Utilizing zeta potential measurements to study the effective charge, membrane partitioning, and membrane permeation of the lipopeptide surfactin.

The effective charge of membrane-active molecules such as the fungicidal lipopeptide surfactin (SF) is a crucial property governing solubility, membrane partitioning, and membrane permeability. We present zeta potential measurements of liposomes to measure the effective charge as well as membrane partitioning of SF by utilizing what we call an equi-activity analysis of several series of samples with different lipid concentrations. We observe an effective charge of -1.0 for SF at pH8.5 and insignificantly lower at pH7.4, illustrating that the effective charge may deviate strongly from the nominal value (-2 for 1 Asp, 1 Glu). The apparent partition coefficient decreases from roughly 100 to 20/mM with increasing membrane content of SF in agreement with the literature. Finally, by comparing zeta potentials measured soon after the addition of peptide to liposomes with those measured after a heat treatment to induce transmembrane equilibration of SF, we quantified the asymmetry of partitioning between the outer and inner leaflets. At very low concentration, SF binds exclusively to the outer leaflet. The onset of partial translocation to the inner leaflet occurs at about 5mol-% SF in the membrane. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.

Design of pH-responsive nanoparticles of terpolymer of poly(methacrylic acid), polysorbate 80 and starch for delivery of doxorubicin.

This work focused on the design of new pH-responsive nanoparticles for controlled delivery of anticancer drug doxorubicin (Dox). Nanoparticles of poly(methacrylic acid)-polysorbate 80-grafted starch (PMAA-PS 80-g-St) were synthesized by using a one-pot method that enabled simultaneous grafting of PMAA and PS 80 onto starch and nanoparticle formation in an aqueous medium. The particles were characterized by FTIR, (1)H NMR, TEM, DLS, and potentiometric titration. Dox loading and in vitro release from the nanoparticles were investigated. The FTIR and (1)H NMR confirmed the chemical composition of the graft terpolymer. The nanoparticles were relatively spherical with narrow size distribution and porous morphology. They exhibited pH-dependent swelling in a physiological pH range. The particle size and magnitude of phase transition were dependent on polymer composition and formulation parameters such as concentrations of surfactant and cross-linking agent and total monomer concentration. The nanoparticles with optimized compositions showed high loading capacity for Dox and sustained Dox release. The results suggest that the new pH-responsive terpolymer nanoparticles are useful in controlled drug delivery.

pH-Dependent doxorubicin release from terpolymer of starch, polymethacrylic acid and polysorbate 80 nanoparticles for overcoming multi-drug resistance in human breast cancer cells.

This work investigated the capability of a new nanoparticulate system, based on terpolymer of starch, polymethacrylic acid and polysorbate 80, to load and release doxorubicin (Dox) as a function of pH and to evaluate the anticancer activity of Dox-loaded nanoparticles (Dox-NPs) to overcome multidrug resistance (MDR) in human breast cancer cells in vitro. The Dox-NPs were characterized by Fourier transform infrared spectroscopy (FTIR), isothermal titration calorimetry (ITC), transmission electron microscopy (TEM), and dynamic light scattering (DLS). The cellular uptake and cytotoxicity of the Dox-loaded nanoparticles were investigated using fluorescence microscopy, flow cytometry, and a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) (MTT) assay. The nanoparticles were able to load up to 49.7±0.3% of Dox with a high loading efficiency of 99.9±0.1%, while maintaining good colloidal stability. The nanoparticles released Dox at a higher rate at acidic pH attributable to weaker Dox-polymer molecular interactions evidenced by ITC. The Dox-NPs were taken up by the cancer cells in vitro and significantly enhanced the cytotoxicity of Dox against human MDR1 cells with up to a 20-fold decrease in the IC50 values. The results suggest that the new terpolymeric nanoparticles are a promising vehicle for the controlled delivery of Dox for treatment of drug resistant breast cancer.

Effects of glycerol and urea on micellization, membrane partitioning and solubilization by a non-ionic surfactant.

We have studied the effect of two cosolvents, urea and glycerol, on the association and interactions of a surfactant, octaethyleneglycol dodecyl ether (C(12)EO(8)) and a phospholipid (POPC). We have measured the CMC, the partition coefficient, the effective mole fractions X(e)(sat) and X(e)(sol) at the onset and completion of the membrane-to-micelle transition (membrane solubilization), and the enthalpies of transfer of surfactant by ITC. Changes in membrane order and dynamics were characterized by time-resolved fluorescence anisotropy measurements of DPH, and micelle sizes and clouding by light scattering. The cosolvents have complex effects that are not governed by the well-known 'salting in' or 'salting out' effects on the solubility alone. Instead, urea and glycerol alter also the intrinsic curvature ('effective molecular shape') of the detergent and the order and packing of the membrane. These curvature effects have an unusual enthalpy/entropy balance and are not additive for lipid and detergent. The results shed light on the phenomena governing lipid-detergent systems in general and have a number of implications for the use of cosolvents in membrane protein studies.