Studies of the Structure and Organization of Cationic Lipid Bilayer Membranes: Calorimetric, Spectroscopic, and X-Ray Diffraction Studies of Linear Saturated P-O-Ethyl Phosphatidylcholines

Abstract

Differential scanning calorimetry, x-ray diffraction, and infrared and (31)P-nuclear magnetic resonance ((31)P-NMR) spectroscopy were used to examine the thermotropic phase behavior and organization of cationic model membranes composed of the P-O-ethyl esters of a homologous series of n-saturated 1,2-diacyl phosphatidylcholines (Et-PCs). Differential scanning calorimetry studies indicate that on heating, these lipids exhibit single highly energetic and cooperative endothermic transitions whose temperatures and enthalpies are higher than those of the corresponding phosphatidylcholines (PCs). Upon cooling, these Et-PCs exhibit two exothermic transitions at temperatures slightly below the single endotherm observed upon heating. These cooling exotherms have both been assigned to transitions between the liquid-crystalline and gel phases of these lipids by x-ray diffraction. The x-ray diffraction data also show that unlike the parent PCs, the chain-melting phase transition of these Et-PCs involves a direct transformation of a chain-interdigitated gel phase to the lamellar liquid-crystalline phase for the homologous series of n > or = 14. Our (31)P-NMR spectroscopic studies indicate that the rates of phosphate headgroup reorientation in both gel and liquid-crystalline phases of these lipids are comparable to those of the corresponding PC bilayers. However, the shape of the (31)P-NMR spectra observed in the interdigitated gel phase indicates that phosphate headgroup reorientation is subject to constraints that are not encountered in the non-interdigitated gel phases of parent PCs. The infrared spectroscopic data indicate that the Et-PCs adopt a very compact form of hydrocarbon chain packing in the interdigitated gel phase and that the polar/apolar interfacial regions of these bilayers are less hydrated than those of corresponding PC bilayers in both the gel and liquid-crystalline phases. Our results indicate that esterification of PC phosphate headgroups results in many alterations of bilayer physical properties aside from the endowment of a positively charged surface. This fact should be considered in assessing the interactions of these compounds with naturally occurring lipids and with other biological materials.

Structural insights on biologically relevant cationic membranes by ESR spectroscopy

“…Interdigitation has been described for cationic lipids presenting large headgroups and short acyl chains or high surface charge density (Pohle et al 2000;Lewis et al 2001;Winter et al 2001;Ryhanen et al 2005). In those cases, the electrostatic repulsion between headgroups played a major role, since addition of neutral lipids or salt affected interdigitation (Ryhanen et al 2005).…”

Differential Scanning Calorimetry Studies of Phospholipid Membranes: The Interdigitated Gel Phase

“…Another modification is the esterification of the phosphate head group, which increases the steric bulk and changes the molecule from zwitterionic to positively charged, allowing interdigitation [95,96]. For example, the P-O-ethyl ester analogue of DPPC, 1,2-dipalmitoyl-sn-glycero-3-ethylphosphocholine (EDPPC or Et-DPPC), is fully interdigitated in the gel phase and has a main transition at 42.5 °C with an enthalpy of 9.6 kcal/mol [95][96][97].…”

“…For example, the P-O-ethyl ester analogue of DPPC, 1,2-dipalmitoyl-sn-glycero-3-ethylphosphocholine (EDPPC or Et-DPPC), is fully interdigitated in the gel phase and has a main transition at 42.5 °C with an enthalpy of 9.6 kcal/mol [95][96][97]. These values are slightly higher than those for DPPC, which has a Tm around 41.3 °C and a corresponding enthalpy of 8.2 kcal/mol ( [7] and references therein).…”

Effect of Gramicidin S on the Dipalmitoylphosphatidyl-glycerol Thermotropic Phase Transition in DPPG/GS Systems: A Mathematical Approach

“…It is well known that this system could show bistable behavior and that in the bistability zone chaotic behavior appears [28,29]. Good results for the behavior of the Duffing system (2) are obtained using the slowly varying amplitude (SVA) approximation.…”