Dynamic lipid lateral segregation driven by lauryl cyclodextrin interactions at the membrane surface

Amphiphilic cyclodextrins, with a cholesterol anchor (βChol) or an aspartic acid moiety esterified by two lauryl acyl chains (βDLC), were designed to combine the inclusion ability of the cyclodextrin cavity with the carrier properties of model membranes. Their insertion in phosphatidylcholine bilayers induces a marked lateral phase separation into a pure lipid phase and a cyclodextrin-rich phase (LCD), organized as a 2D cyclodextrin network stabilized by intermolecular hydrogen bonds between the saccharide headgroups at the membrane surface (Roux, M.; Perly, B.; Djedaïni-Pilard, F. Self-Assemblies of Amphiphilic Cyclodextrins. Eur. Biophys. J.2007, 36, 861-867). We have replaced the dilauryl anchor by a single lauryl chain grafted onto a leucine residue, giving monolauryl-β-cyclodextrin (βMLC), which readily inserts into bilayers of chain-deuterated DMPC-d27. The removal of one lauryl acyl chain leads to a dynamic membrane insertion of this new cyclodextrin derivative, with significant lipid exchange on the deuterium NMR time scale between a loosely packed cyclodextrin-enriched phase (L'CD) and free lipid regions, yielding broadened two-component NMR spectra. Like the LCD phases, the cyclodextrin-enriched L'CD regions remain (partially) fluid below the DMPC-d27 main fluid-to-gel transition but do not undergo a clear transition toward a gel state, as observed at 14.5 °C in the LCD phase induced by the dilauryl derivative. Partially fluid lipids of the βMLC-induced L'CD phase coexist with pure lipids in the Pβ' gel phase with possible exchange between them until all of the lipids undergo a transition toward an Lβ' gel state at around 7 °C. Trimethylated monolauryl-β-cyclodextrins induce only an ordering of the lipid acyl chains just above the main transition, without any lateral phase separation. Similar chain ordering is also observed within the βMLC-induced L'CD phase as a consequence of the deep membrane insertion of the monolauryl nonmethylated cyclodextrin derivative.

Location of chlorhexidine in DMPC model membranes: a neutron diffraction study

Chlorhexidine (CHX) is an effective anti-bacterial agent whose mode of action is thought to be the disruption of the cell membrane. It is known to partition into phospholipid bilayers of aqueous model-membrane preparations. Neutron diffraction data taken at 36 degrees C on the location of CHX in phosphatidylcholine (PC) bilayers is presented. The center of mass of the deuterated hydrocarbon chain of CHX is found to reside 16A from the center of the bilayer in 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (14:0-14:0PC). This places the drug near the glycerol backbone of the lipid, and suggests a mode of action whereby the molecule is bent in half and inserts wedge-like into the lipid matrix. This mechanism is distinct from detergent-like mechanisms of membrane disruption and more similar to some anti-microbial peptide action, where peptides insert obliquely into the bilayer headgroup region to disrupt its structure.

Penetration depth of surfactant peptide KL4 into membranes is determined by fatty acid saturation

KL(4) is a 21-residue functional peptide mimic of lung surfactant protein B, an essential protein for lowering surface tension in the alveoli. Its ability to modify lipid properties and restore lung compliance was investigated with circular dichroism, differential scanning calorimetry, and solid-state NMR spectroscopy. KL(4) binds fluid lamellar phase PC/PG lipid membranes and forms an amphipathic helix that alters lipid organization and acyl chain dynamics. The binding and helicity of KL(4) is dependent on the level of monounsaturation in the fatty acid chains. At physiologic temperatures, KL(4) is more peripheral and dynamic in fluid phase POPC/POPG MLVs but is deeply inserted into fluid phase DPPC/POPG vesicles, resulting in immobilization of the peptide. Substantial increases in the acyl chain order are observed in DPPC/POPG lipid vesicles with increasing levels of KL(4), and POPC/POPG lipid vesicles show small decreases in the acyl chain order parameters on addition of KL(4). Additionally, a clear effect of KL(4) on the orientation of the fluid phase PG headgroups is observed, with similar changes in both lipid environments. Near the phase transition temperature of the DPPC/POPG lipid mixtures, which is just below the physiologic temperature of lung surfactant, KL(4) causes phase separation with the DPPC remaining in a gel phase and the POPG partitioned between gel and fluid phases. The ability of KL(4) to differentially partition into lipid lamellae containing varying levels of monounsaturation and subsequent changes in curvature strain suggest a mechanism for peptide-mediated lipid organization and trafficking within the dynamic lung environment.

