Hydrophobic Modification of Dextran and Applications of Modified Dextrans in Aqueous Polymer Two-Phase Systems

Abstract: In this work benzoyl dextrans with a degree of substitution (DS) ranging from 0.02 to 0.29 and valeryl dextrans with a DS ranging from 0.025 to 0.2 were synthesized. The modified dextrans were characterized by viscosity, UV, HPLC and NMR measurements. The effect of reaction conditions on the DS of modified dextrans was studied. The modified dextrans were used to study the effect of aromatic and aliphatic groups on the formation of two-phase and polyphase systems at room temperature. The influence of benzoyl dextran and valeryl dextran on the partitioning of biological materials in polymer aqueous two-phase systems was investigated. The two-phase systems studied were composed of PEG/benzoyl dextran, PEG/valeryl dextran, Ucon/benzoyl dextran, EO30PO70/benzoyl dextran, dextran/benzoyl dextran, dextran/valeryl dextran and valeryl dextran/benzoyl dextran. The biomaterials studied included amino acids, dipeptides, tripeptides, polypeptides, proteins and thylakoid membrane vesicles. Proteins with high surface hydrophobicity showed interaction with the hydrophobically modified dextrans. The relative hydrophobicity of the proteins studied was estimated by comparing the partition coefficients between two-phase systems without and with hydrophobically modified dextrans. Thylakoid membrane vesicles partitioned to the phases which contained benzoyl dextran or valeryl dextran. Benzoyl dextran with a high DS (DS=0.18) was used to form an aqueous two-phase system with the ethylene oxide-propylene oxide random copolymer, Ucon. The system was used to purify 3-phosphoglycerate kinase from yeast homogenate. Recycling of both phase-forming polymers was achieved by a combination of temperature-induced phase separation of Ucon and salting out of benzoyl dextran. The modified dextrans were also used to study the interactions between tryptophan residues and aromatic (or aliphatic) groups by partitioning a series of tryptophan-containing compounds. The interaction increased with an increasing number of tryptophan residues in the molecule. Salts at different positions in the Hofmeister series were partitioned in two-phase systems containing hydrophobically modified dextrans. The effect of salts on phase behaviour and on the partitioning of proteins was investigated.