RAMAN AND RAMAN OPTICAL ACTIVITY CONFORMATIONAL STUDY OF A CYCLIC HEXAPEPTIDE
J. Hrudíková1,2*, J. Kapitán1,2, V. Baumruk1 and P. Bouř2
1Institute of Physics, Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 5, 121 16 Prague 2
2Institute of Organic Chemistry and Biochemistry, Academy of Sciences, Flemingovo nám. 2, 166 10 Prague 6
Raman optical activity (ROA) measures vibrational optical activity as a difference in the intensity of Raman scattering from chiral molecules for right and left circularly polarized light. Raman and ROA spectral intensities, band positions, and, in the extreme case, ROA signs can be significantly modified by molecular flexibility . All the conformations present in the sample provide unique ROA signal due to the fast molecular response to the light.
A model cyclohexapeptide, cyclo-(Phe-D-Pro-Gly-Arg-Gly-Asp) was selected as a convenient model of b-hairpin induced by the D-Pro-Gly sequence, and a short anti-parallel b-sheet. Its Raman and Raman optical activity (ROA) spectra were used to investigate the structure and flexibility of the side chains, for which the IR and VCD techniques are not very sensitive . This system is accessible to relatively precise computations thanks to its size and a constricted conformation of the backbone . Information about peptide side chains could be extracted from the experiment via combined density functional theory (B3LYP/CPCM/ 6-31G**) and molecular dynamics (MD) simulations of spectral intensities. Spectral averaging for many MD structures provided better spectral profiles than a fixed geometry.
The results suggest that neither the side chains move freely, but oscillate around preferred conformations. According to the simulations the ROA spectrum senses local side chain conformation; in the case of the cyclohexapeptide it senses mostly about the hydrophobic phenylalanine and proline residues, and their interaction.
1. J. Kapitán, V. Baumruk, V. Kopecký Jr., P. Bouř, J. Phys. Chem. A 110 (2006) 4689-4696.
2. P. Bouř, J. Kim, J. Kapitán, R.P. Hammer, R. Huang, L. Wu and T. A. Keiderling, Chirality 20 (2008) 1104-1119.
3. J. Hilario, J. Kubelka, F. A. Syud, S. H. Gellman, T. A. Keiderling, Biospectroscopy 67 (2002) 233236.