Hydrophobic Interaction Determined by Partition in Aqueous Two‐Phase Systems

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Abstract
In this report we describe a new method which is useful for measuring hydrophobic interactions between aliphatic hydrocarbon chains and proteins in aqueous environment. The method is based on partition of proteins in an aqueous two‐phase system containing dextran and poly(ethylene glycol) and different fatty acid esters of poly(ethylene glycol). The partition is measured under conditions where contributions from electrostatic interactions are eliminated. The difference in partition of proteins in phase systems with and without hydrocarbon groups bound to poly(ethylene glycol), Δlog K, where K is the partition coefficient, is taken as a measure of hydrophobic interaction. Δlog K varies with size of hydrocarbon chain and type of protein. The length of the aliphatic chain should be greater than 8 carbon atoms in order to get a measurable effect in terms of Δlog K. Bovine serum albumin, β‐lactoglobulin, hemoglobin and myoglobin have been shown to have different affinities for palmitic acid ester of poly(ethylene glycol). No hydrophobic effect could be observed for ovalbumin, cytochrome c or α‐chymotrypsinogen A.