One of the most distinguished quantities that describes lipid main phase transition, i.e. the transition from the gel (Lβ(')) to the fluid (Lα) phase, is its melting temperature (Tm). Because melting is accompanied by a large change in enthalpy the, Lβ(') → Lα transition can be monitored by various calorimetric, structural and spectroscopic techniques and Tm should be the same regardless of the metric monitored or the technique employed. However, in the case of DPPE multilamellar aggregates there is a small but systematic deviation of Tm values determined by DSC and FTIR spectroscopy. The aim of this paper is to explain this discrepancy by combined UV/Vis spectroscopic and MD computational approach. Multivariate analysis performed on temperature-dependent UV/Vis spectra of DPPE suspensions demonstrated that at 55 ± 1 °C certain phenomenon causes a small but detectable change in suspension turbidity, whereas a dominant change in the latter is registered at 63.2 ± 0.4 °C that coincides with Tm value determined from DSC curve. If this effect should be ignored, the overall data give Tm value the same as FTIR spectra data (61.0 ± 0.4 °C). As the classical MD simulations suggest that about 10° below Tm certain undulations appear at the surface of DPPE bilayers, we concluded that certain discontinuities in curvature fluctuations arise at reported temperature which are to some extent coupled with lipid melting. Ultimately, such events and the associated changes in curvature affect Tm value measured by different techniques.
Keywords: 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC); 1,2-Dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE); MD simulations; Melting temperature (T(m)); Surface undulations; UV/Vis spectroscopy.
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