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Temperature-controlled Raman microscopy to study the phase behavior of synthetic PEGylated lipids and nanovesicles

The study of phase transitions in lipids is important to understand various phenomena such as conformational order, trans-membrane diffusion, vesicle formation and fusion as well as drug-and protein-membrane interactions. Several techniques, including Raman spectroscopy, have previously been employed to investigate the phase behaviour of lipids. In this work, temperature-controlled Raman microscopy has been used to detect and analyze the phase transitions in two newly developed synthetic PEGylated lipids trademarked as QuSomesTM and its nanovesicles in phosphate buffered saline (PBS) suspension. The amphiphiles considered in this study differ in their hydrophobic chain length and contain different units of polyethylene glycol (PEG) hydrophilic head groups. Raman spectra of these new artificial lipids and its nanovesicles have been recorded in the spectral range of 500-3100 cm-1 by using a temperature-controlled sample holder attached to a Raman microscope. The gel to liquid-crystalline phase transitions of the sample lipids, composed of pure 1,2-dimyristoyl-rac-glycerol-3-dodecaethylene glycol (GDM-12) and 1,2-distearoyl-rac-glycerol-3-triicosaethylene glycol (GDS-23), have been detected by examining the changes in Raman spectra of the lipids caused by temperature variation. In the liquid phase both of the studied lipids spontaneously form liposomes (nanovesicles) upon hydration. In this study, we have demonstrated the efficacy of the temperature-controlled Raman microscope system to reveal the main phase transition temperature (Tm) profiles of our sample lipids and its nanovesicles in PBS suspension. The phase changes are detected by plotting peak intensity ratios in the C-H stretching region (~I2935/I2883) versus temperature. These ratios correlate with lateral or inter-chain interactions as well as intra-molecular interactions. In particular, we have found that phase transitions occur at a temperature of approximately 5.2°C and 21.2°C for pure GDM-12 and GDS-23, respectively. However, the phase transition temperature becomes significantly higher for lipid nanovesicles formed in aqueous suspensions. Such information about these PEG coated lipids might find applications in various studies including the development of lipid based novel substances and drug delivery systems.