Evaluation of the Function of Synthetic Bimolecular Membranes and Phospholipids by Spin-Probe Method and Differential Scanning Calorimetry

The amount of heat absorption of synthetic bimolecular membranes and phospholipids, which contained various polar groups and alkyl chain of different length at gel-(liquid crystal) phase transition temperature, the movements of alkyl chain part of the membranes and the differences of incorporation of probe into the membranes were studied by differential scanning calorimetry and the spin-probe method. The variation of physical properties pf the bimolecular membranes at incorporation with cholesterol into the membranes was also investigated. According to the measurements of differential scanning calorimetry, the synthetic bimolecular membranes containing ammonium salt, phosphate and sulfonate as the polar groups had larger amounts of heat absorption and transition entropy than phosphoglycerides bimolecular membrane of phosphatidylcholine type. The incorporation of 2, 2, 6, 6-tetramethyl-l-piperidinyloxyl used as a spin-probe into the phospholipids membranes was 8.7 times higher than those of the synthetic bimolecular membranes. The activation energy, obtained from the measurement of relaxation time by molecular movements using spin-probe of aliphatic acid type, was remarkably influenced by the length of alkyl chains and existence of the unsaturated bond. By the incorporation of cholesterol into the bimolecular membranes, phase transition was eliminated and the incorporation of the probe into the bimolecular membranes was disturbed. These phenomena were interpreted on the basis of a variation of fluidity of the bimolecular membranes.