Two distinct conformers of the cyclic heptapeptide phakellistatin 2 isolated from the Fijian marine sponge Stylotella aurantium

The isolation, structure determination, and solution conformation of two conformers of the cyclic heptapeptide phakellistatin 2 (cyclo-[Phe(1)-cis-Pro(2)-Ile(3)-Ile(4)-CiS-Pro(5)-Tyr(6)-cis-Pro(7)]) isolated from the Fijian marine sponge Stylotella aurantium are reported. The conformers can be isolated separately by HPLC and are stable in methanol solution over a period of weeks as determined by NMR. Their NMR spectra and mass spectral fragmentation patterns differ significantly. Their solution conformations were determined by NOE-restrained molecular dynamics calculations and indicated that the two conformers had different folds, hydrogen bonding patterns, and solvent accessible surfaces. These factors may contribute to the independent stability of the two conformers, and may explain the variable biological activity previously reported for phakellistatin 2.