Molecular structure of a new polymorph of the K⁺, Rb⁺ chelate of mesovalinomycin

The crystal structure of a new polymorph of hydrated mesovalinomycin cyclo[-(D-Val-L-Hyi-L-Val-D-Hyi)3]¹ (C₆₀H₁₀₂N₆O₁₈) was solved by the direct methods, sp. gr. P1, Z = 2, unit-cell parameters a = 14.08, b = 14.48, c = 22.63 Å, α = 95.48°, β = 68.32°, γ = 119.06°. Upon refinement, the standard R factor was 0.109. It was established that, in this modification as well, both independent molecules of mesovalinomycin adopt a rather rigid bracelet conformation with a negatively charged inner cavity and six intramolecular hydrogen bonds. The structural parameters of all the known crystalline modifications of mesovalinomycin are analyzed.