Structural Origins of Cholesterol Accelerated Lipid Flip-Flop Studied by Sum-Frequency Vibrational Spectroscopy

Sum-frequency vibrational spectroscopy, a tutorial: Applications for the study of lipid membrane structure and dynamics

Planar supported lipid bilayers (PSLBs) are an ideal model for the study of lipid membrane structures and dynamics when using sum-frequency vibrational spectroscopy (SFVS). In this paper, we describe the construction of asymmetric PSLBs and the basic SFVS theory needed to understand and make measurements on these membranes. Several examples are presented, including the determination of phospholipid orientation and measuring phospholipid transmembrane translocation (flip-flop).

Passive Translocation of Phospholipids in Asymmetric Model Membranes: Solid-State 1H NMR Characterization of Flip-Flop Kinetics Using Deuterated Sphingomyelin and Phosphatidylcholine

Although lateral and inter-leaflet lipid-lipid interactions in cell membranes play roles in maintaining asymmetric lipid bilayers, the molecular basis of these interactions is largely unknown. Here, we established a method to determine the distribution ratio of phospholipids between the outer and inner leaflets of asymmetric large unilamellar vesicles (aLUVs). The trimethylammonium group, (CH3)3N+, in the choline headgroup of N-palmitoyl-sphingomyelin (PSM) and 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) gave rise to a relatively sharp signal in magic-angle spinning solid-state 1H NMR (MAS-ss-1H NMR). PSM and DOPC have the same headgroup structure, but one phospholipid was selectively observed by deuterating the trimethylammonium group of the other phospholipid. The addition of Pr3+ to the medium surrounding aLUVs selectively shifted the chemical shift of the (CH3)3N+ group in the outer leaflet from that in the inner leaflet, which allowed estimation of the inter-leaflet distribution ratio of the unlabeled lipid in aLUVs. Using this method, we evaluated the translocation of PSM and DOPC between the outer and inner leaflets of the cholesterol-containing aLUVs, with PSM and DOPC mostly distributed in the outer and inner leaflets, respectively, immediately after aLUV preparation; their flip and flop rates were approximately 2.7 and 6.4 × 10-6 s-1, respectively. During the passive symmetrization of aLUVs, the lipid translocation rate was decreased due to changes in the membrane order, probably through the formation of the registered liquid-ordered domains. Comparison of the result with that of symmetric LUVs revealed that lipid asymmetry may not significantly affect the lipid translocation rates, while the lateral lipid-lipid interaction may be a dominant factor in lipid translocation under these conditions. These findings highlight the importance of considering the effects of lateral lipid interactions within the same leaflet on lipid flip-flop rates when evaluating the asymmetry of phospholipids in the cell membrane.

Monitoring membranes: The exploration of biological bilayers with second harmonic generation

Nature's seemingly controlled chaos in heterogeneous two-dimensional cell membranes stands in stark contrast to the precise, often homogeneous, environment in an experimentalist's flask or carefully designed material system. Yet cell membranes can play a direct role, or serve as inspiration, in all fields of biology, chemistry, physics, and engineering. Our understanding of these ubiquitous structures continues to evolve despite over a century of study largely driven by the application of new technologies. Here, we review the insight afforded by second harmonic generation (SHG), a nonlinear optical technique. From potential measurements to adsorption and diffusion on both model and living systems, SHG complements existing techniques while presenting a large exploratory space for new discoveries.

