Synthesis and Chemical Structure of the Cyclic Heptapeptide Stylissamide H

We investigated the chemical structures and conformational
isomers
of the cyclic heptapeptides stylissamide H and euryjanicin A isolated
from marine sources. Despite sharing the same molecular structure,
stylissamide H and euryjanicin A exhibit different conformational
isomers in solution and solid states. The main difference arises
from the cis/trans configurations of the two Pro
residues. We prepared stylissamide H using Fmoc solid-phase peptide
synthesis and investigated its chemical structure in the solution
state and the solid state via NMR spectroscopy and X-ray crystallography,
respectively. The results indicated that the s-trans,cisPro conformer of stylissamide
H is more stable in solution, whereas the s-cis,cisPro conformer is more stable in the solid state.
We concluded that stylissamide H and euryjanicin A are the same molecules.