A systemic formalism is developed that shows how the results for absolute specific volumes of multilamellar lipid dispersions may be combined with results from diffraction studies to obtain quantitative characterizations of the average structure of fully hydrated lipid bilayers. Quantities obtained are the area per molecule, the thickness and volumes of the bilayer, the water layer, the hydrocarbon chain layer and the headgroup layer, and where appropriate, the tilt angle of the hydrocarbon chains. In the case of the C phase of DPPC this formalism leads to the detection of inconsistencies between three data. Results for the G phases of DPPC and DLPE are in reasonable agreement with, though more comprehensive than, previous work that used fewer data and equations. Various diffraction data for the F phase of DPPC are in disagreement and it is shown how this disagreement affects results for the bilayer structure. A recent method of McIntosh and Simon for obtaining fluid phase structure utilizing gel phase structure is slightly modified to obtain results for the F phase of DLPE. Methods of obtaining the average methylene and methyl volumes in the fluid phases are critically examined.
"(5)) when a negative deviation from the Stern–Volmer relationship is observed.   I 0 I = 1 + K SV [Q ] (1 + K SV [Q ]) (1 − f b ) + f b (5) where f b corresponds to the fraction of light arising from the accessible fluorophores to the quencher. "
[Show abstract][Hide abstract] ABSTRACT: Substitution of Ala 108 and Ala 111 in the 107-115 human lysozyme (hLz) fragment results in a 20-fold increased anti-staphylococcal activity while its hemolytic activity becomes significant (30%) at very high concentrations. This analog displays an additional positive charge near the N-terminus (108) and an extra Trp residue at the center of the molecule (111), indicating that this particular amino acid sequence improves its interaction with the bacterial plasma membrane. In order to understand the role of this arrangement in the membrane interaction, studies with model lipid membranes were carried out. The interactions of peptides, 107-115 hLz and the novel analog ([K(108)W(111)]107-115 hLz) with liposomes and lipid monolayers were evaluated by monitoring the changes in the fluorescence of the Trp residues and the variation of the monolayers surface pressure, respectively. Results obtained with both techniques revealed a significant affinity increase of [K(108)W(111)]107-115 hLz for lipids, especially when the membranes containing negatively charged lipids, such as phosphatidylglycerol. However, there is also a significant interaction with zwitterionic lipids, suggesting that other forces in addition to electrostatic interactions are involved in the binding. The analysis of adsorption isotherms and the insertion kinetics suggest that relaxation processes of the membrane structure are involved in the insertion process of novel peptide [K(108)W(111)]107-115 hLz but not in 107-115 hLz, probably by imposing a reorganization of water at the interphases. In this regard, the enhanced activity of peptide [K(108)W(111)]107-115 hLz may be explained by a synergistic effect between the increased electrostatic forces as well as the increased hydrophobic interactions.
"This last feature suggests that Arg perturbs the bilayer packing of PEs causing an increase in the chain mobility and area per molecule , as a consequence of a change in the H-bonding pattern . The enhanced adsorption in gel PE in comparison to gel PC does not correlate with the lower hydration and higher lateral interaction of PEs in comparison to PCs (2 to 4 water molecules per lipid molecule for PEs    in comparison to 7–8 water molecules per lipid molecule for PCs in the same state). Clearly, other structural factors should be considered. "
[Show abstract][Hide abstract] ABSTRACT: The interaction of L-arginine with membranes composed by phospholipids with different degrees of methylation of the ethanolamine group was studied by means of surface and dipole potentials and surface pressure variations. The subsequent methylation of the amine head group appears to hinder the synergic response of the adsorption observed in phosphatidylethanolamine membranes. The kinetics of the binding process denotes that the methyl groups are relevant in regulating the specific interaction of the amino acid with the interface by hydrogen bonds. This response can be put in correlation with the function of signal transduction assigned previously to methyl lipids [F. Hirata and J. Axelrod, 1980] and appears to be relevant to understand the mechanism of insertion of arginine residues in peptides of biological interest.
"For the lipid we choose N H = 2 and N T = 8 (per tail), with the monomer volumes v H = v T = 0.05 nm 3 . With these values, the total volume of our lipid is v L = 0.9 nm 3 , which is on the order of the value for a lipid in a fully hydrated bilayer (v L = 1.232 nm 3 ) as determined by Nagle . The desired surface charge density is experimentally obtained by specifying the composition of lipid species in the membrane ; for simplicity, we choose to distribute the charge evenly among the head monomers and set c H = 0.25 for the (dimensionless ) charge magnitude per head monomer. "
[Show abstract][Hide abstract] ABSTRACT: We employ self-consistent field theory to study the thermodynamics of membrane-particle interactions in the context of gene delivery systems, with the aim to guide the design of dendrimers that can overcome the endosomal escape barrier by inserting into membranes and creating pores. We consider the interaction between a model polyamidoamine dendrimer and a membrane under controlled tension as a function of the separation between the dendrimer and the membrane. In all the cases we have studied, the lowest free energy state corresponds to the membrane partially wrapping the dendrimer. However, with moderate tension, we find that a G5 (or larger) generation dendrimer, through thermal fluctuation, can induce the formation of metastable pores. These metastable pores are subsequently shown to significantly lower the critical tension necessary for membrane rupture, thus possibly enhancing the release of the trapped genetic material from the endosome.
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