Mixed Langmuir monolayers of gramicidin A and fluorinated alcohols

Comparison between the behavior of different hydrophobic peptides allowing membrane anchoring of proteins

Membrane binding of proteins such as short chain dehydrogenase reductases or tail-anchored proteins relies on their N- and/or C-terminal hydrophobic transmembrane segment. In this review, we propose guidelines to characterize such hydrophobic peptide segments using spectroscopic and biophysical measurements. The secondary structure content of the C-terminal peptides of retinol dehydrogenase 8, RGS9-1 anchor protein, lecithin retinol acyl transferase, and of the N-terminal peptide of retinol dehydrogenase 11 has been deduced by prediction tools from their primary sequence as well as by using infrared or circular dichroism analyses. Depending on the solvent and the solubilization method, significant structural differences were observed, often involving α-helices. The helical structure of these peptides was found to be consistent with their presumed membrane binding. Langmuir monolayers have been used as membrane models to study lipid-peptide interactions. The values of maximum insertion pressure obtained for all peptides using a monolayer of 1,2-dioleoyl-sn-glycero-3-phospho-ethanolamine (DOPE) are larger than the estimated lateral pressure of membranes, thus suggesting that they bind membranes. Polarization modulation infrared reflection absorption spectroscopy has been used to determine the structure and orientation of these peptides in the absence and in the presence of a DOPE monolayer. This lipid induced an increase or a decrease in the organization of the peptide secondary structure. Further measurements are necessary using other lipids to better understand the membrane interactions of these peptides.

Miscibility behavior of two-component monolayers at the air-water interface: perfluorocarboxylic acids and DMPE

Surface pressure (pi)-molecular area (A) and surface potential (DeltaV)-A isotherms have been measured for two-component monolayers of four different perfluorocarboxylic acids [FCn; perfluorododecanoic acid (FC12), perfluorotetradecanoic acid (FC14), perfluorohexadecanoic acid (FC16), and perfluorooctadecanoic acid (FC18)] and dimyristoylphosphatidylethanolamine (DMPE) on 0.15M NaCl (pH 2) at 298.2K. The present study is focused on the miscibility and the interfacial behavior for the binary DMPE/FCn monolayers upon compression. From the isotherms, the miscibility has been elucidated in terms of the additivity rule, the interaction parameter, and the interaction energy. The interaction parameter (or energy) is compared with that for the previous dipalmitoylphosphatidylcholine (DPPC)/FCn systems [Colloids Surf. B 41 (2005) 285-298] to understand the effect of phospholipids' polar headgroup on the binary miscibility. Furthermore, the phase behavior of the DMPE/FCn systems has been morphologically examined using fluorescence microscopy (FM) and atomic force microscopy (AFM). These images reveal the different interaction modes among the four systems; DMPE can be miscible with FC12 and FC14 and immiscible with FC16 and FC18 in the monolayer state. These systematic examinations indicate that the miscibility of perfluorocarboxylic acids and phospholipids depends on combination of hydrocarbon and fluorocarbon chain lengths and on phospholipids' polar headgroups within a monolayer.

Dipolar interactions and miscibility in binary Langmuir monolayers with opposite dipole moments of the hydrophilic heads

We investigate unusual binary Langmuir monolayers with the same long CH3(CH2)21 hydrocarbon chains and fluorinated -O-CH2CF3 (FEE) versus nonfluorinated -O-CH2CH3 (EE) hydrophilic heads, whose opposite dipoles assist miscibility, in contrast to the equally oriented polar head dipoles of almost all natural or synthetic amphiphiles that minister to phase separation. Although two-component bulk micelles, lipid bilayers, and monolayers with fluorinated and nonfluorinated chains, which also have opposite dipoles, often show phase separation, we find complete miscibility and nonideality of the FEE-EE mixtures demonstrated via deviation of the composition dependencies of the mean molecular area at fixed surface pressure from the additivity rule. The composition dependencies of the excess molecular areas exhibit minima and maxima which show specific structural changes at particular compositions. They originate from the dipolar and steric interactions between the polar heads, because the interactions between the same chains of FEE and EE do not vary. The pi/A isotherms and the pi/X(FEE) phase diagram reveal that mixtures with molar fractions X(FEE) > or = 0.3 exist in an upright solid phase even in uncompressed state. This result is confirmed by the compressibility values and via Brewster angle microscopy, which does not show optical anisotropy at X(FEE) > or = 0.3. Comparison of the collapse and phase-transition molecular areas with literature data suggests that the upright architecture corresponds to LS-phase or S-phase with more defects as the S-phase in the pure monolayers. The mixtures with X(FEE) < 0.3 exist in tilted L2' phase at low surface pressures. Their mean molecular areas are smaller than the corresponding values in the EE film, which manifests reduction of the tilt of the EE chains with increasing FEE content. We ascribe the chain erection to partial dehydration of the EE heads caused by dipolar attraction between the EE and FEE heads. The excess free energy of mixing deltaG(exc)pi is positive but much smaller than the negative total free energy of mixing AG mix(pi) showing a spontaneous miscibility at all compositions due to an entropy increase. The analysis of the conflict between the deltaG(mix)pi minimum at molar fraction X(FEE) = 0.5 and the minimum and negative value of the excess molecular area A(pi,exc) at X(FEE) = 0.8 shows that the A(pi,exc)/X(FEE) minimum has not an electrostatic but a short-range structural origin.