Inhibitory Effect of Lanthanides on Native Lipid Flip-Flop

The influence of ytterbium ions (Yb³⁺), a commonly used paramagnetic NMR chemical shift reagent, on the physical properties and flip-flop kinetics of dipalmitoylphosphatidylcholine (DPPC) planar supported lipid bilayers (PSLBs) was investigated. Langmuir isotherm studies revealed that Yb³⁺ interacts strongly with the phosphate headgroup of DPPC, evidenced by the increases in shear and compression moduli. Using sum-frequency vibrational spectroscopy, changes in the acyl chain ordering and phase transition temperature were also observed, consistent with Yb³⁺ interacting with the phosphate headgroup of DPPC. The changes in the physical properties of the membrane were also observed to be concentration dependent, with more pronounced modification observed at low (50 μM) Yb³⁺ concentrations compared to 6.5 mM Tb³⁺, suggesting a cross-linking mechanism between adjacent DPPC lipids. Additionally, the changes in membrane packing and phase transition temperatures in the presence of Tris buffer suggested that a putative Yb(Tris)³⁺ complex forms that coordinates to the PC headgroup. The kinetics of DPPC flip-flop in the gel and liquid crystalline (lc) phases were substantially inhibited in the presence of Yb³⁺, regardless of the Yb³⁺ concentration. Analysis of the flip-flop kinetics under the framework of transition state theory revealed that the free energy barrier to flip-flop in both the gel and lc phases was substantial increased over a pure DPPC membrane. In the gel phase, the trend in the free energy barrier appeared to follow the trend in the shear moduli, suggesting that the Yb³⁺-DPPC headgroup interaction was driving the increase in the activation free energy barrier. In the lc phase, activation free energies of DPPC flip-flop in the presence of 50 μM or 6.5 mM Yb³⁺ were found to mirror the free energies of TEMPO-DPPC flip-flop, leading to the conclusion that the strong interaction between Yb³⁺ and the PC headgroup was essentially manifested as a headgroup charge modification. These studies illustrate that the presence of the lanthanide Yb³⁺ results in significant modification to the lipid membrane physical properties and, more importantly, results in a pronounced inhibition of native lipid flip-flop.

Influence of very-long-chain polyunsaturated fatty acids on membrane structure and dynamics

The unique attributes of very-long-chain polyunsaturated fatty acids (VLC-PUFAs), their long carbon chains (n > 24) and high degree of unsaturation, impart unique chemical and physical properties to this class of fatty acids. The changes imparted by VLC-PUFA 32:6 n-3 on lipid packing and the compression moduli of model membranes were evaluated from π-A isotherms of VLC-PUFA in 1,2-distearoyl-sn-3-glycero-phosphocholine (DSPC) lipid monolayers. To compare the attractive or repulsive forces between VLC-PUFA and DSPC lipid monolayers, the measured mean molecular areas (MMAs) were compared with the calculated MMAs of an ideal mixture of VLC-PUFA and DSPC. The presence of 0.1, 1, and 10 mol % VLC-PUFA shifted the π-A isotherm to higher MMAs of the lipids comprising the membrane and the observed positive deviations from ideal behavior of the mixed VLC-PUFA:DSPC monolayers correspond to repulsive forces between VLC-PUFAs and DSPC. The MMA of the VLC-PUFA component was estimated using the measured MMAs of DSPC of 47.1 ± 0.7 Å²/molecule, to be 15,000, 1100, and 91 Å²/molecule at 0.1, 1, and 10 mol % VLC-PUFA:DSPC mixtures, respectively. The large MMAs of VLC-PUFA suggest that the docosahexaenoic acid tail reinserts into the membrane and adopts a nonlinear structure in the membrane, which is most pronounced at 0.1 mol % VLC-PUFA. The presence of 0.1 mol % VLC-PUFA:DSPC also significantly increased the compression modulus of the membrane by 28 mN/m compared with a pure DSPC membrane. The influence of VLC-PUFA on lipid "flip-flop" was investigated by sum-frequency vibrational spectroscopy. The incorporation of 0.1 mol % VLC-PUFA increased the DSPC flip-flop rate fourfold. The fact that VLC-PUFA promotes lipid translocation is noteworthy as retinal membranes require a high influx of retinoids which may be facilitated by lipid flip-flop.

Interfacial hydration determines orientational and functional dimorphism of sterol-derived Raman tags in lipid-coated nanoparticles

Lipid-coated noble metal nanoparticles (L-NPs) combine the biomimetic surface properties of a self-assembled lipid membrane with the plasmonic properties of a nanoparticle (NP) core. In this work, we investigate derivatives of cholesterol, which can be found in high concentrations in biological membranes, and other terpenoids, as tunable, synthetic platforms to functionalize L-NPs. Side chains of different length and polarity, with a terminal alkyne group as Raman label, are introduced into cholesterol and betulin frameworks. The synthesized tags are shown to coexist in two conformations in the lipid layer of the L-NPs, identified as "head-out" and "head-in" orientations, whose relative ratio is determined by their interactions with the lipid-water hydrogen-bonding network. The orientational dimorphism of the tags introduces orthogonal functionalities into the NP surface for selective targeting and plasmon-enhanced Raman sensing, which is utilized for the identification and Raman imaging of epidermal growth factor receptor-overexpressing cancer cells.

