CONFORMATIONAL-ANALYSIS OF AN ANTITUMOR CYCLIC PENTAPEPTIDE, ASTIN-B,FROM ASTER TATARICUS

Conformational analysis of an antitumour cyclic pentapeptide, astin B, isolated from Aster tataricus, was conducted by 2D-NMR techniques, te mperature effects on NH protons, rate of hydrogen-deuterium exchange, vicinal NH-C alpha H coupling constants, and NOE experiments. The meth ods of molecular mechanics and restrained molecular dynamics calculati ons were applied to understand the energetic preferences of various co nformations of astin B. Distances involving in three intramolecular hy drogen bonds and the NOE correlations were used for the refinements us ing AMBER program. These results indicated that the conformation in th e solution state was, on the whole, homologous to that observed in the solid state. Conformational difference between astin B, showing a cis configuration in a proline amide bond, and cyclochlorotine from Penic illium islandicum, showing all trans amide configurations, was also di scussed.