Effect of the addition of alkanols of different topology to dipalmitoyl-phosphatidylcholine vesicles in the presence of gramicidin

Fluorescent styrene maleic acid copolymers to facilitate membrane protein studies in lipid nanodiscs

Fluorescently-labelled variants of poly(styrene-co-maleic acid), SMA, have been synthesised by RAFT copolymerisation. We show that low ratios of vinyl fluorophores, analogous to styrene, can be successfully incorporated during polymerisation without detriment to nanodisc formation upon interaction with lipids. These novel copolymers are capable of encapuslating lipids and the model membrane protein, gramicidin, and hence have the potential to be applied in fluorescence-based biological studies. To demonstrate this, energy transfer is used to probe polymer-protein interactions in nanodiscs. The copolymers may also be used to monitor nanodisc self assembly by exploiting aggregation-caused-quenching (ACQ).

Novel Peptide-Polymer Conjugate with pH-Responsive Targeting/Disrupting Effects on Biomembranes

Conjugating polymers to peptides has become a new strategy of designing functional antitumor agents for their improved stability and enhanced activity. In this paper, a novel peptide-polymer conjugate PEPc-PMAA with pH responsiveness was designed and synthesized. The isoelectric point of PEPc was studied by dynamic light scattering for the targeting effect. Also, the transmittances of PMAA at different pHs were measured using an ultraviolet-visible spectrophotometer for determining the triggering pH of the disrupting effect. The results showed that PEPc-PMAA was hydrophilic under neutral conditions and changed to be amphiphilic composed of positively charged PEPc and hydrophobic PMAA under acidic conditions. The interactions between PEPc-PMAA and mimic cells were investigated by the measurements of membrane fluidity and cargo leakage from 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine and 1,2-dipalmitoyl-sn-glycerol-3-phospho-(1-rac-glycerol) (DPPG) liposomes. It proved that PEPc-PMAA caused a distinct membrane disturbance of the DPPG liposome at pH 5.5, resulting in more serious cargo leakage. Because of its targeting and disrupting effects on negatively charged biomembranes under acidic conditions, PEPc-PMAA showed its good potential as an antitumor agent.

Physicochemical properties of L-alpha dipalmitoyl phosphatidylcholine large unilamellar vesicles: Effect of hydrophobic block (PLA/PCL) of amphipathic diblock copolymers

In the present work, we show how amphipathic diblock copolymers affect the physicochemical properties of the lipid bilayer of DPPC liposome. Diblock copolymers proposed for this study are focused in the difference between PLA and PCL hydrophobic block, because PLA and PCL differ in their glass transition temperature, where a higher ratio of PLA, lowers the flexibility of the diblock copolymer. On the contrary, a greater proportion of PCL makes the diblock copolymer more flexible. This flexibility difference between hydrophobic block would affect the physicochemical properties of lipid bilayer of DPPC. The difference of rigidity or flexibility of hydrophobic block and their interaction with DPPC large unilamellar vesicles (LUVs) was evaluated at low and high copolymers concentration. The copolymer concentrations used were chosen based on their respective cmc. We measure (a) Thermotropic behavior from GP of Laurdan and fluorescence anisotropy of DPH; (b) Relation between wavelength excitation and generalized polarization of Laurdan; (c) Time-resolved fluorescence anisotropy of DPH; (d) Water outflow through the lipid bilayer and (e) calcein release from DPPC LUVs. Furthermore, large unilamellar vesicles in the absence and in the presence of different copolymers were characterized by size and zeta-potential. The results show that the diblock copolymer at high PLA/PCL ratio, that is, greater rigidity of hydrophobic block produces an increase of the phase transition temperature (Tm). For DPPC LUVs, Tm increase 3.5 °C at low and about 4.5 °C at high copolymers concentration, sensed by Laurdan and DPH fluorescent probes, although the DPPC/copolymers molar ratio for Cop4 is higher than Cop3, Cop2 and Cop1. In addition, we observed a decrease in the polarity of microenvironments in the bilayer and an increase in the order of the acyl chains in the bilayer to a high proportion of PLA. Furthermore, the presence of diblock copolymer with high proportion of PLA, decreases water outflow from DPPC liposome and water efflux is slower; leading to a decrease in calcein release from DPPC liposomes. Our results clearly show that the greater the stiffness of the hydrophobic block, greater degree of packaging of the lipid bilayer, greater the order of the acyl chains, and greater retention of water and calcein inside the liposome. Therefore, the presence of AB-type diblock copolymers with a more rigid hydrophobic block, stabilizes the lipid bilayer and would allow a more controlled release of water, and encapsulated molecules inside of the DPPC liposome.

Melting Behavior of Zipper-Structured Lipopeptides in Lipid Bilayer

A zipper-structured lipopeptide is expected to play a role of "intelligent valve" in the lipid bilayer. In this paper, a series of zipper-structured lipopeptides have been designed for preparing thermocontrollable hybrid liposomes. Their conformational transition as a function of temperature in lipid bilayer has been investigated for understanding the influences of molecular structure and bilayer property on biofunction. The melting temperatures T_m of the lipopeptides have been found to depend on their molecular structures. When the lipopeptides have been doped in bilayer, an increase of size of alkyl chain increases the stability of the α-helix resulting in a decrease in fluidity of lipid bilayer. However, an increase of amino groups at N-terminal is found to decrease the stability of the spatial structure. The thermocontrollability of the "valve" in lipid bilayer is confirmed by drug release experiments under different temperatures. Meanwhile, effects of bilayer properties on the thermosensitivity of lipopeptides have also been investigated. Results show the T_m of lipopeptide doped in bilayer decreases with the increase of membrane fluidity. Furthermore, the reversibility of the thermocontrolled "valve" is also proven by release drug under intermittent temperatures. It could be concluded that the molecular structure of the lipopeptide, as well as the property of bilayer, give great influence on the biofunction of the hybrid liposomes.

Rate of solute incorporation to liposomes evaluated from encapsulated enzymes activities

There are numerous studies on systems comprising an enzyme encapsulated in unilamellar liposomes and its substrate initially present in the external aqueous media. Most of these studies are focused on enzyme stability and activity in a restricted media. However, the rate of the process is also determined by the capacity of the substrate to permeate towards the liposome inner pool. In spite of this, there are few studies aimed at a quantitative evaluation of the substrate permeation rate and its lifetime inside the liposome pool. In the present work, we describe, in terms of a very simple mechanism, the permeation of glucose and hydrogen peroxide in DPPC unilamellar liposomes. To this aim, we evaluated the rate of the process employing encapsulated glucose oxidase and catalase in the kinetic diffusion controlled limit. Under this condition, the rate of the process becomes zero order in the enzyme and allows a direct evaluation of the rate constant for the permeation process and the lifetime of a substrate molecule incorporated into the liposome inner pool.