Effect of electrostatic interaction on the leaflet-specific diffusion in a supported lipid bilayer revealed by fluorescence lifetime correlation analysis

Lipid–support electrostatic interaction determines the lipid dynamics in the proximal leaflet of a SLB.

Second Harmonic Generation Spectroscopy of Membrane Probe Dynamics in Gram-Positive Bacteria

Bacterial membranes are complex mixtures with dispersity that is dynamic over scales of both space and time. To capture adsorption onto and transport within these mixtures, we conduct simultaneous second harmonic generation (SHG) and two-photon fluorescence measurements on two different gram-positive bacterial species as the cells uptake membrane-specific probe molecules. Our results show that SHG not only can monitor the movement of small molecules across membrane leaflets but also is sensitive to higher-level ordering of the molecules within the membrane. Further, we show that the membranes of Staphylococcus aureus remain more dynamic after longer times at room temperature in comparison to Enterococcus faecalis. Our findings provide insight into the variability of activities seen between structurally similar molecules in gram-positive bacteria while also demonstrating the power of SHG to examine these dynamics.

Reduction of glass-surface charge density slows the lipid diffusion in the proximal leaflet of a supported lipid bilayer

Understanding the effect of a solid support on the dynamical properties of a supported lipid bilayer (SLB) is a prerequisite for the applications of SLB as a model biomembrane. Here, we applied two-dimensional fluorescence lifetime correlation spectroscopy to examine the effect of solution pH on the diffusion of lipids in the proximal/distal leaflets of a zwitterionic SLB. Leaflet-specific diffusion analyses at various pH revealed that the diffusion of lipids in the proximal leaflet facing a glass surface becomes slower by decreasing pH with the transition pH of ∼7.4. We attributed it to the reduction of the surface charge density of a glass support. Furthermore, the data clearly showed that the lipid diffusion in the distal leaflet facing a bulk solution is insensitive to the change in the diffusion property of the proximal leaflet. This reflects a weak interleaflet coupling between the proximal and distal leaflets of the SLB.

Permeability and Line-Tension-Dependent Response of Polyunsaturated Membranes to Osmotic Stresses

The lipidome of plant plasma membranes-enriched in cellular phospholipids containing at least one polyunsaturated fatty acid tail and a variety of phytosterols and phytosphingolipids-is adapted to significant abiotic stresses. But how mesoscale membrane properties of these membranes such as permeability and deformability, which arise from their unique molecular compositions and corresponding lateral organization, facilitate response to global mechanical stresses is largely unknown. Here, using giant vesicles reconstituting mixtures of polyunsaturated lipids (soy phosphatidylcholine), glucosylceramide, and sitosterol common to plant membranes, we find that the membranes adopt "janus-like" domain morphologies and display anomalous solute permeabilities. The former textures the membrane with a single sterol-glucosylceramide-enriched, liquid-ordered domain separated from a liquid-disordered phase consisting primarily of soy phosphatidylcholine. When subject to osmotic downshifts, the giant unilamellar vesicles (GUVs) respond by transiently producing well-known swell-burst cycles. In each cycle, the influx of water swells the GUV, rendering the membrane tense. Subsequent rupture of the membrane through transient poration, which localizes in the liquid-disordered phase or at the domain boundaries, reduces the osmotic stress by expelling some of the excess osmolytes (and solvent) before sealing. When subject to abrupt hypertonic stress, they deform by nucleating buds at the domain phase boundaries. Remarkably, this incipient vesiculation is reversed in a statistically significant fraction of GUVs because of the interplay with solute permeation timescales, which render osmotic stresses short-lived. This, then, suggests a novel control mechanism in which an interplay of permeability and deformability regulates osmotically induced membrane deformation and limits vesiculation-induced loss of membrane material. Interestingly, recapitulation of such dynamic morphological reconfigurability-switching between budded and nonbudded morphologies-due to the interplay of membrane permeability, which temporally reverses the osmotic gradient, and domain boundaries, which select modes of deformations, might prove valuable in endowing synthetic cells with novel morphological responsiveness.

