Evaluation of carboxylic acid-induced conformational transitions of β-lactoglobulin: Comparison of the alcohol effects on β-lactoglobulin

Conformational transitions of bovine β-lactoglobulin A (β-LG) induced by carboxylic acid were systematically studied by steady-state tryptophan (Trp) fluorescence. The behavior of β-LG denaturation depends upon the species and concentration of carboxylic acid, as well as on the pH of solutions. The order of the effectiveness of the respective carboxylic acids was described as follows: MeCOOH < EtCOOH < PrCOOH < ClEtCOOH < TFA < iBuCOOH < nBuCOOH < PFPA. The conformational change of β-LG through the carboxylic acid-induced transitions of the β-LG conformation were analyzed assuming a two-state mechanism between unfold and native states in order to obtain the m value, a measure of the dependence of the free energy change on the concentration of carboxylic acid. The m values of various carboxylic acids were compared with those of various alcohols based on the role of each group constituting the carboxylic acid and alcohol molecules, namely, the hydrocarbon group, hydroxyl group, halogen substituents, and the carboxyl group. Among these groups, the hydrophobic hydrocarbon groups and halogen substituents contributed positively to the m value, whereas the hydrophilic carboxyl and hydroxyl group contributed negatively. The present results can therefore be interpreted as a simple correlation based on the accessible surface area (ASA) of each groups of carboxylic acids and alcohols. These results suggest that the conformational transition of the protein due to the addition of carboxylic acids and alcohols can be explained both by hydrophobicity as well as clustering effects of each carboxylic acid and alcohol molecule.