Volumetric Properties of Amino Acids and Hen-Egg White Lysozyme in Aqueous Triton X-100 at 298.15 K

The apparent molar volumes, Vφ,2, of glycine, alanine, α-amino-n-butyric acid, valine, leucine, and lysine monohydrochloride have been determined in aqueous solutions of 0.05, 0.1, and 0.4 mol-kg−1 Triton X-100 (TX-100), and the partial specific volume, v0, of hen-egg-white lysozyme in 0.4 mol-kg−1 TX-100 by density measurements at 298.15 K. These data have been used to calculate the infinite dilution apparent molar volumes, V2,m0, for the amino acids in aqueous TX-100 solutions and the standard partial molar volumes of transfer, ΔtrV2,m0, of the amino acids from water to the aqueous surfactant solutions. The linear correlation of V2,m0 for a homologous series of amino acids has been utilized to calculate the contribution of the charged end groups (NH3+, COO−), CH2 group and other alkyl chains of the amino acids to V2,m0. The results on Δ trV2,m0, of amino acids from water to aqueous TX-100 solutions have been interpreted in terms of ion–ion, ion–polar, hydrophilic–hydrophilic and hydrophobic–hydrophobic group interactions. For all the six amino acids studied, the values of ΔtrV2,m0 from water to all the studied concentrations of aqueous TX-100 are small in spite of their different hydrophobic content, indicating an overall balance in interactions of zwitterionic/hydrophilic groups of amino acids with the hydrophilic groups of TX-100, and of hydrophobic and ionic/hydrophilic groups of the amino acids with hydrophobic groups of TX-100. Comparison of the interactions of the amino acids with nonionic, anionic and cationic surfactants has also been made and discussed. The partial specific volume of transfer of lysozyme from water to aqueous TX-100 solutions also indicates a balance of the hydrophobic and hydrophilic interactions in the protein–nonionic surfactant system.