Proteins are the most abundant molecules found in living organisms, and, from a functional point of view, also the most diverse. They consist of long chains of amino acids and their conformation, structure and stability can be influenced by many parameters. Aqueous solutions of proteins usually contain salts, most often in the form of a buffer. In addition to proteins, salt, and water, other macromolecules (lipids, sugars, nucleic acids) are often present in biological systems. In laboratory conditions, influence of these macromolecules can be mimicked by the addition of the neutral polymer polyethylene glycol.
Salts and other molecules affect the interaction between protein molecules. One way to determine these interactions is by measuring the zeta potential. In this work, I measured the zeta potential in lysozyme protein solutions, where polyethylene glycol was also present in different concentrations (0, 100 and 150 g/L) and at three pH values (4,1, 7 in 9). 10 mM NaBr salt was used in all cases.
The viscosity of polyethylene glycol solutions increases with its concentration and also with the length of the polymer chain. As the concentration increases, the dielectric constant of the solution decreases. The results show that the zeta potential increases with polyethylene glycol concentration at all three pH values. The zeta potencial also increases with the length of the polymer chain.
In addition to lysozyme, I also used the protein bovine serum albumin. For this protein, the zeta potencial increases in absolute value with increasing polyethylene glycol concentration. At pH 7, the charge of bovine serum albumin is negative.
The results of zeta potential measurements in systems without polyethylene glycol are comparable to those in the literature. However, measurements in the presence of polyethylene glycol have not yet been made.
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