Thermodynamics of the interactions of some amino acids and peptides with dodecyltrimethylammonium bromide and tetradecyltrimethylammonium bromide

• Interactions of amino acids and peptides were studied with two cationic surfactants.
• Partial molar properties and hydration numbers did not change significantly.
• Measured properties indicate balance of polar and non-polar interactions.
• Peptide bonds did not strengthen the extent of polar interactions with surfactant.
• Results provide quantitative fine details of cationic surfactant–amino acids/peptides interactions.

The values of apparent molar volume V2,ϕV2,ϕ and apparent molar adiabatic compressibility KS,2,ϕKS,2,ϕ of amino acids glycine, l-alanine, dl-α-amino-n-butyric acid, l-valine, l-leucine and peptides glycyl-glycine, glycyl-glycyl-glycine and glycyl-leucine have been determined in aqueous solutions of cationic surfactants dodecyltrimethylammonium bromide (DTAB) and tetradecyltrimethylammonium bromide (TTAB) by means of density and sound velocity measurements. The heat evolved or absorbed (q) during the course of interactions of amino acids and peptides with the aqueous solutions of surfactants were determined by isothermal titration calorimetry at T = 298.15 K. The values of standard partial molar volume V2,m0 and standard partial molar adiabatic compressibility Ks,2,m0 at infinite dilution were calculated from the values of V2,ϕV2,ϕ and KS,2,ϕKS,2,ϕ. Similarly the values of limiting enthalpies of dilution (ΔdilH0ΔdilH0) of the amino acids/peptides were calculated from heat evolved or absorbed during calorimetric experiments. The standard partial molar quantities of transfer from water to aqueous surfactant solutions have been used to identify the interactions of amino acids and peptides with surfactants in terms of ionic–ionic, ionic–hydrophobic and hydrophobic–hydrophobic group interactions.

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