Conformational Analysis of the Cyclic Pentadepsipeptide Cyclo(Tro-Aib-Aib-Aib-Aib) in the Solid State and in Solution

Abstract

The cyclic 16-membered pentadepsipeptide cyclo(Tro-Aib-Aib-Aib-Aib) (1) was crystallized from MeOH/AcOEt/CH2Cl2, and its structure was established by X-ray crystallography (Fig. 1). There are two symmetry-independent molecules with different conformations in the asymmetric unit. Two intramolecular H-bonds stabilize two beta-turns in each molecule. On the other hand, two of the four Aib residues are forced to assume a nonfavorable nonhelical conformation in each of the symmetry-independent molecules (Table 1). The conformational study in CDCl3 solution by NMR spectroscopy and molecular dynamics (MD) simulations indicate that the averaged structure (Fig. 3) is almost the same as in the solid state.