bands vary with temperatu re, reflecting different proportions of helical and disordered conform ations. However, amide-I bands compositions of the bound peptides are more complex and include marker bands typical of the alpha-helix withi n the hydrophobic environment of a protein.