CRYSTAL AND MOLECULAR-STRUCTURE OF THE (ENNIATIN B)(2) KI COMPLEX

Single crystals of a complex of enniatin B with KI grown from an alcoh ol solution at room temperature were studied by X-ray structure analys is [a = b = 14.540(10) Angstrom, c = 66.517(15) Angstrom; sp. gr. P6(1 ); Z = 6; [C33H57N3O9](2). KI . H2O; 93 independent non-hydrogen atoms ; CuKalpha-radiation; theta(min) = 3.75 degrees, theta(max) = 42.03 de grees; 2646 reflections; direct method and difference syntheses 2F(obs )-F-calc; R = 0.077 based on 1857 reflections with I greater than or e qual to 2 sigma(I)]. Two independent enniatin B molecules with amide c arbonyl groups facing each other form a 2 : 1 sandwich complex. The K ion is located in the inner cavity of the complex. The coordination s phere of the K+ ion is a distorted trigonal prism formed by the amide carbonyl groups of both molecules. The hydrophobic environment of the complex is formed by side radicals and N-methyl groups of the amino ac id residues of an antibiotic molecule. Both enniatin molecules adopt s imilar conformations, which are close to those observed in the Na,Ni-c omplexe studied previously; whence it follows that the ''bottom'' of t he molecule with N-methyl groups is slightly open if this molecule bin ds the K+ ion, which hinders the membrane transport of a sandwich comp lex because of disruption of its hydrophobic shell.