Cyclopentanoid analogs of dipalmitoyl phosphatidic acid: effect of backbone geometry on thermotropic properties

Seven geometrical or positional isomers of dipalmitoyl cyclopentanophosphoric acid (DPCPA) have been synthesized and studied: 1,3/2-1P (I); 1,2/3-1P (II); 1,2/3-3P (III), 1,2,3/0-P (IV); 1,2,3/0-2P (V); 1,3/2-2P (VI); 1,2/3-2P (VII). When dispersed in 0.1 M Tris-HCl at pH 7.4, I-VII gave thermal transitions (T_c) of 60.0°, 59.0°, 56.8°, 55.3°, 38.3°, 36.8° and 34.0°C, respectively, as measured by differential scanning calorimetry (DSC). When the lipids were dispersed at pH 9.5 in 0.1 M borate, T_c of I T_c of I-IV decreased, whereas T_c of V-VII increased. In contrast, at pH 1.5 in 0.1 M HCl/KCl, T_c of 1-IV decreased slightly, but T_c of V-VII rose markedly. To determine the effect of head group geometry and substitution pattern on acyl chain motion, EPR spectra of 1-palmitoyl1,2[16-doxylstearoyl]-glycero-3-phosphoric acid in bilayers of DPCPA isomers were acquired. Abrupt spectral changes occurred at temperatures closely correlating with transition temperatures observed by DSC. These results have led to the conclusions that: (i) isomers I-IV containing vicinal acyl chains form bilayers that exhibit structural transitions at temperatures higher than those at which transition are exhibited by isomers V-VII which have a polar phosphate group interposed between the two chains; (ii) the effects of differences in backbon structure are transmitted down the entire length of the acyl chains; (iii) the orientation of the cyclopentane ring in the isomers I-IV is significantly different from that in isomers V-VII at pH values where the phosphate group is doubly negatively charged.