HYDRATION-INDUCED GEL STATES OF THE DIENIC LIPID ,4-OCTADECADIENOYL)-SN-GLYCERO-3-PHOSPHORYLCHOLINE AND THEIR CHARACTERIZATION USING INFRARED-SPECTROSCOPY

The lyotropic phase behavior of the dienic lipid ,4-octadecadienoyl)-s n-glycero-3-phosphorylcholine (DODPC) has been investigated by means o f IR spectroscopy at 25 degrees C. Gradual hydration has been realized exposing the lipid to an atmosphere of variable relative humidity (RH ). Upon scans of decreasing RH, the liquid crystalline lipid undergoes the chain-freezing transition to a metastable gel state. By storage o f the sample at low RH, the gel transforms to a crystalline subgel des ignated as SG(I). Subsequent hydration induces the conversion to a sec ond subgel (SG(II)). The subgel phases are characterized by the dense packing of the acyl chains as indicated by the correlation field split ting of the CH2 rocking and bending modes. Band shifts of phosphate gr oup vibrations as well as the splitting of the carbonyl stretching mod e are correlated with the hydration of the polar region of the bilayer given in terms of the molar ratio of water to lipid. The nu(1,3)(OH) absorption band of water yields qualitative information about the wate r-lipid interaction. The drastic sharpening of this band in the SG(I) phase was attributed to the reduction of water binding sites on the li pid, leading to a more uniform population of water molecules adsorbed onto the lipid headgroup. The external conditions of phase transformat ion of DODPC were compared with corresponding data of dimyristoylphosp hatidylcholine (DMPC) having the same number of subsequent methylene s egments in the acyl chains. Apparent differences can be attributed to the influence of the diene groups representing a rigid spacer inserted between the methylene chains and the ester groups of the lipid, i.e., in a position near the polar/apolar interface of the bilayer.