BILAYER PACKING CHARACTERISTICS OF MIXED CHAIN PHOSPHOLIPID DERIVATIVES - RAMAN-SPECTROSCOPIC AND DIFFERENTIAL SCANNING CALORIMETRIC STUDIES OF 1-STEAROYL-2-CAPRYL-SN-GLYCERO-3-PHOSPHOCHOLINE (C(18)C(10)PC) AND 1-STEAROYL-2-CAPRYL-SN-GLYCERO-3-PHOSPHO-N-TRIMETHYLPROPANOLAMINE (C(18)C(10)TMPC)

Raman spectroscopy and high-sensitivity differential scanning calorimetry (DSC) were used to compare the effects of headgroup conformation on the acyl chain packing arrangements in two highly asymmetric phosphatidylcholine (PC) analogues, 1-stearoyl-2-capryl-sn-glycero-3-phosphocholine (C(18):C(10)PC) and a polar headgroup derivative of C(18):C(10)PC, 1-stearoyl-2-capryl-sn-glycero-3-phospho-N-trimethylpropanolamine (C(18):C(10)TMPC), which contains an additional methylene group within the choline moiety; namely, -P-O-(CH2)(3)-N(CH3)(3). The C(18):C(10)TMPC headgroup exhibits an extended trans conformation which is independent of bilayer phase. A comparison of gel phase spectral order parameters of the two lipid species indicates a mixed interdigitated state characteristic of three chains per headgroup for C(18):C(10)TMPC. A more intermolecularly ordered liquid crystalline phase is observed, however, for the C(18):C(10)TMPC bilayers. The phase transition cooperative unit size estimated for the C(18):C(10)PC bilayers (similar to 140 molecules per unit) is about 7-fold greater than that for the C(18):C(10)TMPC dispersions (similar to 20 molecules per unit). We suggest that the extended headgroup for C(18):C(10)TMPC induces a slight tilt in the gel phase packing arrangements for the acyl chains, which may persist in the partially interdigitated liquid crystalline phase bilayer. Macroscopically, tighter packed multilamellar dispersions of C(18):C(1O)TMPC occur for systems prepared first in the presence of a higher ionic strength medium. The stacked bilayers may then be transferred to a lower ionic strength environment without loss of their more closely packed adjacent lamellae.