The x-ray structure of the monoclinic crystal form of [o-hyi2, l-hyi4]/nesovalinomycin

The conformation and intennolecular association of[o-Hyi2, L-Hyi4]msso-valinomycin (cyclo[-D-Val-D-Hyi-L-Val-L-Hyi-(D-Val-L-Hyi-L-Val-D-Hyi)2-], C,so//,02W(As/ in a crystal fonn obtained from ethanol solution has been determined by x-ray crystallography. Two depsipeptides and one ethanol molecule per asymmetric unit crystallize in space group P2, (Z = 4); a = 14.579, b = 39.795, c = 13.928A, = 116.90, Rl = 0.0757. The molecular conformation is very similar to that observed in the trigonal P32 crystal fonn obtained from acetone solution [V. Z Pletnev et al. (1991) Biopolymers, Vol. 31, pp. 409-415]. Both independent molecules in the crystal adopt a similar distorted bracelet structure with a sterically inaccessible, partially formed, ion-binding center that is stabilized by six 4 -1 type H bonds. The observed conformation accounts for the inability of the molecule to complex ions. Close examination of the three crystallographically independent molecules reveals that differences in the backbone conformation associated with solvent interaction are significantly larger than those associated with Hydrophobie van der \Vaals interactions of crystal packing.