Effect of amino acids on aggregation behaviour of sodium deoxycholate in solution: a fluorescence study

The influence of three alkaline amino acids, L-lysine (L-Lys), L-arginine (L-Arg) and L-histidine (L-His), on the aggregation behaviour of sodium deoxycholate (NaDC) in phosphate buffer, pH 7.0, was studied at 25 °C. The fluorescence probe technique based on pyrene was employed to determine accurately the critical aggregation concentration (cac), polarity of the microenvironment and aggregation numbers for the NaDC aggregates. The added amino acids can effectively reduce the cac values and micropolarity of NaDC, indicating that it is easier for NaDC to aggregate in a compact manner in the presence of amino acids. The aggregation numbers of NaDC were increased, indicating that more NaDC molecules connect together to form stable aggregates. The performance of L-Arg is similar to that of L-His, and both have a smaller effect on the above parameters than L-Lys. In view of this, it may be inferred that both electrostatic and hydrophobic interaction are responsible for the interaction between NaDC and amino acids in aqueous solution.