The effect of univalent electrolyte concentration on protein-polyelectrolyte complex formation has been measured by frontal analysis continuous capillary electrophoresis (FACCE) and turbidimetry for the interaction of bovine serum albumin (BSA) with a synthetic hydrophobically modified polyacid, for BSA with (porcine mucosal) heparin (Hp), a highly charged polyanion, and for Hp and insulin. All three highly diverse systems display maxima or plateaus in complex formation in the range of ionic strength 5 < I < 30 mM, confirmed in the case of BSA-Hp by multiple techniques. Similar maxima are reported in the literature, but with little discussion, for BSA-poly(dimethyldiallylammonium chloride), lysozyme-hyaluronic acid, and lysozyme-chondroitin sulfate, always in the I range 5-30 mM. While inversion of salt effect has been discussed specifically for the interaction of gelatin and sodium polystyrenesulfonate with gelatin(28) and with beta-lactoglobulin,(10) the general nature of this phenomenon, regardless of polyelectrolyte origin, molecular weight, and charge sign has not been recognized. The position of the maxima and their occurrence when protein and polyelectrolyte have the same net charge imply that they arise when Debye lengths extend, at low I, beyond half the protein diameter so that addition of salt screens repulsions, as well as attractions. This appears to be a general effect caused by electrostatic repulsions that can coexist simultaneously with hydrophobic interactions. Modeling of protein electrostatics via Delphi is used to visualize this effect for BSA, lysozyme, insulin, and beta-lactoglobulin.

The aggregation of beta-lactoglobulin (BLG) at ambient temperature was studied using turbidimetry and dynamic light scattering in the range 3.8

The effect of a polyelectrolyte's chain stiffness on its interaction with an oppositely charged colloid particle was studied by measuring the relative affinity of two polyelectrolytes for (1) mixed cationic/nonionic micelles (DTAB/TX100), and (2) the protein serum albumin. The binding affinity as manifested, respectively, in the critical ionic surfactant mole fraction required for polyelectrolyte-micelle complex formation, and in the critical pH for polyelectrolyte-protein association, was determined by turbidimetric titrations over a range of ionic strengths. Binding was generally weaker for the stiffer chain, hyaluronic acid (HA), relative to the more flexible chain, a copolymer of acrylamidomethylpropanesulfonate (AMPS) and acrylamide (AAm), chosen to have the same linear charge density as HA at neutral pH. In the case of serum albumin, comparisons were also made to AMPS-AAm copolymers of higher charge densities, and to heparin, a highly charged and flexible biopolyelectrolyte. The results are discussed in terms of the ionic strength dependence of the relevant persistence lengths.

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Highly charged polyelectrolytes adsorbed on oppositely charged colloidal particles are investigated by electrophoresis and dynamic light scattering. The dependence of the adsorbed amount and of the hydrodynamic layer thickness on the molecular mass and the salt level is analyzed. The adsorbed amount increases with increasing salt level and decreases with increasing molecular mass. The hydrodynamic layer thickness is independent of the molecular mass at low salt levels, but increases with the molecular mass as a power law with an exponent 0.10 ± 0.01 at high salt. The same behavior was observed for different polyelectrolytes and substrates and therefore is suspected to be generic. Due to semi-quantitative agreement with computer simulations carried out by Kong and Muthukumar in 1998, the observed behavior is interpreted with conformational changes of single adsorbed polyelectrolyte chains.

We report on the interaction of pulmonary surfactant composed of phospholipids and proteins with nanometric alumina (Al 2 O 3 ) in the context of lung exposure and nanotoxicity. We study the bulk properties of phospholipid/nanoparticle dispersions and determine the nature of their interactions. The clinical surfactant Curosurf®, both native and extruded, and a protein-free surfactant are investigated. The phase behavior of mixed surfactant/particle dispersions was determined by optical and electron microscopy, light scattering and zeta potential measurements. It exhibits broad similarities with that of strongly interacting nanosystems such as polymers, proteins or particles, and supports the hypothesis of electrostatic complexation. At a critical stoichiometry, micron sized aggregates arising from the association between oppositely charged vesicles and nanoparticles are formed. Contrary to the models of lipoprotein corona or of particle wrapping, our work shows that vesicles maintain their structural integrity and trap the particles at their surfaces. The agglomeration of particles in surfactant phase is a phenomenon of importance since it could change the interactions of the particles with lung cells. keywords:Pulmonary surfactant -Curosurf® -Aluminum nanoparticles -Electrostatic complexationMultilamellar vesicles Corresponding authors: jean-francois.berret@univ-paris-diderot.fr Accepted at Langmuir: Tuesday, June 16, 2015 I -Introduction Pulmonary surfactant, the fluid lining the epithelium of the lungs is a complex surface-active fluid that contains phospholipids and lipids (85% and 5%, respectively) and 10% proteins (SP-A, SP-B, SP-C, SP-D and serum proteins). 1-2 The biophysical functions of pulmonary surfactant are to prevent the collapse of small alveoli during expiration and the overexpansion of large alveoli during inspiration. It also preserves bronchiolar patency during normal and forced respiration. 1,[3][4] Furthermore, it has an important immunological role of protecting the lungs from injuries and infections caused by inhaled particles, including microorganisms, particulate matter or engineered particles. [5][6][7][8][9][10] More specifically, particles of sizes less than 100 nm end up significantly deposited in the alveoli, and are susceptible to interact with the lung fluid. [11][12] To evaluate the risks of exposure to inhaled nanomaterials, recent studies have been focusing on the interaction of particles with membranes, more specifically on model systems made of DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) or DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine) unilamellar vesicles. [13][14][15][16][17][18][19][20] The review of the different interaction potentials ! 2! between particles and membranes revealed the importance of the interplay between particle/vesicle attraction and bilayer bending energy. 17 For diameters lower than a critical size (order of 10 nm for silica), the particles decorate the outer surface of the membrane, and induce aggregation. 17-18 For larger particle diamet...

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The aggregation of insulin near its isoelectric point and at low ionic strength was suppressed in the presence of heparin. To understand this effect, we used turbidimetry and stopped-flow to study the pH-and ionic strength (I)-dependence of the aggregation of heparin-free insulin. The results supported the role of interprotein electrostatic interactions, contrary to the commonly held view that such forces are minimized at pH ) pI. Electrostatic modeling of insulin (DelPhi) revealed that attractive interactions arise from the marked charge anisotropy of insulin near pI. We show how screening of the interprotein attractions by added salt lead to maximum aggregation near I ) 0.01 M, corresponding to a Debye length nearly equal to the diameter of the insulin dimer, consistent with a dipole-like protein charge distribution. This analysis is also consistent with suppression of aggregation by heparin, a strong polyanion that by binding to the positive domain of one protein, inhibits its interaction with the negative domain of another.