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.

Reconstitution of the zone-block model of biomembranes. 3. Synthesis and investigation of the structure of phosphatidylcholine analog isobutyl-2-(trimethylammonia)ethyl phosphate in its monohydrate crystals and monohydrate complexes with isobutanol.

An isobutyl analog of phosphatidylcholine--isobutyl-2-(trimethylammonia)ethyl-phosphate (IPC)--was synthesized to accomplish the physical reconstitution of the zone-block model of biomembranes. X-ray analysis was used to study the IPC monohydrate (IPC.H2O) crystals grown from water or dimethylformamide with 2% water and the IPC.H2O complex with isobutanol (IPC.H2O.IB) formed by IPC crystallization from isobutanol with 0.5% water. The two compounds were shown to belong to the monoclinic syngony (Sp. gr. P2(l)/a): IPC.H2O-a = 11.075(4), b = 9.679(3), c = 13.921(5) A; beta = 110.61(4) degrees, Z = 4; IPC.H2O.IB-a = 11.370(6), b = 9.397(5), c = 18.12(1) A, beta = 99.42(5) degrees, Z = 4. In IPC.H2O crystals, IPC molecules formed chains due to the system of hydrogen bonds between water molecules and IPC phosphate groups. In turn, ionic bonds between trimethylammonium and phosphate groups of the bipolar heads made these chains to form layers. Another system of ionic bonds (also between -N+(CH3)3 and -O-P = O groups) arranged the layers in a bilayer structure. The bonds were formed between the pairs of molecules oppositely directed and located in neighbouring monolayers. The structure of the bilayer in IPC.H2O.IB crystals was similar. The only difference was the presence of isobutanol molecules between the IPC isobutyl radicals. C-OH groups of isobutanol formed hydrogen bonds with water molecules. However, the interlayer gaps in these two cases differed significantly. In IPC.H2O, the bilayers partially overlapped, whereas in IPC.H2O.IB, they were located at a considerable distance. The structures of the investigated compounds confirm in principle the possibility of the formation of ionic bonds between trimethylammonium and phosphate groups within the bilayer as well as in the interzonal region with apolar radicals that was predicted by the zone-block model of biomembrane. However, the phosphocholine heads were parallel but not perpendicular (as presumed in the model) to the bilayer plane.