Structural Calorimetry of Main Transition of Supported DMPC Bilayers by Temperature-Controlled AFM

Abstract

Atomic force microscopy at high temperature resolution (DeltaT < or approximately 0.1 K) provided a quantitative structural calorimetry of the transition from the fluid (Lalpha)- to the gel (Pbeta')-phase of supported dimyristoylphosphatidylcholine bilayers. Besides a determination of the main transition temperature (T0) and the van't Hoff transition enthalpy (DeltaHvH), the structural analysis in the nm-scale at T close to T0 of the ripple phase allowed an experimental estimation of the area of cooperative units from small lipid domains. Thereby, the corresponding transition enthalpy (DeltaH) of single molecules could be determined. The lipid organization and the corresponding parameters T0 and DeltaHvH (DeltaH) were modulated by heptanol or external Ca2+ and compared with physiological findings. The size of the cooperative unit was not significantly affected by the presence of 1 mM heptanol. The observed linear relationship of DeltaHvH and T0 was discussed in terms of a change in heat capacity.

Thermal Expansion of Microstructured DMPC Bilayers Quantified by Temperature‐Controlled Atomic Force Microscopy

“…The formation of the ripple-gel-phase P b' as a pre-melting transition [12,13] was not detected in the experiments. The large image sizes and the surface roughness of the glass substrate impede the visualization of ripple phases, but we have experimental evidence from lipid patch analysis on mica at higher magnification, to show that the P b' phase occurs during the melting process (results not shown) in accordance with the results of Kolb and coworkers.…”

“…Characteristic lipid patches, visible on the left hand side of the bilayer stripe, help to locate the image and its magnification, allowing images of the same size and position to be produced At low temperature (15.6 8C) an average bilayer thickness d ave , of 4.70 AE 0.02 nm was found which is in good agreement with the study from Kolb and coworkers who found a bilayer thickness of 4.8 nm in the gel phase also employing AFM. [12] Upon heating, the structured bilayer maintains its initial surface coverage of 0.59 AE 0.03 and average thickness of 4.7 AE 0.02 nm up to 26.4 8C. However, the AFM image at 26.4 8C shows weak structural changes of the microstructured bilayer (arrow).…”

“…Bilayer thickness as a function of temperature is frequently used as a measure of the chain melting process during the main phase transition from the gel phase (L C ) to the fluid crystalline phase (L a ). [5,[8][9][10][11][12] Apart from height analysis, thermal expansion during the heating or the cooling of the SSMs, which can be detected as a closing or opening of defect areas within the SSM, is also used to extract thermodynamic quantities from temperature-controlled AFM. [5][6][7][8][9][10][11][12][13] Measuring thermal expansion directly upon heating is, however, hampered by the lack of free area and only a few studies provide quantitative information.…”

“…The large image sizes and the surface roughness of the glass substrate impede the visualization of ripple phases, but we have experimental evidence from lipid patch analysis on mica at higher magnification, to show that the P b' phase occurs during the melting process (results not shown) in accordance with the results of Kolb and coworkers. [12] It is instructive to monitor not only the changes in bilayer thickness and surface coverage, but also to evaluate fluctuations of the membrane/glass boundary. Figure 4 shows that the fluctuation amplitude (s 2 ), obtained from contour line analysis of the boundary increases considerably within the phase transition regime (see Supplementary Information).…”

Ion Dynamics in Cationic Lipid Bilayer Systems in Saline Solutions

“…The main transition temperature, T m , at atmospheric pressure is 296 K for a pure DMPC bilayer, 83 while it is around 311 K for a pure DMTAP bilayer. 84 For DMPC/DMTAP mixtures, T m depends on the molar fraction of DMTAP, TAP ; experiments at 3 bar 85 have shown that T m ( TAP ) is downward concave and has a global maximum of 310 K at around TAP ∼ 45 mol %.…”