Determination of principal components and their directions of 15N chemical shift tensor and N-H bond lengths in simple peptides as parameters for characterization of N-H•••O=C hydrogen bonds

The principal components and their direction of N-15 chemical shift tensor, isotropic N-15 chemical shifts and N-H bond lengths for amide nitrogen were determined for a variety of simple peptides as parameters to characterize (NHO)-O-...=C hydrogen bonds. Principal components and their directions of N-15 chemical shift tensors were evaluated by recording proton-decoupled and N-15-H-1 dipolar coupled N-15 CP-static powder pattern NMR spectra, respectively. It is emphasized that determination of the direction of the principal component is very important as encountered for reversal of the magnitudes of delta(22) (perpendicular to the peptide plane) and delta(33) (parallel to C-N bond) components for Gly-Pro-D-Leu-[N-15]Gly, as compared with those of other peptides. The N-H bond lengths necessary for the latter experiments were determined by analyzing Pake type doublet patterns recorded by either conventional two-dimensional separated local dipolar field (2D-SLDF) or newly developed spin echo double resonance combined with multiple pulse homonuclear decoupling (MPSEDOR) methods, utilizing pi refocusing pulses applied both in H-1 and in N-15 channels to recouple the heteronuclear dipolar interaction while elimination of chemical shift interaction. It is found that the observed N-H bond lengths based on the N-H dipolar interaction could be overestimated, in some instances, when amplitude of a thermal vibration of N-H moiety is significantly enhanced as encountered for weakly hydrogen bonded systems. This means that special care should be exercised when one compares the NNM-derived N-H distances involved in different types of hydrogen bond systems as a parameter to characterize hydrogen bond system. (C) 2002 Elsevier Science B.V. All rights reserved.