Alternating current calorimetry of dimyristoylphosphatidylcholine multilayers: hysteresis and annealing near the gel to liquid-crystal transition

Abstract

The gel to liquid-crystal phase transition in dimyristoylphosphatidylcholine liposomes was studied with 0.4-Hz ac calorimetry. The ac heat capacity on heating scans exhibited a peak in the vicinity of 23.9 degrees C with a full width at half-maximum of 0.15-0.20 degrees C. The enthalpy change was 4.8 kcal/mol, in good agreement with conventional differential scanning calorimetry. On cooling scans, the peak shifted to lower temperature by 0.1-0.5 degrees C, the width increased to 0.25-0.40 degrees C, and the apparent enthalpy change was only 40% of that observed on heating. Both the heating and cooling heat capacities were stable for at least 20 min in quasi-isothermal conditions. Following a 1 h anneal at 10 degrees C, the heating scans were quite reproducible. The results have been interpreted in terms of the nucleation and subsequent annealing of small ordered domains in the bilayer on freezing the acyl chains. No peak associated with the pretransition was observed, as expected since the relaxation time for the degrees of freedom the produce the pretransition is much longer than the period of the 0.4-Hz temperature wave.

Characterisation of a new ceramide EOS species: synthesis and investigation of the thermotropic phase behaviour and influence on the bilayer architecture of stratum corneum lipid model membranes

“…A hysteresis of about 10 C found upon cooling the sample corroborates this assumption of slow reformation of the gel structure during the cooling process, which is in line with other reports in the literature. 45,46 The native CER[EOS] species with the linoleic acid in u-position shows one pretransition at 77 C during the second heating scan, which is only visible as a small shoulder during the third sample heating. The temperature of the main phase transition shifts to slightly higher temperatures, from 83 C to 85 C. Either partial conformational change from cis to trans or degradation due to thermal influences presumably accounts for this observation.…”

Physical properties of the fluid lipid-bilayer component of cell membranes: a perspective

“…Since Fig. 25 is a plot of purely thermal fluctuations, it should really be compared with the ac specific heat (Black & Dixon, 1981;Hatta et al 1984;Van Osdol et al 1989) which is only sensitive to equilibrium energy fluctuations. Secondly, the specific heat function has significant wings away from the transition signalling pronounced precursor effects to the transition: the transition is pseudo-critical (Mitaku et al 1983 ;Hatta et al 1984;Mouritsen & Zuckermann, 1985), with remnants of critical behaviour at the phase transition.…”

Frequency spectrum of enthalpy fluctuations associated with macromolecular transitions.

“…[4] RT '2 RT22 According to the fluctuation-dissipation theorem, the same equation describes the response of the system to a small external perturbation. In the case of a small periodic temperature perturbation, it is possible to observe the relaxation of the heat capacity and define a frequency-dependent "heat capacity."…”