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Anisotropy of Chemical Shielding and its Relation to Secondary Structure Elements 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]. Density Functional Theory (DFT) calculations were used to quantitatively predict the influence of secondary structure (phi and psi angles) on CSA tensors as a prerequisite to structural interpretation of experimental CSA values. The Gaussian98 program package was used employing implemented methods, basis sets and approaches (e.g., the 6 311G(d, p) basis set for geometry optimization, as well as frequency calculation and 6 311++G(3df, 3pd) basis set for NMR parameters calculation). The Becke's three parameter hybrid functional with the Lee, Yang and Parr correlation functional (B3LYP) was employed in DFT calculation. The gauge independence requirement for NMR parameter calculation was treated with the Gauge Invariant Atomic Orbital (GIAO) approach. Molecular models used in simulation of secondary structure elements and CSA calculations were capped alanine dipeptide and octapeptide. The results of DFT simulations of 1H, 13C and 15N CSA showing dependence on secondary protein structure will be demonstrated by way of Ramachandran-CSA diagrams for the dipeptide model. A chain length and hydrogen bonding influence on CSA in alpha-helices were tested with the octapeptide model. Acknowledgement This work was supported in part by the Austrian Science Fond (FWF) Lise Meitner fellowship (M677) and FWF project P15380, Austrian Croatian joint research project WTZ 13/2002, and the Ministry of Science and Technology Republic of Croatia P0098059. 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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