Commensurate molecules in isostructural crystals of cholesteryl cis- and trans-9-hexadecenoate.

Abstract

At 295 K, crystals of form I of cholesteryl cis-9-hexadecenoate (palmitoleate) and cholesteryl trans-9-hexadecenoate (palmitelaidate) are difficult to distinguish by X-ray diffraction. Both form monoclinic thin plates, space group P21 with two molecules (C43H74O2) A and B in the asymmetric unit. Unit cell dimensions for cholesteryl palmitelaidate (I) are a = 12.827(4), b = 9.075(4), c = 35.67(1) A, beta = 93.42(3) degrees, very similar to those of the palmitoleate crystals. Other crystals (form II) of the palmitelaidate ester are described. The crystal structure of form I of cholesteryl palmitelaidate has been determined from 3657 reflections (sin theta/lambda less than 0.46 A-1) measured at 295 K using CuK alpha X-radiation and refined to give Rw(F) = 0.095. The molecular packing arrangement is isostructural to that of the previously determined crystal structure of cholesteryl palmitoleate. In both crystals, the fatty acid chains of the A molecules are kinked at the double bond but are nearly straight. The chains of B molecules have more complicated dislocations and are bent. It is remarkable that, neglecting their detailed conformations, corresponding fatty acid chains in the two crystal structures have similar overall shapes, although palmitoleate chains have cis-ethylenic groups and palmitelaidate chains have trans groups.