Nitrogen-15 NMR studies of solid dipeptides

The first part of this study deals with the systematic determination of the 15N chemical shift tensor for the series of dipeptides N-acetyl[1-13C]-glycyl-[15N]-X-amide (GlyX: X = alanine, glycine, tyrosine), [1-13C]-glycyl[15N]-glycine/center dot/HCL and [1-13C]-ananyl-[15N]-alanine (AlaAla). The principal values and polar angles relating the chemical shift tensor to the peptide C-N bond are given for all dipeptides studied. The isotropic values for 15N varied from 105 ppm up to 119.7ppm for all the dipeptides studied and the angle β/sub CN/ relating σ33to the C-N bond varied from 98/degree/ to 106/degree/. GlyAla and AlaAla showed striking differences in asymmetry parameter, .37 vs. .13, and anisotropy, 165 vs. 144, yet the isotropic values for these two are identical. The 13C chemical shift principal values and polar angles are determined for AlaAla. The second part of the study deals with an experiment to determine unambiguously the orientation of the chemical shift tensor in the molecular frame