The structure of [D‐Hyi², L‐Hyi⁴]meso‐valinomycin revealed by X‐Ray analysis

Direct x‐ray analysis has been used to determine the crystal structure of [D‐Hyi², L‐Hyi⁴]meso‐valinomycin {cyclo[‐D‐Val‐D‐Hyi‐L‐Val‐L‐Hyi‐(D‐Val‐L‐Hyi‐L‐Val‐D‐Hyi)₂‐], C₆₀H₁₀₂N₆O₁₈}, which crystallized from acetone with two solvent molecules. The crystals are trigonal, space group P3₂, number of molecules per unit cell Z = 3, cell parameters a = b = 15.2085 (8) Å, c = 29.3250 (9) Å, γ = 120°. The standard (R)and weighted (R_w) reliability factors after refinement of the atomic coordinates for C, N, and O atoms in the anisotropic thermal motion approximation, allowing for isotropic H atom contributions, were 0.070 and 0.082, respectively. The molecule adopts a distorted bracelet structure which is stabilized by six NH … OC 4 → 1 type intramolecular hydrogen bonds. The side chains predominantly occupy external pseudoaxial positions relative to the cylindrical axis of the molecule. In contrast to meso‐valinomycin, only four of the six Val carbonyl oxygen atoms are directed inwards to form a coordination centre for the molecule, and the carbonyl oxygen atoms of residues D‐Val¹ and L‐Val³ are twisted outward and point away from the centre of the molecule. Although the analogue has a partially formed ion‐binding center, it is inaccessible because the hydrophobic isopropyl groups of the D‐Hyi² and L‐Hyi⁴ residues screen the molecular cavity on both sides.