Crystallography of linear molecule binary solids. X-ray structure of a cholesteryl myristate/cholesteryl pentadecanoate solid solution

As the first example of a cholesteryl ester solid solution packing in the bilayer polymorph, the X-ray crystal structure of a nearly 1:1 cholesteryl myristate/cholesteryl pentadecanoate pair, with components differing by one methylene unit, was determined at room temperature. The non-centrosymmetric unit cell is monoclinic, space group C2, with a = 102.6 Å, b = 7.59 Å, c = 10.20 Å, β = 92.1°, and Z = 8 (two molecules in the asymmetric unit). The final weighted R-factor for 5450 measured intensities was 0.129. Although there may well be some partial occupancy of the terminal methyl group in the longer component, the layer packing attempts to isolate the two components to preferred sites in the asymmetric unit. (Similar behavior was observed previously for the crystal structure of two esters in the monolayer I polymorphic form, but only when the chains differed by two methylene units.) Compared to the pure myristate component, the solid solution packs with oxygen to oxygen contacts in the ester linkage that are somewhat shifted and closer to accommodate the additional methylene unit in the bimolecular layer.