RESUMEN
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.