Cross-peptide bond 13C---15N coupling constants by 13C and J cross-polarization 15N NMR

Abstract

Comparative 13C---15N coupling constants are reported for the linear dipeptide tBoc--[U-13C]Ala-[15N]GlyOMe and the corresponding cyclic diketopiperazine, both in dimethylsulfoxide (DMSO) and, upon removal of the tBoc group, in water solutions. Spectra were obtained by 13C NMR and by the first application of J cross-polarization (JCP) 15 NMR, which greatly reduces the time required to accumulate 15N NMR spectra. In DMSO there was evidence for the formation of coplexed species which were not present in water. The values obtained for the cross-peptide bond coupling constant 2J13C[alpha]-15N were consistently less (by 2.2 Hz in DMSO, 4.3 Hz in water) for the cyclic than for the linear peptide, which may be related to the cross-peptide bond conformation. The 15N resonance for the cyclic peptide was shifted only 2 ppm downfield only 2 ppm downfield from the linear peptide chemical shift value in both solvents.