Biological thermodynamic data for the calibration of differential scanning calorimeters: dynamic temperature data on the gel to liquid crystal phase transition of dialkylphosphatidylcholine in water suspensions

Abstract

Heat capacity measurements of the gel to liquid crystalline transition of phosphocholine lipid suspensions in buffer solutions were performed using a differential scanning calorimeter (DSC) over a range of experimental conditions. Since the temperatures at the transition peak maxima, i.e. transition temperatures, were observed to increase linearly with scan rate due to the response time of the DSC, they were corrected to zero scan rate and found to agree closely with the literature values. The transition temperatures and enthalpies of the seven dialkylphosphatidylcholine lipids investigated ranged from 286.6 ± 0.1 K and 15.9 ±0.6 kJ mol −1 for 1,2-ditridecanoyl- sn-glycero-3-phosphocholine to 353.7 ± 0.1 K and 69.1 ±3.3 kJ mol −1 for l,2-ditetracosanoyl- sn-glycero-3-phosphocholine. At the slowest scan rates of 3–5 K h −1, the transition widths at half the peak maximum were close to 0.2 K. The transition temperatures were found to be independent of the following: the ionic strength of the sodium phosphate buffer from 0.01–0.02 M, the pH of the buffer from 6 to 8, the concentration of the suspension from 0.1–1.0 mass%, the commercial source of the lipid, and storage of the lipid in a freezer for 4 years. Suspensions of the dialkylphosphatidylcholine lipids in sodium phosphate buffer solutions can be used as sources of dynamic temperature data for the calibration of DSCs.