Application of the TORO technique of 1H NMR to the structural analysis of cyclic peptide isomers having a slightly distorted symmetry from C2

The extended TORO technique was applied to the structural analysis of endo-D-Tyr-gramicidin S, cyclo(-Val-Orn-Leu-D-Phe-D-Tyr-Pro-Val-Orn-Leu-D-Phe-Pro-), which has a slightly distorted symmetry from C2, by the insertion of D-Tyr and equivalent alpha-proton chemical shifts in the 1H NMR spectrum. All NMR signals of the two dominant isomers of this antibiotic with trans-trans prolines were determined by using the extended TORO technique with TOCSY and ROESY spectra. This technique is generally applicable for distinguishing overlapped signals of alpha- and amide protons from the main chains of peptides.