A comparison of lateral diffusion in supported lipid monolayers and bilayers

Abstract

Lipid monolayers and bilayers exist in distinct physical states differentiated by the differences in the manner in which translational fluidity relates to their phase transition and how cholesterol influences the two. Work presented here suggests that intra-leaflet diffusion and cholesterol interactions are modulated by the nature of inter-leaflet coupling. Our results also provide an important practical caveat in the comparisons of membrane physical properties deduced using the two, mono-and bilayer, model membrane configurations.Understanding the physical basis of lateral fluidity of bilayer-forming lipids in biological membranes is important because of its central role in many important functions including signaling, transport, and intermembrane interactions. 1,2 A significant body of work now establishes that the membrane fluidity at physiological temperatures is primarily determined by two key factors: (1) physical properties of the constituent lipids (e.g., degree of unsaturation, length of the acyl tail, head-group electrostatics) and (2) the amount of cholesterol. The bulk of our understanding, to this end, comes from studies of temperature-dependent diffusivity of single lipids (and their mixtures with cholesterol) typically using model membrane configurations all of which present symmetric bilayers (e.g., vesicles, black lipid membranes, supported bilayers). These studies treat long-range lateral diffusion of lipids as the reciprocal of the structural order and of the lateral packing of acyl chains. This in turn relates lateral diffusion to the gel (low fluidity phase) to liquid-crystalline or l.c. (high fluidity phase) main phase transition and attendant chain conformational transition of the bilayer 3,4 The role of cholesterol, within this framework, is understood to be that of a substitutional ''impurity'' which tends to decouple the conformational transition from the packing transition 5 of the host bilayer. Specifically, introduction of cholesterol induces a condensing effect on disordered lipids resulting in denser packing and conversely fluidizes ordered lipids by disrupting their packing habit. 6,7 Implicit in these studies is the assumption that diffusion of membrane lipids is directly linked to their thermotropic phase transition. 8The main phase transition in pure lipids is a cooperative process, strongly influenced by the smectic coupling of two apposed monolayers comprising the bilayer. 9-11 Indeed, previous theoretical efforts show that the inter-monolayer interactions are critical for the transitions, otherwise absent in uncoupled monolayers. 11 But biological membranes are both compositionally and structurally asymmetric across the two leaflets. 12 For instance, in red blood cells, phosphatidylethanolamines and serines are located primarily in the cytosolic leaflet whereas cholines and glycolipids tend to concentrate in the exoplasmic monolayer. Moreover the inner leaflet is also structurally constrained by the underlying cytoskeleton. These structural and compositional asymmetries in ce...

Microcontact Printing for Creation of Patterned Lipid Bilayers on Tetraethylene Glycol Self-Assembled Monolayers

“…The mobility of the lipid bilayer is approximately two times greater than the lipid monolayer which is in good agreement with previous studies. 33 The difference in mobilities also indicates that the bilayers are isolated from one another by the lipid monolayers.…”

Lipid Monolayers at the Air–Water Interface

“…There are a few reports comparing the results for the same system forming different types of model membranes [133][134][135][136][137][138][139][140][141][142][143], and they indicate that the similarities and differences depend on the system analyzed and on which property is explored. The property of membranes that can probably be compared most easily between different models is the temperature for the phase transition.…”

“…This temperature appears not to change [144] or to alter only slightly [134,139,145,146] in vesicles compared to that in supported membranes. In supported bilayers, however, two different transition temperatures for the distal (to the support) and the proximal leaflets have been reported [137,147].…”

Formation of Alkanethiol Supported Hybrid Membranes Revisited

“…It is important to note that these methods only provide indirect evidence for the formation of a fused monolayer, as they either average over a large surface area (e.g., SPR) or, in the other extreme, only sample a small area (AFM); a diffusion measurement such as fluorescence recovering after photobleaching (FRAP) can prove that the layer is continuous. However, if the fluorophores are too close to a conductive surface, fluorescence is quenched via Förster resonance energy transfer (FRET) and hence reports of successful FRAP measurement on a lipid monolayer used silica surfaces modified with an n ‐octadecyltrichlorosilane SAM …”