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# Crystal and Molecular Structure of Didemnin B, an Antiviral and Cytotoxic Depsipeptide

M. B. Hossain, D. Van Der Helm, J. Antel, G. M. Sheldrick, S. K. Sanduja and A. J. Weinheimer
Proceedings of the National Academy of Sciences of the United States of America
Vol. 85, No. 12 (Jun. 15, 1988), pp. 4118-4122
Stable URL: http://www.jstor.org/stable/31728
Page Count: 5
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## Abstract

Didemnin B, a highly active depsipeptide isolated from a Caribbean tunicate, crystallizes from chloroform/benzene in the orthorhombic space group C2221 with cell parameters a = 14.990 ± 0.003 angstrom, b = 22.574 ± 0.004 angstrom, c = 41.112 ± 0.009 angstrom, V = 13911.7 angstrom 3 at 138 K and a calculated density of 1.143 g/cm3 based on C57H89N7O$_{15}\cdot$1.5C6H6· H2O and eight formula units per cell. The overall agreement factor R=0.052 for 7699 reflections, 2θ max = 150 degrees, Cu K$\overline{\alpha}$ radiation. The structure determination revealed that didemnin B contains an isostatine residue instead of a statine residue. The conformation of the 23-membered depsipeptide ring is stabilized by one transannular hydrogen bond. The ring does not show the antiparallel β -pleated-sheet structure but, instead, has a fold in the shape of a bent figure-eight. The linear peptide moiety, containing N-methylleucine and lactylproline, forms a β (II)-bend and is folded back toward the cyclic backbone, giving the overall molecule a globular character. Comparison with the structure of cyclosporin A shows distinct stereochemical differences between the two molecules. It is suggested that didemnin B and cyclosporin A are unlikely to have a common receptor binding site.

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