Cholesteryl myristate: structures of the crystalline solid and mesophases

Abstract

Crystals of the title compound are monoclinic, space group A2, with lattice parameters a= 10.260(8), b= 7.596(8), c= 101.43(9)Å, β= 94.41(5), and Z= 8 [2 independent molecules (A) and (B)]. The structure was determined from 2 807 observed X-ray intensities, measured by diffractometer. The phase problem was solved by Patterson rotation and translation methods. Refinement by block-diagonal least-squares gave R 0.079. Molecules (A) and (B) have almost fully extended conformations, but differ significantly in the rotation about the ester bond and in the C(17) chains. The crystal structure consists of stacked bilayers 50·7 Å thick. The C(17) chain ends, which lie at the bilayer surfaces, are virtually liquid. Within a bilayer cholesterols pack with cholesterols, and myristate chains pack with myristate chains. The chain packing has an orthorhombic (O_a) subcell. The inner diffraction ring at sin θ/λ 0.015 Å^–1, 0.016 Å^–1 which has been observed for the smectic A and cholesteric mesophases respectively, may be a result of multiple almost parallel 33 Å vectors which exist between atoms of cholesterols on opposite sides of a bilayer. Thus, we infer that the mesophases retain some characteristics of the crystal structure.