Proteinstruktur und Proteinfaltungsanalysen mit Hilfe der Ramanspektroskopie: Untersuchungen an Modellpeptiden zur Optimierung der Methodik

In this study we investigated small model peptides by visible and UV resonance Raman spectroscopy to obtain a more detailed understanding about the normal modes giving rise to the protein secondary structure marker bands in Raman and infrared spectra. They predominantly arise from vibrations of the peptide groups. Additionally, the spectra of b-sheet and ´random coil´ structures display a resonance enhanced band at about 1400cm-1 which is generally assigned to the CaH bending (b) vibration. By analyzing the structure marker bands in the UV-resonance Raman spectrum it is possible to directly determine protein secondary structures in solution. Furthermore, the utility of UV resonance Raman spectroscopy to probe the earliest events of thermal unfolding of an a-helical peptide was demonstrated. In this present study we investigated di- and tripeptides, containing one and two amide linkages respectively, to obtain a deeper insight into the origin of the structure sensitive bands. The results of these investigations allow us to characterize the enhancement pattern of the CaH band in a protein or polypeptide. We show that only the N-terminal CaHb and not the C-terminal CaHb band of a peptide group is revealed in the UV-Raman spectrum. For longer peptides we found that the intensity of the CaHb depends linearly on the number of amino acid residues minus the C-terminal residue. Hence, this band can be used as a direct measure of the length of secondary structure motifs within a more complex protein structure. With respect to AIII, we provide compelling evidence that this vibration is localized in various tripeptides which sample a significant portion of the Ramachandran plot. Furthermore, we show that the AIII eigenvector contains internal coordinates of the CaHb group and that this contribution mainly determines the AIII frequency position in UV-Raman spectra of investigated molecules.