With the aim to gain insight into the possible applicability of protein-filled polyelectrolyte complex micelles under physiological salt conditions, we studied the behavior of these micelles as a function of salt concentration. The micelles form by electrostatically driven co-assembly from strong cationic block copolymers poly(2-methyl vinyl pyridinium) 41 -block-poly(ethylene oxide) 205 , weak anionic homopolymers poly(acrylic acid) 139 , and negatively charged lipase molecules. The formation and disintegration of these micelles were studied with dynamic light scattering (DLS), by means of composition and salt titrations, respectively. The latter measurements revealed differences between disintegration of lipase-filled and normal polyelectrolyte complex micelles. These data, together with small angle neutron scattering (SANS) measurements provide indications that lipase is gradually released with increasing salt concentration. From the SANS data a linear relation between the intensity at q ¼ 0 and the volume of the cores of the micelles at different salt concentrations was derived, indicating a loss of volume of the micelles due to the release of lipase molecules. It was estimated that beyond 0.12 M NaCl all lipase molecules are released.
Encapsulation of enzymes by polymers is a promising method to influence their activity and stability. Here, we explore the use of complex coacervate core micelles for encapsulation of enzymes. The core of the micelles consists of negatively charged blocks of the diblock copolymer PAA42PAAm417 and the positively charged homopolymer PDMAEMA150. For encapsulation, part of the positively charged homopolymer was replaced by the positively charged globular protein lysozyme. We have studied the formation, structure, and stability of the resulting micelles for three different mixing ratios of homopolymer and lysozyme: a system predominantly consisting of homopolymer, a system predominantly consisting of lysozyme, and a system where the molar ratio between the two positively charged molecules was almost one. We also studied complexes made of only lysozyme and PAA42PAAm417. Complex formation and the salt-induced disintegration of the complexes were studied using dynamic light-scattering titrations. Small-angle neutron scattering was used to investigate the structures of the cores. We found that micelles predominantly consisting of homopolymer are spherical but that complex coacervate core micelles predominantly consisting of lysozyme are nonspherical. The stability of the micelles containing a larger fraction of lysozyme is lower.
The structure of dispersions of TEMPO-oxidised cellulose nanofibrils (OCNF), at various concentrations, in water and in NaCl aqueous solutions, was probed using small angle X-ray scattering (SAXS). OCNF are modelled as rod-like particles with an elliptical cross-section of 10 nm and a length greater than 100 nm. As OCNF concentration increases above 1.5 wt%, repulsive interactions between fibrils are evidenced, modelled by the interaction parameter νRPA > 0. This corresponds to gel-like behaviour, where G' > G'' and the storage modulus, G', shows weak frequency dependence. Hydrogels can also be formed at OCNF concentration of 1 wt% in 0.1 M NaCl(aq). SAXS patterns shows an increase of the intensity at low angle that is modelled by attractive interactions (νRPA < 0) between OCNF, arising from the screening of the surface charge of the fibrils. Results are supported by ζ potential and cryo-TEM measurements.
To obtain insight into the accumulation of proteins into macroscopic complex coacervate phases, the lysozyme concentration in complex coacervates containing the cationic polyelectrolyte poly-(N,N dimethylaminoethyl methacrylate) and the anionic polyelectrolyte polyacrylic acid was investigated as a function of the mixing ratio, protein concentration and ionic strength. Maximal protein enrichment of the complex coacervate phase was observed to require the presence of all three macromolecules. Under optimized conditions the protein concentrations in the complex coacervate were as high as 200 g L(-1). Such high concentrations are comparable to the protein concentration in the cytosol, suggesting that these interesting liquid phases may serve a suitable model system for the phase behavior of the cytosol and genesis and function of membrane-less organelles. The high stability of the complexes and the salt dependent uptake of protein suggest that complex coacervates may provide a way to store hydrated proteins at high concentrations and might therefore be of interest in the formulation of high protein foods.
In this study, the formation and disintegration of polyelectrolyte complex micelles is studied by dynamic light scattering titrations with the aim to assess the extent to which these complexes equilibrate. Also, the time evolution of samples at fixed (electroneutral) composition was followed to obtain information about the relaxation time of the complex formation. We find that, in 3.5 mM phosphate buffer with pH 7, polyelectrolyte complex micelles consisting of the positively charged homopolymer PDMAEMA(150), the negatively charged diblock copolymer PAA(42)-PAAm(417) (both having a pH-dependent charge), as well as the positively charged protein lysozyme slowly equilibrate with a relaxation time of about 2 days. The same structures were obtained, independent of the way the polymers and proteins had been mixed. In contrast, polyelectrolyte complex micelles (at the same pH) consisting of (pH-dependent) negatively charged homopolymer PAA(139), the pH-independent positively charged diblock copolymer P2MVP(41)-PEO(205), and the negatively charged protein alpha-lactalbumin did not equilibrate. The way in which solutions containing these macromolecules were mixed yielded different results that did not change over the period of at least a week.
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