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...

The synthetic-method determination of liquid-liquid coexistence curves using semiautomated light scattering instrumentation and stirred samples is based on identifying the coexistence curve transition temperatures (T(cx)) from sudden changes in turbidity associated with droplet formation. Here we use a thorough set of such measurements to evaluate the accuracy of several different analysis methods reported in the literature for assigning T(cx). More than 20 samples each of weakly opalescent isobutyric acid+water and strongly opalescent aniline+hexane were tested with our instrumentation. Transmitted light and scattering intensities at 2 degrees , 24 degrees , and 90 degrees were collected simultaneously as a function of temperature for each stirred sample, and the data were compared with visual observations and light scattering theory. We find that assigning T(cx) to the onset of decreased transmitted light or increased 2 degrees scattering has a potential accuracy of 0.01 K or better for many samples. However, the turbidity due to critical opalescence obscures the identification of T(cx) from the light scattering data of near-critical stirred samples, and no simple rule of interpretation can be applied regardless of collection geometry. At best, when 90 degrees scattering is collected along with transmitted or 2 degrees data, the accuracy of T(cx) is limited to 0.05 K for near-critical samples. Visual determination of T(cx) remains the more accurate approach in this case.

This Letter describes Fourier-transform infrared spectroscopy evidence for the evolution of conformational order and coverage during the formation of n-alkanethiol monolayers on microdroplets of mercury from the solution phase. At the highest coverages obtained by self-assembly, the monolayer is characterized by predominantly all-trans conformational order. For partial monolayers obtained at arbitrarily quenched incubation periods, we find a continuous evolution of the chain conformational order with monolayer coverage. Analyzing these results in light of previously reported models from X-ray scattering reveals a complex self-assembly process in which the density-dependent evolution of the chain conformational order is coupled with that of molecular orientation and density.

We have devised an infrared spectromicroscopy based experimental configuration to enable structural characterization of buried molecular junctions. Our design utilizes a small mercury drop at the focal point of an infrared microscope to act as a mirror in studying metal-molecule-metal (MmM) junctions. An organic molecular monolayer is formed either directly on the mercury drop or on a thin, infrared (IR) semi-transparent layer of Au deposited onto an IR transparent, undoped silicon substrate. Following the formation of the monolayer, films on either metal can be examined independently using specular reflection spectroscopy. Furthermore, by bringing together the two monolayers, a buried molecular bilayer within the MmM junction can be characterized. Independent examination of each half of the junction prior to junction formation also allows probing any structural and/or conformational changes that occur as a result of forming the bilayer. Because our approach allows assembling and disassembling microscopic junctions by forming and withdrawing Hg drops onto the monolayer covered metal, spatial mapping of junctions can be performed simply by translating the location of the derivatized silicon wafer. Finally, the applicability of this technique for the longer-term studies of changes in molecular structure in the presence of electrical bias is discussed.