engleski

DFT Calculations of CSA Tensors in Model Peptides. Dependence on Secondary Structure

Density Functional Theory (DFT) calculations were used to asses the dependence of backbone-atom chemical shielding anisotropy (CSA) tensors on backbone conformation. The molecular models used in the simulation of protein secondary structure elements and CSA calculations were a capped alanyl-alanine di-peptide and a capped ala(8) octa-peptide. The Gaussian98 program package with the DFT (B3LYP) method was used for geometry optimizations and NMR parameter calculations (GIAO) with the 6-311G(d, p) basis set for the former and 6-311++G(3df, 3pd) for the latter. For each backbone atom of the dipeptide the CSA tensor was analyzed by itself and, additionally, in the context of cross-correlated relaxation with selected dipolar interactions. This was to allow comparison to both solid state and liquid state NMR experiments. The octa-peptide model served to account for the hydrogen bonding influences in alpha helices. The results obtained on backbone 1H, 13C and 15N CSA tensors showing dependence on protein secondary structure are visualized by way of Ramachandran-type diagrams for the dipeptide model. Comparisons of predicted CSAxDD cross-correlation rates using symmetric and fully asymmetric CSA tensors are shown. The relative influences of hydrogen bonding and conformation on CSA tensors in an α -helical secondary structure are compared. Acknowledgement This work was supported in part by the Lise Meitner fellowship of the Austrian Science Fund (FWF M677), FWF project P15380, Austrian Croatian joint research project 911-02/03-06 (ÖAD-Project 13/04), and the Ministry of Science, Education and Sport of the Republic of Croatia project P0098059.

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