Thermal Response Analysis of Phospholipid Bilayers Using Ellipsometric Techniques

Abstract

Biomimetic planar artificial membranes have been widely studied due to their multiple applications in several research fields. Their humectation and thermal response are crucial for reaching stability; these characteristics are related to the molecular organization inside the bilayer, which is affected by the aliphatic chain length, saturations, and molecule polarity, among others. Bilayer stability becomes a fundamental factor when technological devices are developed—like biosensors—based on those systems. Thermal studies were performed for different types of phosphatidylcholine (PC) molecules: two pure PC bilayers and four binary PC mixtures. These analyses were carried out through the detection of slight changes in their optical and structural parameters via Ellipsometry and Surface Plasmon Resonance (SPR) techniques. Phospholipid bilayers were prepared by Langmuir-Blodgett technique and deposited over a hydrophilic silicon wafer. Their molecular inclination degree, mobility, and stability of the different phases were detected and analyzed through bilayer thickness changes and their optical phase-amplitude response. Results show that certain binary lipid mixtures—with differences in its aliphatic chain length—present a co-existence of two thermal responses due to non-ideal mixing.

Quartz Crystal Microbalance With Dissipation Monitoring: A Versatile Tool to Monitor Phase Transitions in Biomimetic Membranes

“…DPPC SVLs display a large maximum in d( f /n)/dT as a signature of the main phase transition at T m ∼ 41.5 • C. This temperature agrees well with previous differential scanning calorimetric measurements of multilamellar DPPC vesicle dispersions (T m ∼ 41.6 • C) (Benesch et al, 2011;Redondo-Morata et al, 2012). This temperature is slightly lower than the observed for planar supported lipid bilayers using AFM and ellipsometry (Leonenko et al, 2004;Redondo-Morata et al, 2012;González-Henríquez et al, 2017), since the presence of the solid surfaces induces a stronger decoupling between the two lipid leaflets during melting, as compared to 3-D supported vesicle layers and multilamellar vesicle dispersions. In SVLs containing 32.5% mole fraction of cholesterol no transition is detectable, indicating the presence of a homogeneous liquid ordered phase, in agreement with previous calorimetric measurements of multilamellar DPPC + chol vesicles (Mannock et al, 2010).…”

Lyophilization of Liposomal Formulations: Still Necessary, Still Challenging

“…An alternative technique is the fluorescence spectroscopy that can permit the measurement of the dependence of the membrane fluidity on the variation of temperature also in presence of cholesterol [ 24 ]. Furthermore, small-angle X-ray scattering (SAXS) or electron spin resonance [ 25 ], along with ellipsometric techniques [ 26 ] and Raman spectroscopy [ 24 ], can provide useful information on the structural changes of the bilayers over a broad range of conditions.…”

Morphological Behavior of DPPC-DMPC Two-dimensional Mixed Monolayer on Water Surface by Dropping Method

“…On the temperature range 24 to 41 ºC, DPPC-DMPC mixture (vesicle) is transferred from the gel phase to the liquid-crystal phase via the two-phase coexistence region with an increase in x DMPC . It is well known that alkyl chains in vesicle are in a state close to crystal state (all-trans type) in the gel phase, a state close to liquid (gauche type) in the liquid-crystal phase, a mixed state including both gel and liquid-crystal phases exists in coexistence region [1], [4], [7]- [11]. The 3-dimensional macroscopic phase diagram is not always enough to explain 2-dimensional monolayer state.…”

“…In mixed DPPC-DMPC multilamellar phase diagram in water the preferential appearance of gel or liquid crystal phase depends on the mole fractions of each lipid and at specific mole fraction and temperature the two phases coexist [7]- [11]. Apart from multilamellar phase, the DPPC-DMPC twodimensional mixed monolayer on water surface can be investigated from the viewpoint of observing the changes in monolayer versus mole fractions of the two lipids.…”