The Ins and Outs of Lipid Flip-Flop

Our current view of cellular membranes centers on the fluid-mosaic model, which envisions the cellular membrane as a "liquidlike" bilayer of lipids, cholesterol, and proteins that freely diffuse in two dimensions. In stark contrast, the exchange of materials between the leaflets of a bilayer was presumed to be prohibited by the large enthalpic barrier associated with translocating hydrophilic materials, such as a charged lipid headgroup, through the hydrophobic membrane core. This static picture with regard to lipid translocation (or "flip-flop" as it is affectionately known) has been a long-held belief in the study of membrane dynamics. The current accepted membrane model invokes specific protein flippase (inward moving), floppase (outward moving), and scramblase (bidirectional) enzymes that assist in the movement of lipids between the leaflets of cellular membranes. The low rate of protein-free lipid flip-flop has also been a cornerstone of our understanding of the bilateral organization of cellular membrane components, specifically the asymmetric distribution of lipid species found in the luminal and extracellular leaflets of the plasma membrane of eukaryotic cells. Much of the previous work contributing to our current understanding of lipid flip-flop has utilized fluorescent- or spin-labeled lipids. However, there is growing evidence that these lipid probes do not accurately convey the dynamics and thermodynamics of native (unlabeled) lipid motion. This Account summarizes our research efforts directed toward developing a deep physical and chemical understanding of protein-free lipid flip-flop in phospholipid membrane models using sum-frequency vibrational spectroscopy (SFVS). Our use of SFVS enables the direct measurement of native lipid flip-flop in model membranes. In particular, we have explored the kinetic rates and activation thermodynamics of lipid translocation as a means of deciphering the underlying chemical and physical directors governing this process. By means of transition state theory, the contributions from enthalpy and entropy on the activation energy barrier to lipid flip-flop have been explored in detail for a variety of lipid species and membrane compositions. Specifically, the effect of lipid structure and packing and the inclusion of cholesterol and transmembrane peptides on the rates and thermodynamics of lipid translocation have been investigated in detail. It is our hope that these studies will provide a new perspective on lipid translocation in biological membranes and the role of lipid flip-flop in generating and maintaining cell membrane lipid asymmetry.

Transport and Organization of Cholesterol in a Planar Solid-Supported Lipid Bilayer Depend on the Phospholipid Flip-Flop Rate

Understanding the transport behavior of the cholesterol molecules within a cell membrane is a key challenge in cell biology at present. Here, we have applied sum frequency generation vibrational spectroscopy to characterize the transport and organization of cholesterol in different kinds of planar solid-supported lipid bilayers by combining achiral- and chiral-sensitive polarization measurements. This method allows us to distinguish the organization of cholesterol in tail-to-tail, head-to-tail, head-to-head, and side-by-side manners. It is found that the movement of cholesterol in the lipid bilayer largely depends on the flip-flop rate of the phospholipid. The flip-flop dynamics of the phospholipid and cholesterol are synchronous. In the solid-supported zwitterionic phosphocholine lipid bilayer, the cholesterol molecules flip quickly from the distal leaflet to the neutral proximal leaflet of the bilayer and form tail-to-tail organization on both leaflets. The phosphocholine lipid and cholesterol show the same flip-flop rate. However, when the proximal leaflet is prepared using negative glycerol phospholipids, cholesterol organizes itself by mainly forming an α-β structure on the distal leaflet. Because of the strong interaction between the glycerol phospholipid and the substrate, no or only partial cholesterol molecules flip from the distal leaflet to the negatively charged proximal leaflet. However, the cholesterol molecules undergo flip-flop in the presence of salt solution because the ions weaken the interaction between the negative phospholipid and the substrate.