Modelling the influence of hydrogen bond network on chemical shielding tensors description. GIAO-DFT study of WALP23 transmembrane α-helix as a test case

International audienceDensity Functional Theory (B3LYP/6-31G(d,p)) calculations of 15 N amide and 13 C carbonyl NMR chemical shielding tensors have been performed on WALP23 trans-membrane -helix peptide and 10 compared to solid state NMR experiment performed on [ 13 C1-Ala13, 15 N-Leu14] specifically labelled peptide powder sample. Using either theoretical result obtained on the whole peptide or experimental data as reference, several simplest chemical models have been explored in order to reduce the computational cost while maintaining good theoretical accuracy. From this study, it appears that the hydrogen bond (N-H…O=C) network that exists in the -helix has a major 15 influence on the chemical shielding tensor and more specifically on the carbonyl 13 C 22 eigenvalue. We show that a small truncated WALP_7 model is not adequate for 13 C1 NMR description. The application of an external electric field in order to model the hydr ogen bond network allows calculating chemical shielding tensors with accurate eigenvalues while the associated eigenvectors are largely modified. Finally, a 23 residues polyglycine peptide that 20 includes the Alanine and Leucine residues for which NMR parameters must be calculated is proposed as the chemical model. This model is sufficient to mostly reproduce the calculation performed on WALP23 with major gain in computational time. Moreover, the application of an external electric field allows reaching the experimental accuracy for the determination of the eigenvalues. 2