MONITORING OF ZWITTERIONIC PROLINE AND ALANINE CONFORMATIONAL SPACE BY RAMAN OPTICAL ACTIVITY

J. Kapitán^1,2, P. Bouř¹ and V. Baumruk²

¹Institute of Organic Chemistry and Biochemistry, Flemingovo nám. 2, 166 10 Prague 6,

e-mail: kapitan@karlov.mff.cuni.cz

²Institute of Physics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 121 16 Prague 2

Raman optical activity (ROA) measures vibrational optical activity by means of a small difference in the intensity of Raman scattering from chiral molecules in right and left circularly polarized incident laser light. The ROA spectra of a wide range of biomolecules in aqueous solutions can be measured routinely. Because of its sensitivity to the chiral elements, ROA provides new information about solution structure and dynamics, complementary to that supplied by conventional spectroscopic techniques [1].

Natural targets for ROA technique are amino acids and peptides. However, the interpretation of the spectra is almost entirely dependent on ab-initio modeling of vibrational frequencies and spectral intensities, which imposes limits on molecular size and overall accuracy.

Incident circular polarization (ICP) ROA instrument has been built at the Institute of Physics following the design of the instrument constructed in Glasgow [2]. Combination of experimental and computational approaches represents unique and powerful tool for studying structure and interactions of biologically important molecules.

Computation of ROA is a complex process, including evaluation of equilibrium geometry, molecular force fields and polarizability tensor derivatives. In case of zwitterionic amino acids and peptides many complications arise also from their conformational flexibility and strong interaction with the solvent, which has to be taken into account in the modeling. For our ROA simulations we used continuum solvent models and solvation with explicit molecules of water [3].

Conformational space of L-alanine was investigated in detail by rotating the NH₃⁺, CH₃ and COO^- groups. Our calculations suggest that NH₃⁺ group is freely rotating while CH₃ and COO^- groups rotate only limitedly. Proline molecule contains a non-planar five-member ring and exhibits two major conformations with very similar energies. Conformational space of L-proline was examined by puckering the ring and also rotating COO^- group. Weighted average spectra that were constructed can explain natural broadening of several spectral bands in particular in the low wavenumber region.

Finally we have shown that the simulation techniques requiring consideration of system dynamics and averaging over molecular conformations and solvent configurations are able to provide realistic ROA spectra of flexible and polar molecules.

[1] L.D. Barron, L. Hecht, E.W. Blanch, A.F. Bell, Prog. Biophys. Mol. Biol. 73 (2000) 1-49.

[2] L. Hecht, L.D. Barron, E.W. Blanch, A.F. Bell, L.A. Day, J .Raman Spectrosc. 30 (1999) 815-825.

[3] P. Bour, T.A. Keiderling, J. Chem. Phys. 119 (2003), 11253-11262.