Configurational and conformational analyses of a cyclic octapeptide, lyciumin A, from Lycium chinense Mill

Configurational and conformational analysis of a unique cyclic octapeptide, lyciumin A, showing an inhibitory activity on angiotensin-converting enzyme, was made by the spectroscopic and computational chemical methods. The homo- and heteronuclear 2D NMR analysis at 600 MHz in pyridine-d(5) enable us to determine the complete stereostructure of lyciumin A, which agrees with the structure obtained by the Monte Carlo (MC) and restrained molecular dynamics (MD) calculation using AMBER* force field. A major solution form of lyciumin A in pyfidine-d(5), analyzed by NH-C alpha H coupling constants, temperature dependence on NH protons, and NOE constrained MD calculations, was shown to have a type II beta-turn-like conformation between the Val and Gly residues constituting the cyclic backbone.