Peptides shielding tensor

Most of the ab initio studies of NMR chemical shifts in peptides and proteins have focused on the average or isotropic value of the NMR chemical shift. The chemical shift is a tensor quantity and is, therefore, capable of providing six independent pieces of information, namely, the magnitude and direction of each of the three principal components. In general, the shielding tensor can be antisymmetric, leading to nine independent components. However, only the symmetric part of the shielding tensor is relevant to the experiments... 

Figure 1. Model peptide fragment showing the spatial relationship between N shielding tensors of adjacent residues and the dihedral angle...

It is a well-known fact that the magnitudes of the peptide backbone " N, and amide chemical shielding tensors and their orientations relative to the peptide planes are quite similar for all amino acids. Although the details of the tensor parameters may vary, this feature is fundamental for powerful new techniques exploiting residual anisotropies in liquid-state NMR of partially aligned samples, and for the general utility... 

The dipole-dipole coupling parameters are generated straightforwardly from the atomic coordinates based on the intemuclear distances and the orientations of the intemuclear vectors. The peptide plane chemical shielding tensors are established according to the type of atom and its typical orientation relative to the peptide plane (Fig. 2). If amide protons and H are not present (as would be the case with X-ray... 

Fig. 3. (a) A typical spin system of three peptide planes with chemical shielding tensors and... 

As novel solution and solid state experiments enabling the determination of shielding tensor quantities in peptides and proteins become widespread, it is strongly recommended that practitioners stick to precise definitions of terms that have accepted meanings within the solid state NMR community. As a reminder, we list again the modern terms The difference between the most shielded (ass) and the least shielded (an) tensor component is the span. 

There are different parameters that can provide insights into the hydrogen bond interaction in the solid state. Solid state 13C-NMR studies of protonated and deprotonated carboxylates in amino acids have shown that the values of the principal elements of the nuclear shielding tensor change significantly with the protonation state of the carboxylic groups [54—59]. The orientation of the three CSA tensors for the COOH and COO" groups in a peptide is depicted in Fig. 3.2.21. 

Fig. 4. Orientation of the shielding tensor elements in the molecular frame on alanyl-alanine, cTn lies essentially in the plane of the peptide linkage as do the N-H, N-Ca,...

In order to establish the orientation of a peptide backbone or side-chain residue from chemical interactions, the orientation of the shielding tensor in the molecular frame needs to be determined. 

The first four N- H systems, all peptide linkages, yield similar results to [l-13C]glycyl[15N]glycine HCl-H20 where the an principal axis of the shielding tensor is rotated approximately 20° from the unique principal axis... 

The determination of polymer structure at the atomic level is possible by analyzing orientation-dependent NMR interactions such as dipole-dipole, quadrupole and chemical shielding anisotropy as mentioned above. The outline of the atomic coordinate determination for oriented protein fibers used here is described more fully in Ref. [30]. The chemical shielding anisotropy (CSA) interaction for N nucleus in an amide (peptide) plane can be interpreted with the chemical shielding tensor transformation as shown in Fig. 8.3. [31, 32]. 

Figure 2.38 Correlations of experimental and calculated shielding tensors (5n/ 22 and 33) of carboxyl (C=0) carbons for various amino acids and peptides obtained from MM, PMl, and PM2 models. The solid line across the diagonal axis represents a perfect match between the experimental and calculated values. Reprinted from Ref [185]. Copyright 2009 Wiley Periodical, Inc.

A. Zheng, S.-B. Liu, F. Deng, C shielding tensors of crystalhne amino acids and peptides theoretical predictions based on periodic structure modeb, J. Comput. Chem. 30 (2009) 222-235. 

J. Czernek, T. Pawlak, M.J. Potrzebowski, Benchmarks for the C NMR chemical shielding tensors in peptides in the solid state, Chem. Phys. Lett. 527 (2012) 31-35. 

---