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Determination of the Amide‐Plane Orientations in a Cyclo‐ β ‐Peptide by Magic‐Angle‐Spinning Deuterium‐Correlation Spectroscopy, and Comparison with the Powder X‐Ray Structure
Author(s) -
Le Hongbiao C.,
Hintermann Tobias,
Wessels Thomas,
Gan Zhehong,
Seebach Dieter,
Ernst Richard R.
Publication year - 2001
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/1522-2675(20010131)84:1<208::aid-hlca208>3.0.co;2-b
Subject(s) - chemistry , deuterium , magic angle spinning , dihedral angle , principal axis theorem , crystallography , spin diffusion , nuclear magnetic resonance spectroscopy , magic angle , molecule , molecular physics , stereochemistry , hydrogen bond , diffusion , atomic physics , geometry , physics , organic chemistry , mathematics , thermodynamics
Previously, it has been shown that the correlation of deuterium quadrupolar tensors by spin diffusion under slow magic‐angle‐spinning conditions can provide accurate measurements of their relative orientation. In the present work we apply the technique to the cyclo‐ β ‐peptide cyclo[( S )‐ β ‐homoalanyl‐( R )‐ β ‐homoalanyl‐( S )‐ β ‐homoalanyl‐( R )‐ β ‐homoalanyl] with its amide hydrogens labeled by deuterons. From the 2D spin‐diffusion spectrum, it is possible to determine the mutual orientation of the amide deuteron quadrupolar coupling tensors. Assuming that the molecule has four‐fold molecular symmetry, the polar angles of the symmetry axis in the principal‐axis frames of the deuterium electric‐field‐gradient tensors are found to be θ =15.7°±1.0° or θ =164.3°±1.0°, and ϕ =±72°±10°. They are used to deduce possible conformations of the peptide based on the result of a previous measurement that correlated the deuterium principal quadrupolar frame with a local molecular frame. We found eight conformations that are all consistent with the NMR measurement. Three of these have acceptably small van der Waals contact energies. One of the three structures agrees, within a rmsd of 6° for the backbone dihedral angles, with an X‐ray conformation.