NEW ANTITUMOR BICYCLIC HEXAPEPTIDES, RA-VI AND RA-VIII FROM RUBIA-CORDIFOLIA - CONFORMATION-ACTIVITY RELATIONSHIP II

The structures of new antitumor bicyclic hexapeptides, RA-VI and -VIII from Rubia cordifolia were elucidated by the spectroscopic and chemical methods. A combination of two-dimentional NMR techniques and NOE relationships showed that the amino acids constituting the beta-turn of RA-VI are Ser-2 and D-Tyr-3 and those of RA-VIII, Thr-2 and Tyr-3. By the conformational analysis of RA-VI in its crystalline state using the X-ray diffractometric technique, RA-VI was shown to have, in its solid state, a type V beta-turn structure at the residues Ser-2 and D-Tyr-3, while other RAs have type II beta-turns. Further, by 2D-NMR techniques, temperature effects on NH protons and NOE experiments, in solution of CDCl3, RA-VI was shown to exist only as conformer A and RA-VIII as conformers A, B and C. The difference between the solid state and solution state conformations of RA-VI was also shown by the refinement of the restrained molecular dynamics calculations using AMBER program. RA-VIII, having a smaller population of conformer A with type II beta-turn than other RAs, showed a reduced biological activity, and the N-methyl derivative of RA-VIII, whose conformer A content is further reduced, gave a further reduced activity, suggesting that conformer A contributes to the activity. However, RA-VI, existing in solution 100% as conformer A, showed a very low activity and N-methylation increased the activity. This shows that the stereochemistry and molecular mobility of the aromatic side chain of Tyr-3 over this turn, as elucidated by the C-13 spin lattice relaxation times, plays a more important role in the antitumor activity of the compounds of this series in addition to the type II beta-turn structures.