New antitumour bicyclic hexapeptides, RA-IX and -X from Rubia cordifolia. Part 3. Conformation–antitumour activity relationship

Abstract

RA-IX and -X, new antitumour bicyclic hexapeptides were isolated from Rubia cordifolia and their structures were elucidated by spectroscopic and chemical evidences. RA-X containing a glutamic acid at residue 2 showed strong antitumour activity. However, RA-IX with conformation restricted to a type II β-turn at residues 2 and 3 showed almost no antitumour activity. Their conformation–antitumour activity relationships are discussed.