The effective viscosity was calculated using the Kelvin-Voigt-based model. The curves of concentrations in between two pure compounds reveal combined effects of the two respective individual gels to liquid-disordered phase transitions, whose contribution scales with the concentration of the respective compound. To some extent, the tendency to engage in these larger shape transitions depends on the local curvature of the vesicle, which depends on the local phospholipid composition and fluidity of the membrane. [ "article:topic", "authorname:jjakubowskih", "showtoc:no", "license:ccbyncsa" ], College of St. Benedict/St. Adiabatic scanning calorimetry is a high-resolution calorimetric technique that enables studies of phase transitions and critical phenomena of condensed and soft matter systems. In order to minimize arbitrary choices, we followed a systematic procedure based on an equivalent approach to the one used in previous studies known as the tangent method [33]. In this work, we have shown that Peltier-element-based adiabatic scanning calorimetry and quartz-crystal microbalance with dissipation monitoring can be used to study the phase transitions of lipid vesicles. Apart from the coexistence region, pASC enabled the study of low-temperature transitions, mapping the phase boundary between the gel-type phases for both types of lipid mixtures. This work can be considered as a departure point for further investigations on more complex lipid mixtures displaying relevant phases such as the liquid-ordered phase and solid-lipid interfaces with biologically functional importance. Hydrophobic mismatch and long-range protein/lipid interactions in bacteriorhodopsin/phosphatidylcholine vesicles. The gel to ripple phase transition, on the other hand, becomes virtually invisible, a consequence of the much smaller amount of heat involved and its more diffuse character. For further details on the structure and functioning of the pASC setup we refer to [34, 35]. The cholesterol helps prevent ordered packing of the acyl chains of the PL's, which increases their freedom of motion. The approach for determining phase transition temperatures from the analysis of both quantities is equivalent, corroborating the compatibility of both methods. Please enable it to take advantage of the complete set of features! The total heat and the degree of cooperativity CU were also determined from pASC measurements. Please enable it to take advantage of the complete set of features! In our case, it was especially useful to monitor the formation of an intact vesicle layer, since it senses the water coupled to the adsorbed layer. For DMPC + DSPC, mixtures with a mole fraction step of 0.1 covering the whole composition range were prepared, while, for DMPC + DPPC, a step of 0.2 was chosen. All lipids have a characteristic temperature at which they undergo a transition (melt) from the gel to liquid phase. Membrane permeability and fusogenicity simultaneously increase. During the transition the temperature rate is substantially reduced, the basic advantage of pASC. At the melting or main phase transition, DMPC + DPPC mixtures exhibit a clear phase coexistence region, and the single peak characteristic of the main phase transition is shifted towards higher temperatures and broadens upon addition of DPPC. As regards substrate influence, it strongly affects supported lipid bilayers, where the melting of the two leaflets is decoupled due to the strong interactions of the lower leaflet with the substrate. In contrast, no thermal bleaching was observed below 60 °C for any sample kept in the dark. Large scale conformational transitions can occur in lipid bilayers. is the total transition heat, the enthalpy difference over the entire jump, and it includes the contribution from the latent heat on the one hand and from contributions of pretransitional fluctuations on the other hand. The sample is driven out of equilibrium, and the curves are deformed according to the specifics of the instrument and the experiment. John's University), TM di16:0 PC and di16:0 PA - Energy Diagram. On the other hand, the viscosity as measured by QCM-D appears to be an insufficiently sensitive probe for these transitions. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. The concept of an ASC experiment is summarized in Figure 1 for the case of a fairly weak transition with considerable pretransitional changes in the heat capacity. Proteins. These transition temperatures were close to the main transition temperature of reconstituted vesicles measured by calorimetry, indicating that the lipid phase transition brought about protein disaggregation. Polymorphism of the bilayer membranes in the ordered phase and the molecular origin of the lipid pretransition and rippled lamellae. Both files from Scott Feller, Wabash College. In these techniques, the sample is forcibly heated at a fixed rate. Biochim Biophys Acta. J Biochem. That one does not observe two separate peaks in or separate steps in the specific enthalpy curves at the respective transition temperatures of the pure compounds is a clear indication that one does not have a system with two types of single-component vesicles, but rather with vesicles with both types of lipids present in the membranes (roughly in proportions to respective the lipid mole fractions). When the AC voltage is turned off, the oscillation amplitude decays exponentially, this decay is recorded, and the frequency () and the energy dissipation factor () of different overtones are extracted [36].