Crystallization of the isobutylphosphocholine-cholesterol-isobutanol (1:3:3) complex and its investigation by X-ray analysis: interaction of phopholipid headgroups with cholesterol

A crystal complex consisting of the isobutyl analog of phosphatidylcholine (PC) (isobutylphosphocholine), cholesterol, and isobutanol with molecular ratio 1:3:3 was obtained and investigated by means of X-ray analysis. The complex was shown to correspond to the monoclinic system (sp. gr. P2(1)): a=16.994(10), b=11.314(7), c=28.164(15), beta=104.07(3), V=5252.63 A(3), Z=2, D(calc)=1.0273 g/cm(3). The isobutylphosphocholine molecule is the key component of the complex. Pairs of hydrogen bonds are formed between the (-delta)O-P-O(delta-) group of the isobutylphosphocholine molecule and C-OH groups of two cholesterol and two isobutanol molecules. The third molecules of cholesterol and isobutanol are H-bonded with the (-delta)O-P-O(delta-) group of the isobutylphosphocholine molecule via C-OH groups of isobutanol and cholesterol, respectively. The crystal structure is built up by translation of the complex in multiplicate along the two-fold axis in the direction of axis b. It contains bands formed by isobutylphosphocholine molecules alternately changing their direction. They are fixed by virtue of two zones of electrostatic interactions of the type (-delta)O-P-O(delta-)ellipsis(+)N(CH(3))(3) and are more or less parallel to the bc plane. The structure also contains three-layer domains formed by cholesterol molecules perpendicular to isobutylphosphocholine bands. In the direction of the c-axis isobutylphosphocholine bands alternate with the layers of cholesterol molecules herewith reproducing repeated blocks. The obtained structure is compared with that of crystals of phospholipids and cholesterol and its derivatives.