Lipid membranes orientation

Hollars C W and Dunn R C 2000 Probing single molecule orientations in model lipid membranes with near-field scanning optical microscopy J. Phys. Chem 112 7822-30... 

All of the transport systems examined thus far are relatively large proteins. Several small molecule toxins produced by microorganisms facilitate ion transport across membranes. Due to their relative simplicity, these molecules, the lonophore antibiotics, represent paradigms of the mobile carrier and pore or charmel models for membrane transport. Mobile carriers are molecules that form complexes with particular ions and diffuse freely across a lipid membrane (Figure 10.38). Pores or channels, on the other hand, adopt a fixed orientation in a membrane, creating a hole that permits the transmembrane movement of ions. These pores or channels may be formed from monomeric or (more often) multimeric structures in the membrane. 

Figure 3.3 Molecular structure of G-protein-coupled receptors. In (a) the electron density map of bovine rhodopsin is shown as obtained by cryoelectron microscopy of two-dimensional arrays of receptors embedded in lipid membrane. The electron densities show seven peaks reflecting the seven a-helices which are predicted to cross the cell membrane. In (b) is shown a helical-wheel diagram of the receptor orientated according to the electron density map shown in (a). The diagram is seen as the receptor would be viewed from outside the cell membrane. The agonist binding pocket is illustrated by the hatched region between TM3, TM5 and TM6. (From Schertler et al. 1993 and Baldwin 1993, reproduced from Schwartz 1996). Reprinted with permission from Textbook of Receptor Pharmacology. Eds Foreman, JC and Johansen, T. Copyright CRC Press, Boca Raton, Florida...

Figure 22.3 The basic construction of phosphodiglyceride molecules within lipid bilayers. The fatty acid chains are embedded in the hydrophobic inner region of the membrane, oriented at an angle to the plane of the membrane surface. The hydrophilic head group, including the phosphate portion, points out toward the hydrophilic aqueous environment.

Glaser RW, Sachse C, Durr UHN, Wadhwani P, Ulrich AS (2004) Orientation of the antimicrobial peptide PGLa in lipid membranes determined from F-19-NMR dipolar couplings of 4-CF3-phenylglycine labels. J Magn Reson 168 153-163... 

In order to verify that the adsorbed lipid membrane indeed forms a bilayer film, another experiment is conducted with an aim to detect the formation of a monolayer lipid. It starts with a piranha-cleaned micro-tube treated with silane to render its inner surface hydrophobic. POPC liposome is then injected into the microtube. It is known that POPC lipid will form a monolayer to such a surface by orienting their hydrophobic tails toward the hydrophobic wall. The experimental results using a mode with similar sensitivity as the previous experiment are shown in Fig. 8.39. The resonance shift in this case is 22 pm, which is about half of that observed for the adsorption of a lipid bilayer. These two experiments suggest that the microtube resonator is capable of accurately determining an adsorbed biomolecular layer down to a few nm thicknesses. 

The association of host defense peptides with lipid bilayers has been observed to be directly related to the ratio of peptide to lipid. At low peptide/lipid ratios, peptides are oriented parallel to the membrane. As the ratio increases, the peptides reorient themselves perpendicular to the membrane, ultimately inserting into the bilayer. Following membrane insertion transmembrane pores are formed. The insertion of peptides into the lipid membrane and subsequent translocation of peptides into the cytoplasm or formation of transmembrane pores has been described by multiple models of host defense peptide insertion. 

Most of the above membrane-oriented studies were carried out for peptides in multilayer systems that were collapsed or transferred onto a sample cell surface. An alternative and very interesting way to study membrane systems is by IRRAS (infrared reflection absorption spectroscopy) at the air-water interface. In this way, unilamellar systems can be studied as a function of surface pressure and under the influence of various membrane proteins and peptides added. Mendelsohn et al.[136] have studied a model series of peptides, [K2(LA) ] (n = 6, 8, 10, 12), in nonaqueous (solution), multilamellar (lipid), and unilamellar (peptide-IRRAS) conditions. In the multilamellar vesicles these peptides are predominantly helical in conformation, but as peptide only monolayers on a D20 subphase the conformation is (1-sheet like, at least initially. For different lengths, the peptides show variable surface pressure sensitivity to development of some helical component. These authors further use their IR data to hypothesize the existence of the less-usual parallel (i-sheet conformation in these peptides. A critical comparison is available for different secondary structures as detected using the IRRAS data for peptides on H20 and D20 subphasesJ137 ... 

The bimolecular lipid membrane (BLM) produced in aqueous solution described in this paper is of considerable interest for two main reasons. First, the BLM is a new type of interfacial film of ultrathinness. The limiting thickness of BLM is 40-130 A. as estimated from various measurements. The values obtained by optical methods are probably most reliable, indicating that the thickness of the BLM is equal to about twice the length of the lipid molecules. The environment in which the BLM is formed and the molecular orientation at the biface lends itself as a promising tool in understanding some outstanding problems in colloid and interfacial chemistry such as Van der Waals attraction and... 

Fig. 5a—c. Orientation of amphiphilic compounds in model membranes33 a) monolayer at the gas/water interface b) bimolecular lipid membrane (BLM) c) liposomes. Between b and c a cross section through the bilayer of BLM or liposomes is shown... 

Bouquet -shaped molecules based on either a polyether macrocycle [8.205] or a /3-cyclodextrin (/3-CD) [8.206] core, such as 114 and 115 respectively, have been synthesized and their polycarboxylate forms have been incorporated into vesicle bilayer membranes [8.207]. They present structural features suitable for studies of chundle-type molecular channels (1) the functionalized cyclic annulus possesses substrate selection properties (2) it bears axially oriented bundles of oxygen-containing chains, which provide binding sites for metal cations and are long enough for the molecule to span a typical lipid membrane (the overall length with the chains... 

Enhanced photovoltage and photocurrent signals were observed by the authors of Refs. [183,184] with linked porphyrin-quinone molecules in planar bilayer lipid membranes (BLM) as compared with preparations containing the non-Iinked components. They interposed BLM between two aqueous compartments containing a secondary electron donor on one side and a secondary acceptor on the other side. The efficiency of PET increased when the P-L-Q molecules were oriented in the membrane. 

The interaction of various cholesterol ancestors with lipid membranes on oriented bilayers has also been studied by 2H- and 31P-NMR [100]. 

Figure 21 Effect of macroscopic orientation on 3,P NMR spectra of PL membranes. (A) Multi-lamellar PL vesicles yield 31P NMR spectra that are characterised by an axially symmetric shielding tensor. If lamellar lipid membranes are oriented between glass plates, a single 3,P NMR signal is obtained for any given orientational angle between the membrane normal and the external magnetic field B0. 31P NMR spectra for (B) perpendicular and (C) parallel orientations are shown. Taken from Ref. [86].

A membrane-puncturing theory has been proposed by Davis (Dravnieks 1967). According to this theory, the odorous substance molecules are adsorbed across the interface of the thin lipid membrane, which forms part of the cylindrical wall of the neuron in the chemoreceptor and the aqueous phase that surrounds the neuron. Adsorbed molecules orient themselves with the hydrophilic end toward the aqueous phase. When the adsorbed molecules are desorbed, they move into the aqueous phase, leaving a defect. Ions... 

Ipsen, J. H., Mouritsen, O. G. and Bloom, M. (1990). Relationships between lipid membrane area, hydrophobic thickness and acyl-chain orientational order. The effects of cholesterol. Biophys. J. 57 405. 

---