Physical and Chemical Stability of Binary Lipid Mixtures
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In biological membranes, it is thought that lipid heterogeneity and lipid domains play a role in membrane functions, such as protein regulation. Also, chemical stability is a concern with lipid structures such as liposome suspensions used for drug delivery. To test both phase segregation in a partially miscible lipid mixture and its stability, two phospholipids of different acyl chain lengths, DPPC and DAPC are combined to create a model system. Since the chains differ by only four carbon lengths, the driving force for phase separation is relatively small. As a result, the system is expected to approach equilibrium only after a substantial amount of time. Differential scanning calorimetry (DSC) is used to observe how these domains form and to determine whether a stable equilibrium state emerges. In this mixed system, significant changes in the main transition temperature (Tm) suggest that the composition of the bilayer rearranges with time. Furthermore, changes in DSC peak shape and hysteresis between the heating and cooling scans provide evidence that the reversibility of the Tm changes over time. In addition, 31P nuclear magnetic resonance (NMR) spectroscopy results suggest that the bilayer transforms to a different macroscopic shape, such as vesicles or micelles. This is reinforced by the change in color from a milky-white solution to clear over the course of the experiment. 1H NMR and DSC experiments on unmixed pure lipids suggest that the main factor driving the changes in the mixed system is pH and temperature dependent hydrolysis, not phase separation.