Conformational changes of cholesteryl palmitoleate in the crystal structure at low temperature

At 123 K, crystals of cholesteryl cis-9-hexadecenoate (cholesteryl palmitoleate, C45H74O2) are monoclinic, space group P2(1) with cell dimensions a = 12.917(7), b = 8.910(5), c = 34.04(1) A, beta = 94.95(7) degrees [lambda(CuK alpha) = 1.5424 A] having two independent molecules (A and B) per unit cell. The crystal structure has been determined from 6178 reflections with sin theta/lambda less than or equal to 0.56 A-1, of which 3406 gave [F] greater than 3 sigma. Structure refinement by alternating cycles of Fourier syntheses and block diagonal least squares gave R = 0.24 for all reflections, R = 0.13 for reflections [F] greater than 3 sigma. At 123 K, the crystal structure consists of closely packed layers very similar to those at 295 K. However, there are major conformational differences in the layer interface region, which affect the ester chain of molecule B and the C(17) tail of molecule A. Although the electron density is diffuse in this region, the B-chain, which is bent, appears to be ordered at 123 K and has a different conformation from the disordered B-chains at 295 K. The change in the A-tail, which is twisted at 123 K and extended at 295 K, is very similar to that which occurs in two of the molecules when anhydrous cholesterol undergoes phase transition. Measurements of the unit cell dimensions at twelve temperatures (295 K to 123 K) indicate that the major changes in the crystal structure of cholesteryl palmitoleate occur in a 10 K range near 173 K.