Cai, Ling, Fushman, David, and Kosov, Daniel S. (2009) Density functional calculations of chemical shielding of backbone 15N in helical residues of protein G. Journal of Biomolecular NMR, 45 (3). pp. 245-253.

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Abstract

We performed density functional calculations of backbone 15N chemical shielding tensors in selected helical residues of protein G. Here we describe a computationally efficient methodology to include most of the important effects in the calculation of chemical shieldings of backbone 15N. We analyzed the role of long-range intra-protein electrostatic interactions by comparing models with different complexity in vacuum and in charge field. Our results show that the dipole moment of the α-helix can cause significant deshielding of 15N; therefore, it needs to be considered when calculating 15N chemical shielding. We found that it is important to include interactions with the side chains that are close in space when the charged form for ionizable side chains is adopted in the calculation. We also illustrate how the ionization state of these side chains can affect the chemical shielding tensor elements. Chemical shielding calculations using a 8-residue fragment model in vacuum and adopting the charged form of ionizable side chains yield a generally good agreement with experimental data.

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