Calculation of the Effective Chemical Shielding Anisotropy in L-Alanyl-L-alanine, conformational and charge dependence study

Ladislav Benda¹, Petr Bouř¹, Miloš Buděšínský¹, Norbert Müller², Vladimír Sychrovský¹

¹Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nam. 2, 166 10 Praha 6, Czech Republic

²Institute of Organic Chemistry, Johannes Kepler University, Altenbergerstraße 69, 4040 Linz, Austria

ladislav.benda@marge.uochb.cas.cz

Biomolecular structure can be determined by the high-resolution NMR spectroscopy. Theoretical modeling of NMR parameters via modern quantum chemistry methods allows accurate predictions of structural parameters and dynamical behavior of important biomolecular compounds. Recently we determined the structure of the L-Alanyl-L-alanine di-peptide on the basis of complete assignment of isotropic chemical shifts and scalar coupling constants [1]. Another important NMR parameter, the cross-correlated relaxation rate, still requires theoretical modeling, in particular the dependence of the chemical shielding anisotropy (CSA) on the molecular structure [2,3].

In this work we correlated the effective CSA calculated for atoms along the di-peptide backbone with its major descriptors, the torsion angles f, y. Further we investigated the dependence of the effective CSA on total charge of the di-peptide, i.e. for its anion, zwitterion, and cation forms that can be experimentally accessed at different pH. The geometry was optimized at the BPW91 / 6-311++G** level employing the PCM solvent model for all forms of the di-peptide. The NMR parameters were calculated using the B3LYP / IGLO-III / PCM approach. The calculated surfaces of the effective CSA can be readily utilized for more accurate interpretation of the cross-correlated relaxation rates.

1. P. Bouř, M. Buděšínský, V. Špirko, J. Kapitán, J. Šebestík, V. Sychrovský, J. Am. Chem. Soc., 127, (2005), 17079.

2. V. Sychrovský, N. Müller, B. Schneider, V. Smrečki, V. Špirko, J. Šponer, L. Trantírek, J. Am. Chem. Soc., 127, (2005), 14663.

3. V. Smrečki, N. Müller, In preparation