Disentangling the events that constitute the main phase transition of predominantly neutral lipid bilayers containing anionic lipids by measuring their macroscopic properties.

The thinning of lipid bilayers during their main phase transition, as the outcome of the weakening of van der Waals interactions between their hydrocarbon chains and the hydration of the polar headgroup region, displays a peak at the specific temperature (Tm). The most common technique used for Tm determination is differential scanning calorimetry (DSC), which provides the required information straightforwardly and rapidly, but it cannot provide any details on the molecular-level events that underlie the main phase transition. To uncouple the impact of hydrogen bonding and (de)protonation in the polar headgroup region on the measured Tm, in this work, we determined the latter of anionic lipid-containing predominantly neutral bilayers (10:90% molar ratio) in Britton-Robinson buffers with pH values 4, 7, and 9 using macroscopic techniques. Specifically, the temperature-dependent UV-Vis spectra combined with the measurement of refractive indices allowed the modeling of the bilayer thickness (d) change during the main phase transition. Significantly, the alteration in the hydrogen bonding pattern affected the change in d, unlike the deprotonation of anionic lipids. A molecular polarizability displayed an abrupt decrease upon the main phase transition, but its magnitude and associated uncertainty level were found to be acceptable only for a lipid mixture in which anionic lipid does not exchange protons with aqueous milieu, i.e., a pH-dependent (de)protonation obscures more precise determination of Tm. Besides the determination of Tm values, this new methodology enabled us to rank the contributions to Tm value: van der Waals interactions > hydrogen bonding > (de)protonation.