The Carbonyl ¹³C Chemical Shift Tensors of Five Peptides Determined from ¹⁵N Dipole-Coupled Chemical Shift Powder Patterns

The¹³C chemical shift tensors have been determined for the glycine carbonyl carbon in a homologous series of peptides of the general form N-acetyl[l-¹³C]glycyl-X-amide, where X was [¹⁵N]glycine, dl-[15n] <-tyrosine, l-[15n] <phenylalanine and DL-[¹⁵N]alanine. The principal values and molecular orientations of the tensors were extracted from¹⁵N dipole-coupled¹³C powder spectra. The shift tensor of a powdered sample of [l-^I3C]glycyl[¹⁵N]glycine•HClsH₂O was determined by the same method and was found to agree to within 1 ppm in principal values and 2° in orientation with the previous single crystal measurements of R. E. Stark et. al. (J. Magn. Reson. 1983, 55, 266). The shift tensors of the five peptides were found to be significantly different in both principal values and molecular orientation. However, the isotropic chemical shifts of the end-protected peptides in D₂O were nearly identical. From these data it is concluded that lattice environment has a significant effect on the chemical shift tensors of peptide carbonyl carbons. An approach to approximating carbonyl¹³C chemical shift tensors of peptides in proteins with use of the isotropic chemical shift in the molecule of interest is proposed. In addition, the utility of the powder pattern technique for accurately determining the chemical shift tensors of peptide carbonyl carbons is demonstrated.