Thermodynamics of Biomacromolecular Interactions in Aqueous Solutions

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Abstract

An understanding of the interactions between polyelectrolytes and proteins is vital to determine structure and functionality of materials constructed of these two components. Possible applications for the protein-polyelectrolyte composites are ranging from materials used to deliver drugs to the methods of protein stabilisation for storage of therapeutics, biosemsors, and encapsulation of medicines for triggered release. The binding of globular proteins to the polyelectrolyte chains can prevent undesired protein aggregation and may help to extend the shelf-life of the protein-containing food. The aim of this project is to study the mechanism of non-covalent binding between proteins and polyelectrolytes, responsiveness of the proteinpolyelectrolyte composites to external stimuli such as changing pH, presence of salt of different types and concentrations or influence of enzyme on the integrity of protein-polyelectrolyte multilayer film.Our study was focused on the effects of different mono- and multivalent salts on binding affinitybetween a negatively charged polyelectrolyte - poly(styrene sulfonate) PSS and bovine serumalbumin BSA or myoglobin. The complex formation between these polymers was examinedusing the static light scattering (SLS), turbidimetric and potentiometric titrations, differentialscanning calorimetry (DSC) and theoretical studies based on molecular dynamics simulations. We established that the inter- and intramolecular interactions between proteins and polyelectrolytes are primarily driven by the electrostatic forces at the conditions when thepolymers are in low ionic strength solutions and attractive or repulsive relations are based upon the charge density and its distribution. When proteins are interacting with polyelectrolytes in solutions of high ionic strength the electrostatic interactions are screened by the salt originated co-ions. In these conditions there is a competition between salting-out effect on proteins leading to protein aggregation or protein-polyelectrolyte complex formation, which can prevent undesired protein-protein association. The forces driving the attractive interactions at high ionic strength are of non-electrostatic origin, these are mainly hydrophobic forces. The computer simulation study shows that more flexible polyanionic chains are stronger binders to the positive patches on protein surface than these of a more rigid backbone. Also a total energy of binding depends on a sum of electrostatic and non-electrostatic energies.The formation of multilayers composed of a protein and a polyelectrolyte, where componentswere: poly-L-lysine – a positively charged homopolypeptide and polygalacturonic acid - apolysaccharide was examined using a quartz crystal microbalance with dissipation monitoring. A 10 and 11 layer film, deposited on the charged surface, exhibited the linear growth pattern for first 5 layers and exponential growth for a flowing 5 (or 6) layers. The influence of pectinase enzyme on digesting the polygalacturonic acid component of the multilayer was most effective for 1 AU/mL concentration of pectinase. After the enzyme was applied the multilayer film was fully disintegrated within the period of 20 minutes for pectinase at 1 AU/mL and the time of disintegration was extended to 120 minutes for pectinase at 0.1 AU/mL.Silk fibroin aqueous solutions were tested rheologically for their structural properties involvingthe existence of fibroin aggregates. We examined the process of ageing of fibroin solutions and solid-liquid transformations taking place within the fluid. The transitions between viscous and elastic behaviour of the fibroin’s semi-dilute solutions were initiated by strain, shear frequency and temperature. We highlighted that the irreversible change in secondary structure of the silk fibroin in aqueous solutions are taking place after the 48 hour period of time since the preparation of protein fluids. We recommend that further processing of silk fibroin such aselectrospinning should be completed within the 48 hour after dissolution.

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