Thermotropic phase behavior of model membranes composed of phosphatidylcholines containing dl-methyl anteisobranched fatty acids. 1. Differential scanning calorimetric and 31P NMR spectroscopic studies.
The thermotropic phase behavior of aqueous dispersions of nine dl-methyl branched anteisoacylphosphatidylcholines was studied by differential scanning calorimetry and 31P nuclear magnetic resonance spectroscopy. The calorimetric studies demonstrate that these compounds all exhibit a complex phase behavior, consisting of at least two minor, low-enthalpy, gel-state transitions which occur at temperatures just prior to the onset of the gel/liquid-crystalline phase transition. In addition, at still lower temperatures, anteisobranched phosphatidylcholines containing fatty acyl chains with an odd number of carbon atoms show a major, higher enthalpy, gel-state transition, which was assigned to a conversion from a condensed to a more loosely packed gel phase. No such transition was observed for the even-numbered compounds in aqueous dispersion, but when dispersed in aqueous ethylene glycol, a major gel-state transition is clearly discernible for two of the even-numbered phospholipids. The major gel-state transition exhibits heating and cooling hysteresis and is fairly sensitive to the composition of the bulk aqueous phase. 31P NMR spectroscopic studies indicate that the major gel-state transition is accompanied by a considerable change in the mobility of the phosphate head group and that, at temperatures just prior to the onset of the gel/liquid-crystalline phase transition, the mobility of the phosphate head group is comparable to that normally exhibited by the liquid-crystalline state of most other phospholipids. The temperatures at which the gel/liquid-crystalline phase transition occurs and the enthalpy change associated with this process are considerably lower than those of the saturated n-acyl-PC's of comparable acyl chain length.(ABSTRACT TRUNCATED AT 250 WORDS)
D002151:Calorimetry; D002152:Calorimetry, Differential Scanning; D005227:Fatty Acids / Q000032:analysis; D008051:Lipid Bilayers / Q000138:chemical synthesis / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy; D008954:Models, Biological; D008968:Molecular Conformation; D010713:Phosphatidylcholines / Q000032:analysis / Q000138:chemical synthesis / Q000494:pharmacology*; D010761:Phosphorus Radioisotopes; D013696:Temperature; D013816:Thermodynamics
D005227:Fatty Acids; D008051:Lipid Bilayers; D010713:Phosphatidylcholines; D010761:Phosphorus Radioisotopes