RAI-III and VI, conformational isomers of antitumor cyclic hexapeptides, RA-III and VI from Rubia cordifolia

RAI-III and VI, conformational isomers of antitumor bicyclic hexapeptides, RA-III and VI, respectively, were isolated from Rubia cordifolia. By the conformational analysis of them using spectroscopic and computational chemical methods, they were shown to have γ-turn structures at residues 2,3, and 4, which were stabilized by a hydrogen bond between Ser-2-OH and D-Ala-1-CO.

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