On the microscopic and mesoscopic perturbations of lipid bilayers upon interaction with the MPER domain of the HIV glycoprotein gp41.

The effect of the 665-683 fragment of the HIV fusion glycoprotein 41, corresponding to the MPER domain of the protein and named gp41MPER, on the microscopic structure and mesoscopic arrangement of palmitoyl oleoyl phosphatidylcholine (POPC) and POPC/sphingomyelin (SM)/cholesterol (CHOL) lipid bilayers is analyzed. The microscopic structuring of the bilayers has been studied by Electron Spin Resonance (ESR) spectroscopy, using glycerophosphocholines spin-labelled in different positions along the acyl chain. Transitions of the bilayer liquid crystalline state have been also monitored by Differential Scanning Calorimetry (DSC). Changes of the bilayers morphology have been studied by determining the dimension of the liposomes through Dynamic Light Scattering (DLS) measurements. The results converge in showing that the sample preparation procedure, the bilayer composition and the peptide/lipid ratio critically tune the lipid response to the peptide/membrane interaction. When gp41MPER is added to preformed liposomes, it positions at the bilayer interface and the lipid perturbation is limited to the more external segments. In contrast, if the peptide is mixed with the lipids during the liposome preparation, it assumes a trans-membrane topology. This happens at all peptide/lipid ratios for fluid POPC bilayers, while in the case of rigid POPC/SM/CHOL membranes a minimum ratio has to be reached, thus suggesting peptide self-aggregation to occur. Peptide insertion results in a dramatic increase of the lipid ordering and bilayer stiffening, which reflect in significant changes in liposome average dimension and distribution. The biological implications of these findings are discussed.

On the mechanism of ion transport through lipid membranes mediated by PEGylated cyclic oligosaccharides (CyPLOS): an ESR study.

The mechanism underlying the ionophoric activity of CyPLOS (cyclic phosphate-linked oligosaccharide, 2), a carbohydrate-based synthetic ion transporter decorated with four tetraethylene glycol (TEG) chains, has been investigated by an integrated electron spin resonance (ESR) approach. The mode of interaction of the ionophore with lipid bilayers has been studied by quantitatively analyzing the perturbations in the ESR spectrum of an ad hoc synthesized spin-labeled CyPLOS analog (6), and, in parallel, in the spectra of spin-labeled lipids mixed with 2. The results point to a positioning of the cyclic saccharide backbone close to the lipid headgroups, largely exposed to the aqueous medium. The TEG chains, carrying a terminal benzyl group, are deeply inserted among the lipid acyl chains, showing good mobility and flexibility. As a consequence, the order of the acyl chain packing is significantly reduced, and water penetration in the bilayer is enhanced. The resulting asymmetric perturbation of the bilayer leads to its local destabilization, thus facilitating, through a non-specific mechanism, the ion transport through the membrane.

Destabilization of lipid membranes by a peptide derived from glycoprotein gp36 of feline immunodeficiency virus: a combined molecular dynamics/experimental study.

Viral fusion glycoproteins present a membrane-proximal external region (MPER) which is usually rich in aromatic residues and exhibits a marked tendency to stably reside at the membrane interfaces, leading, through unknown mechanisms, to a destabilization of the bilayer structure. This step has been proposed to be fundamental for the fusion process between target membrane and viral envelope. In the present work, we investigate the interaction between an octapeptide (C8) deriving from the MPER domain of gp36 of feline immunodeficiency virus and POPC bilayers by combining experimental results obtained by neutron reflectivity, electron spin resonance, circular dichroism, and fluorescence spectroscopy with molecular dynamics simulations. Our data indicate that C8 binds to the lipid bilayer adsorbing onto the membrane surface without deep penetration. As a consequence of this interaction, the bilayer thickness decreases. The association of the peptide with the lipid membrane is driven by hydrogen bonds as well as hydrophobic interactions that the Trp side chains form with the lipid headgroups. Upon peptide-bilayer interaction, C8 forms transient secondary structures ranging from 3(10) helices to turn conformations, while acyl chains of the peptide-exposed POPC molecules assume a more ordered packing. At the same time, lipid headgroups' hydration increases. The asymmetric lipid bilayer perturbation is proposed to play a fundamental role in favoring the membrane fusion process.

A novel environmental indicator for monitoring of pesticides.

The identification of an indicator for monitoring pesticides is a very effective analytical approach because it allows one to schedule and simplify the analytical routine. In this study, a new indicator has been designed, which has to be able to define a scale of priorities in the pesticides monitoring. A starting equation was developed involving the escaping tendency of a given substance from a phase (based on the Mackay model I level). The reliability of the indicator was tested by application to a model system, consisting of a defined and limited area, choosing water as matrix over a period of 6 years. A group of marker compounds was also defined to implement the predictive efficacy of the indicator. The results obtained by modeling were compared to those obtained by experimentation of the same model system. The indicator was subsequently and appropriately modified creating a new equation, including a kinetic factor, which considers the environmental degradation processes. The effect of the rectified indicator was consistent with the sales data list of compounds, when applied to the markers. The indicator developed in this study, tested as a model on specific area-phase-period (Province of Siena, water phase, 2000-2006), is applicable to any other area-phase-period, adjusting the partition value of the Mackay model for the case under study.

Design and photochemical characterization of a biomimetic light-driven Z/E switcher.

Protonated Schiff bases (PSBs) of polyenals constitute a class of light-driven switchers selected by biological evolution that provide model compounds for the development of artificial light-driven molecular devices or motors. In the present paper, our primary target is to show, through combined computational and experimental studies, that it is possible to approach the design of artificial PSBs suitable for such applications. Below, we use the methods of computational photochemistry to design and characterize the prototype biomimetic molecular switchers 4-cyclopenten-2'-enylidene-3,4-dihydro-2H-pyrrolinium and its 5,5'-dimethyl derivative both containing the penta-2,4-dieniminium chromophore. To find support for the predicted behavior, we also report the photochemical reaction path of the synthetically accessible compound 4-benzylidene-3,4-dihydro-2H-pyrrolinium. We show that the preparation and photochemical characterization of this compound (together with three different N-methyl derivatives) provide both support for the predicted photoisomerization mechanism and information on its sensitivity to the molecular environment.