Interactions of the C-terminus of lung surfactant protein B with lipid bilayers are modulated by acyl chain saturation

Lung surfactant protein B (SP-B) is critical to minimizing surface tension in the alveoli. The C-terminus of SP-B, residues 59-80, has much of the surface activity of the full protein and serves as a template for the development of synthetic surfactant replacements. The molecular mechanisms responsible for its ability to restore lung compliance were investigated with circular dichroism, differential scanning calorimetry, and (31)P and (2)H solid-state NMR spectroscopy. SP-B(59-80) forms an amphipathic helix which alters lipid organization and acyl chain dynamics in fluid lamellar phase 4:1 DPPC:POPG and 3:1 POPC:POPG MLVs. At higher levels of SP-B(59-80) in the POPC:POPG lipid system a transition to a nonlamellar phase is observed while DPPC:POPG mixtures remain in a lamellar phase. Deuterium NMR shows an increase in acyl chain order in DPPC:POPG MLVs on addition of SP-B(59-80); in POPC:POPG MLVs, acyl chain order parameters decrease. Our results indicate SP-B(59-80) penetrates deeply into DPPC:POPG bilayers and binds more peripherally to POPC:POPG bilayers. Similar behavior has been observed for KL(4), a peptide mimetic of SP-B which was originally designed using SP-B(59-80) as a template and has been clinically demonstrated to be successful in treating respiratory distress syndrome. The ability of these helical peptides to differentially partition into lipid lamellae based on their degree of monounsaturation and subsequent changes in lipid dynamics suggest a mechanism for lipid organization and trafficking within the dynamic lung environment.

Use of inverse theory algorithms in the analysis of biomembrane NMR data

Treating the analysis of experimental spectroscopic data as an inverse problem and using regularization techniques to obtain stable pseudoinverse solutions, allows access to previously unavailable level of spectroscopic detail. The data is mapped into an appropriate physically relevant parameter space, leading to better qualitative and quantitative understanding of the underlying physics, and in turn, to better and more detailed models. A brief survey of relevant inverse methods is illustrated by several successful applications to the analysis of nuclear magnetic resonance data, yielding new insight into the structure and dynamics of biomembrane lipids.

Changes in molecular order across the lamellar-to-inverted hexagonal phase transition depend on the position of the double-bond in mono-unsaturated phospholipid dispersions

A series of mono-unsaturated phosphatidylethanolamine (PE) model membranes is studied using deuterium nuclear magnetic resonance (NMR) and differential scanning calorimetry. As the position of the double bond is systematically changed, the internal conformational motions are monitored through the bilayer-to-inverted-hexagonal phase transition. The order parameter profiles extracted from the NMR spectra report on the conformational order of the lipid and on the way this order is changed by structural reorganizations of the membrane. The calculation of a ratio of renormalized order parameter profiles is presented here as an attempt to distill the essential features of these changes into dimensionless descriptions of "shape" functions. This variation of the extent of molecular disorder along the long molecular axis of the phospholipids appears to be a recurring motif, modulated by temperature, structural rearrangement, and chemical composition of the membrane. The reported experimentally measured changes in the shape of the order parameter profile can be compared to those obtained during molecular dynamics simulation studies.

Determination of orientational order parameters from 2H NMR spectra of magnetically partially oriented lipid bilayers

The partial orientation of multilamellar vesicles (MLVs) in high magnetic fields is known to affect the shape of 2H NMR spectra. There are numerical methods for extracting either the orientational order parameters of lipid molecules for a random distribution of domain orientations in the sample, or the distribution of orientations for a known set of spectral anisotropies. A first attempt at determining the orientational order parameters in the presence of an unknown nonrandom distribution of orientations is presented. The numerical method is based on the Tikhonov regularization algorithm. It is tested using simulated partially oriented spectra. An experimental spectrum of a phospholipid-ether mixture in water is analyzed as an example. The experimental spectrum is consistent with an ellipsoidal shape of MLVs with a ratio of semiaxes of approximately 3.4.

Smoothed orientational order profile of lipid bilayers by 2H-nuclear magnetic resonance

A new method has been developed to determine the complete orientational order profile of lipid bilayers using 2H-NMR. The profile is obtained from a single powder spectrum of a lipid which has a saturated chain fully deuteriated. The smoothed order profile is determined directly from the normalized dePaked spectrum assuming a monotonic decrease of the order along the acyl chain. The oscillatory variations of the order at the beginning of the chain are not described by this method. However the smoothed order profile reveals in a straightforward way the crucial features of the anisotropic order of the bilayer.

Acyl chain orientational order in the hexagonal HII phase of phospholipid-water dispersions

The deuterium nuclear magnetic resonance (2H NMR) spectrum of perdeuterated tetradecanol in a mixture of 1-palmitoyl-2-oleoyl-phosphatidylethanolamine (POPE) and water was used to compare the variation of the acyl chain orientational order parameter, S(n), with carbon position, n, in the liquid crystalline lamellar (L alpha) and hexagonal (HII) phases. The characteristics independence of S(n) with n (plateau) normally observed in the L alpha phase is replaced by a more rapid decrease of S(n) with n in the HII phase. It is suggested that as a consequence of the geometrical characteristics of the HII phase, there is an increase in conformational freedom available to different parts of the acyl chain.