First-principles studies of C-13NMR chemical shift tensors of amino acids in crystal state

A series of gauge-independent atomic orbital (GIAO) magnetic shielding calculations with ab initio Hartree-Fock (HF) and densityfunctional theory (DFT) methods were performed on alanine, asparagine, aspartic acid, cysteine, glutamine, glycine, histidine, serine, and tyrosine in crystal state to study their C-13 NMR chemical shift tensors. The GIAO calculations on the amino acids were performed with and without inclusion of the crystal environment. Compared to previous computational studies reported in literature, this study accounts for more complete crystal environmental effects on the C-13 NMR chemical shifts and, therefore, provides calculated results in better agreement with the experimental data. For each amino acid considered, the crystal environment included in the calculations includes all molecules having hydrogen bonds with the target molecule. It has been shown that the C-13 NMR chemical shift tensors, particularly the principal components 811 and 22, calculated without inclusion of the crystal environment significantly differ from the corresponding experimental data for the carbon atoms that are close to a hydrogen bond in the crystal, although the calculated isotropic C-13 chemical shifts are in good agreement with the experimental data. The GIAO calculations including the crystal environment with both the HF and DFT methods can significantly improve the calculated C-13 chemical shift tensors. The results calculated with the DFT method are slightly better than the corresponding results with the HF method. The good agreement between the calculated and experimental results indicate that the significant crystal environmental effects on the principal components of C-13 chemical shift tensors of the amino acids are mainly attributed to the hydrogen bonding of amino acid with its environment. When all the hydrogen bonds of amino acid with its environment are included explicitly in the GIAO calculation with a supermolecule model, the calculated C-13 chemical shift tensors, are in good agreement with the corresponding experimental data. This insight obtained from studying the crystal environmental effects on C-13 chemical shift tensors of amino acids in crystal state could also be useful for future computational studies of the protein environmental effects on C-13 chemical shift tensors of amino acid residues of proteins. (C) 2004 Elsevier B.V. All rights reserved.