Peptides chemical shifts tensors

As mentioned above, the principal values of chemical shift tensor give information about three dimensional electronic state of a molecule. However, in order to understand behavior of the principal values, one should obtain information about the orientation of the principal axis system of a chemical shift tensor with respect to the molecular fixed frame. The orientations of the principal axis systems of the chemical shift tensors of L-alanine Cp -carbons in some peptides were calculated, whose L-alanine moieties have different main-chain dihedral-angles, (( >,v /H-57.40,-47.50)[aR-helix], (-138.8°,134.7°)[ pA-sheet], (-66.3°,-... 

In this paper, we aim to elucidate the correlation between the chemical shift tensors and the conformation of peptides, by carrying out NMR shielding calculations using the ab initio GIAO-CHF MO with 6-31G basis set, in order to understand the 13C chemical shift and chemical shift tensor behavior of the peptides and polypeptides. 

The dependence of the principal components of the nuclear magnetic resonance (NMR) chemical shift tensor of non-hydrogen nuclei in model dipeptides is investigated. It is observed that the principal axis system of the chemical shift tensors of the carbonyl carbon and the amide nitrogen are intimately linked to the amide plane. On the other hand, there is no clear relationship between the alpha carbon chemical shift tensor and the molecular framework. However, the projection of this tensor on the C-H vector reveals interesting trends that one may use in peptide secondary structure determination. Effects of hydrogen bonding on the chemical shift tensor will also be discussed. The dependence of the chemical shift on ionic distance has also been studied in Rb halides and mixed halides. Lastly, the presence of motion can have dramatic effects on the observed NMR chemical shift tensor as illustrated by a nitrosyl meso-tetraphenyl porphinato cobalt (III) complex. 

Similar to the situation for 13C, isotropic 15N chemical shifts and the principal components of 15N chemical shift tensors have been used to study N-H- -0=C hydrogen bonds in peptides. It has been shown that isotropic 15N chemical shifts of proton donors (such as N-H) are displaced downfield by ca. 15 ppm, whereas those of proton acceptors are shifted upfield by ca. 20 ppm [110-112]. Amongst the CSA components, S33 (parallel to the C-N bond) has been shown to be most sensitive to the hydrogen bond strength, as reflected by the N- -O distance [113]. Detailed studies of the principal components and orientations of 15N chemical shift tensors for amide nitrogens in simple peptides have been reported recently [114]. This work confirmed that S33 and Siso are the 15N chemical shift parameters that are the most sensitive to details of the hydrogen bonding. It was also found that N-H... 

To illustrate the power of PISA wheels and dipolar waves to determine the structure of helical peptides and proteins in uniaxiaUy oriented lipid bilayers. Fig. 6a-c show SIMPSON/SIMMOL-simulated PISEMA spectra of an ideal 18-residue a-helix with a tilt angle of 10°-30° relative to Bq. In these simulations, we have tried to mimic experimental conditions by including a random distribution of the principal components of the chemical shift tensor and the dipolar coupling. The chemical shift distribution is 6 ppm for each principal element and has been established as follows we obtained — 85000 N isotropic chemical shifts reported to the BioMagResBank and selected only the — 31000 located in helical secondary stractures to have a data set independent on secondary chemical shifts. The standard deviation on the N chemical shifts for these resonances was — 6 ppm. With the lack of other statistically reliable experimental methods to establish such results for the individual principal elements of the N CSA tensor, we assumed the above variation of 6 ppm for all three principal elements. The variation of the H- N dipolar coupling was estimated by investigating the structures for a small number of a-helical membrane proteins for which the structures were established by liquid-state NMR spectroscopy. These showed standard deviations... 

In order to establish structural constraints on proteins and peptides from solid-state NMR, it is important to consider all aspects from appropriate labeling of the sample, selection of the experiments providing the desired information, and to have appropriate reference data available to allow extraction of structural data from the (anisotropic interaction) parameters determined by the experiment. As an example. Cross and co-workers investigated the conformation of the ion channel gramicidin A using selectively N-labeled peptides in uniaxiaUy oriented lipid bilayers c.f.. Section 4.2). To translate the measured " N chemical shifts in the oriented samples into stractural constraints, it is necessary to determine the magnitude and orientation of the " N chemical shift tensors... 

Fig. 8. Orientations of the principal axies of the N chemical shift tensor relative to the N-H bond and the peptide plane. The angle / is defined by the direction of the <533 tensor component and its projection onto the peptide plane, fi is the angle between the direction of S33 and the N-H bond direction, and a is the angle between <5 1 and the projection of the N-H bond vector onto the i5i 1- 22 plane.

Fig. 9. Dependence of the C chemical shift tensor in glycine on different types of peptide secondary structure motifs, from an experimental investigation of tripeptides.

Figure 3.2.21 The orientation of the 13C chemical shift tensors with respect to the molecular frame of the carboxylic group of a peptide.

Fig. 22.8. Plots of the observed chemical shift tensor components for (a) 6u (b) 822, and (c) 533for the amide carbonyl carbons in the Gly (solid circle), Ala (solid square), Val (open diamond), Leu (open triangle) and Asp (open circle) residues in peptides against the Rw o-The experimental errors of 5n and 633 are indicated by error bars.

Figure 2.43 The full optimized Leu-enkephalin structure with enlarged parts of both systems. Selected interatomic distances are indicated. (A) The plots show the correlation of experimental isotropic chemical shift values (5 so) and calculated nuclear shielding values ( 7 so). (B)The plots represent the correlation of experimental chemical shift tensor values Sii) and calculated nuclear shielding parameters of the enkephalin peptides. (C) The correlations of the experimental versus the computed parameters are shown for Leu-enkephalin. Reprinted from Ref. [96]. Copyright 2014 American Chemical Society.

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