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A Density Functional Theory Study of Chemical Shielding Anisotropy Dependence on Secondary Structure in Model Peptides

Chemical shielding (or shift) anisotropy (CSA) is attracting increasing scientific interest in particular in the context of structure determination of biomolecules. Current NMR techniques enable more accurate determination of larger numbers of CSA's not only in solid state NMR of powder protein samples [1], but also in liquid state NMR both in isotropic solution [2] and in partially oriented fluid media [3]. Renewed interest in cross-correlation phenomena involving CSA has arisen since these can be exploited to derive structural constraints [4 6]. As a prerequisite to structural interpretation of experimental CSA values we used Density Functional Theory (DFT) calculations in order to quantitatively predict the influence of secondary structure ( and  angles) on CSA tensors. The DFT calculations were conducted using the Gaussian98 program package employing implemented methods, basis sets and approaches (e.g., the 6 311G(d, p) basis set for geometry optimization and frequency calculation, as well as 6 311++G(3df, 3pd) basis set for NMR parameters calculation). The Becke's three parameter hybrid functional using the Lee, Yang and Parr correlation functional (B3LYP) was employed in DFT calculation., while the gauge independence requirement for NMR parameter calculation was treated with the Gauge Invariant Atomic Orbital (GIAO) approach. Capped oligopeptides were used to assess the fundamental dependence of CSA tensors on secondary structure elements. The potential of CSA measurements for secondary structure determination is demonstrated by way of Ramachandran CSA diagrams. References [1] Y. Wei, D. K. Lee, and A. Ramamoorthy, J. Am. Chem. Soc., 2001, 123, 6118 26. [2] N. Tjandra and A. Bax, J. Am. Chem. Soc., 1997, 119, 8076 82. [3] G. Cornilescu and A. Bax, J. Am. Chem. Soc., 2000, 122, 10143 54. [4] R. Brüschweiler, C. Griesinger, and R. R. Ernst, J. Am. Chem. Soc., 1989, 111, 8034 5. [5] R. Brüschweiler and R. R. Ernst, J. Chem. Phys., 1992, 96, 1758 66. [6] B. Reif, M. Hennig, and C. Griesinger, Science, 1997, 276, 1230 3.

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