{'fraction_non_alphanumeric': 0.02276176024279211, 'fraction_numerical': 0.002276176024279211, 'mean_word_length': 5.625628140703518, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Evidence for interleaflet slip during spreading of single lipid bilayers at hydrophilic solids.

Slip—relative mobility of the two leaflets comprising lipid bilayers—represents a generic mechanism that contributes to a broad range of mechanical–dynamical processes in bilayer lipid membranes. Some examples include extrusion of tethers from giant vesicles experiencing local deformations, fingering of vesicles due to thermal fluctuations and advancing or spreading of bilayers in response to surface shear or at solid surfaces. Moreover, curvature, compositional (as well as density) differences between the two leaflets, binding of the distal (or outer) leaflet receptors by proteins, and any coupling between the substrate and the lower (or proximal) leaflet accentuate conditions for non-trivial interleaflet slip. Current methods detect slip by measuring the force required to introduce it. Here we describe non-perturbative optical methods, involving fluorescence photo-bleaching and bead tracking, to determine slip between molecular leaflets during spontaneous self-spreading of lipid bilayers at a hydrophilic surface. We find the existence of a substantial interleaflet slip during bilayer advance reflecting that the surface-distal leaflet spreads faster than the surface-proximal one and that the excess material from the faster leaflet must “roll-over” at the spreading front. Because the interleaflet slip is opposed by the interleaflet friction, these results also implicate the energy dissipative character of the hydration-induced membrane spreading process. When a stack of dried phospholipids is hydrated near a hydrophilic surface, a single lipid bilayer spreads at the solid– liquid interface. This bilayer progressively covers the surface with square-root of time dependent kinetics until the excess at the source is depleted. The spreading bilayer has two possible slip planes: between the lower leaflet and the substrate, and between the two leaflets of the bilayer. To detect interleaflet slip, the advancing lipid bilayers (see Experimental Section) are exposed to an intense laser spot, rapidly bleaching the illuminated fluorophores in a radially symmetric shape adapted from the well-known fluorescence recovery after photobleaching (FRAP) approach. A fiber-coupled laser (10 mW 532 nm) produces a radially symmetric, roughly Gaussian-shaped dark spot by photobleaching the fluorescent lipids within the advancing bilayer. The time-lapse sequence of images shown in Figure 1 b reveals two salient changes. First, the circular spot dramatically transforms into an elliptical shape in the direction of bilayer