1Institute
of Physics, Charles University in Prague, Ke Karlovu 5, 121 16 Prague 2
2Institute
of Organic Chemistry and Biochemistry, Flemingovo nám. 2, 166 10 Prague 6
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 solution can now be measured
routinely. Because of its sensitivity to the chiral elements of biomolecular
structure, ROA provides new information about solution structure and dynamics complementary to that
supplied by conventional spectroscopic techniques [1].
Amino acids and peptides represent natural targets for ROA technique especially. However, the interpretation of the spectra is almost entirely dependent on ab initio modeling of vibrational frequencies and spectral intensities and so far imposes limits on molecular size and overall accuracy.
Computation of ROA is
a complex process, including evaluation of equilibrium geometry, molecular
force field and polarizability tensor derivatives. Currently only a slow finite
difference methods can be used for accurate evaluation of the tensors. For
zwitterionic amino acids and peptides many complications arise also from their
conformational flexibility and strong interaction with the solvent, which
cannot be modeled with the usual procedures developed for vacuum. Many of these
obstacles can be avoided in the modeling. For our ROA simulations we used
simplified representation of the polarizabilities [2], Cartesian transfer
techniques for the molecular tensors [3] and the COSMO continuum solvent model.
Incident circular polarization (ICP) ROA instrument has been built at Institute of Physics following the design of the instrument constructed in Glasgow [4].
However, there are currently serious limitations of the method, with respect to limited instrumental sensitivity and artifacts appearing in the spectra. Nowadays we develop program allowing to process signal in image mode and to identify and eliminate artifacts. We are going to replace conventional spectrograph in Czerny-Turner configuration with a fast stigmatic imaging spectrograph (equipped with a transmission holographic grating) [5], which enables 8 times faster acquisition of ROA signal and thus makes study of large (bio)molecular systems at lower concentration possible.
Combination of experimental and computational approaches represents unique and powerful tool for studying structure and interaction of biologically important molecules.
References
[1] L.D. Barron, L. Hecht, E.W. Blanch, A.F. Bell, Prog. Biophys. Mol. Bio. 73 (2000), 1-49.
[2] P. Bouř,
J.
Comp. Chem. 22 (2001), 426–435.
[3] P. Bouř,
J. Kapitán, V. Baumruk, J.
Phys. Chem. A 105 (2001), 6362-6368.
[4] L. Hecht, L.D. Barron, A.R.Gargaro, Z.Q.Wen, W.Hug, J. Raman Spectrosc. 23 (1992), 401-411.
[5] L. Hecht, L.D. Barron, E.W. Blanch, A.F. Bell, L.A. Day, J .Raman Spectrosc. 30 (1999), 815-825.