Solution forms of antitumor cyclic pentapeptides with 3,4-dichlorinated proline residues, astins A and C, from Aster tataricus.

Conformational analysis of antitumor cyclic pentapeptides, astins A (1) and C (3), was made by a combination of NMR and computational techniques. These results indicated that the backbone conformations of 1 and 3, with lower activity than astin B (2), were different from that of 2. The backbone conformation together with a cis 3,4-dichlorinated proline residue was considered to play an important role in the antitumor activities of astins.