Effect of Temperature on the Nanomechanics of Lipid Bilayers Studied by Force Spectroscopy

Abstract

The effect of temperature on the nanomechanical response of supported lipid bilayers has been studied by force spectroscopy with atomic force microscopy. We have experimentally proved that the force needed to puncture the lipid bilayer (Fy) is temperature dependent. The quantitative measurement of the evolution of Fy with temperature has been related to the structural changes that the surface undergoes as observed through atomic force microscopy images. These studies were carried out with three different phosphatidylcholine bilayers with different main phase transition temperature (TM), namely, 1,2-dimyristoyl-sn-glycero-3-phosphocholine, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, and 2-dilauroyl-sn-glycero-3-phosphocholine. The solid-like phase shows a much higher Fy than the liquid-like phase, which also exhibits a jump in the force curve. Within the solid-like phase, Fy decreases as temperature is increased and suddenly drops as it approaches TM. Interestingly, a "well" in the Fy versus temperature plot occurs around TM, thus proving an "anomalous mechanical softening" around TM. Such mechanical softening has been predicted by experimental techniques and also by molecular dynamics simulations and interpreted in terms of water ordering around the phospholipid headgroups. Ion binding has been demonstrated to increase Fy, and its influence on both solid and liquid phases has also been discussed.

Phase transition behaviors of the supported DPPC bilayer investigated by sum frequency generation (SFG) vibrational spectroscopy and atomic force microscopy (AFM)

“…2). This is in agreement with previous studies on the supported bilayer, 24,[27][28][33][34][35] but different from a sharp phase transition process for lipid vesicles in solution (<1°C), 5,[16][17][18] implying the unique feature for the phase transition on the supported lipid bilayer system.…”

“…1b). 33,35,41 It is interesting to note that pits and holes in the bilayer always appear on the supported DPPC bilayer surface in the gel phase (domain (i), see Fig. 4a-c).…”

“…Similar phenomena have also been observed on the supported lipid bilayers prepared by either LB method or vesicle fusion method. 30,33,35 It is noteworthy that these pits are observed only on the lipid bilayers in the gel-phase, but completely disappear in the liquid-phase when the temperature is higher than the Tm (see, for example, Figs. 4f and 4a).…”

Atomic Force Microscopy Imaging and Force Spectroscopy of Supported Lipid Bilayers

“…These properties depend on lipid packing (liquid or gel phase) and structure (acyl chain length and degree of unsaturation) and altered by membrane-active agents 16 . The theory behind the rupture has been explained 17 and other experimental parameters such as cantilever softness, tip radius and approach speed also affect the breakthrough force 15,16,18 . Force spectroscopy has been used to analyze properties of different lipid phases 11,19 .…”

“…This also demonstrates the importance of mixing the right lipid ratio -a critical factor in reproducibility of results. Furthermore, using different lipids (chain length, and degree of unsaturation) also changes the bilayer properties 6,15,18 . While plotting the distribution of breakthrough forces is useful and can already show differences in the bilayer properties, further analysis of the breakthrough force distribution can be used to deduce other mechanical properties including line tension and spreading pressure 11,17,21 .…”