Tunicyclin E (1), a new cyclic heptapeptide, cyclo(Pro1–Ser2–Trp3–Leu4–Val5–Gly6–Ser7), was
isolated from the root of Psammosilene tunicoides. The presence of two sets of resonance signals in its
NMR spectra (1a:1b, y3 : 1 abundance) indicated that it has two conformations in solution, while
only one conformation was found in its crystal state by X-ray diffraction. To explore the molecular
basis of the two conformations of 1 in solution and their interconversion mechanism, X-ray
diffraction, NMR experiments, and theoretical calculations were performed. The results disclosed
that two conformers of 1 in solution were derived from the cis/trans isomers of the Ser7–Pro1 peptide
bond (1a, trans; 1b, cis). The fast interconversion of the two conformations in solution is explained by
an intramolecular catalysis mechanism and solvent effects. Furthermore, the existence of several
unusual pseudo turns characterized for the first time plays a key role for dominant trans conformation
in solution.