Molecular conformation of prostaglandin A₁ monoclinic cyrstalline polymorph

The molecular conformation of the monoclinic crystalline polymorph of prostaglandin A₁ has been determined by X-ray diffraction techniques. The space group is P2₁ with a = 13.637 (2), b = 7.567 (1), I c = 10.576 (2) Å, β = 107.37 (3)°; D_c = 1.073 g·cm^-3 for Z = 2. The molecular conformation is characterized by the nearly parallel arrangement of the C₁-C₇ and C₁₃-C₂₀ side chains, with a general flattening of the overall structure when compared with the orthorhombic polymorph. The cyclopentenone moiety assumes a C₈ envelope conformation with C₈ and O₉ displaced +0.29 Å and -0.18 Å from the C₉-C₁₀=C₁₁-C₁₂ plane respectively. Concerted, small variations of the torsion angles, primarily about the C₈-C₁₂, C₁₄-C₁₅ and C₁₆-C₁₇ bonds, bring the monoclinic and orthorhombic conformations into coincidence.