Thermotropic phase transition of dehydrated glycerophospholipids monitored by drop coating deposition Raman spectroscopy (DCDRS)
Abstract
Glycerophospholipids as one of the main components in the cell membrane are extensively studied to understand their properties and behavior. To explore the properties of lipids in biological membranes, liposomes composed of one type of lipids are commonly used. Liposomes undergo physical changes over temperature range denoted as the thermotropic phase transitions. The most studied transition is from a gel phase to a liquid crystalline phase known as a main transition attributed to the "chain-melting" of the hydrocarbon tails. It is characterized by a temperature T(m) specific for each lipid. Many techniques are available to study phase transitions including differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR), fluorescence spectroscopy, or vibrational spectroscopy. Raman spectroscopy is a rapid and non-invasive analytical tool providing unique information about the vibrational motion of molecules. The method requires minimal sample preparation and is sensitive to structural changes present during the phase transition of lipids. Raman spectra obtained from the gel and liquid crystalline state of lipids can reveal relevant changes related to the structural alternations during the transition.
Here we focused on the thermotropic phase transition of lipids in a dry state studied by drop coating deposition Raman spectroscopy (DCDRS). The method is based on the evaporation of a solvent from a small droplet of a studied solution or suspension deposited on a hydrophobic substrate. The evaporation of a droplet leads to the preconcentration of the analyte into a dried pattern ("coffee-ring" for lipids) from which the Raman spectra are acquired. DCDRS technique enables to obtain Raman spectra from small sample volumes at very low initial concentrations in comparison with the conventional Raman measurements. Raman spectra from dried deposits of different glycerophospholipids (differing in acyl chain length and type of the head group) were acquired in the temperature range covering the phase transition. Changes in spectral bands (intensity ratio, wavenumber shift) related to the phase transition were observed mainly in spectral regions for C-H stretching vibration (2800-3100 cm(-1)) or C-C stretching vibration (1050-1150 cm(-1)) of alkyl chains.