Thermodynamics of the interactions of a homologous series of some amino acids with trimethylamine N-oxide: Volumetric, compressibility, and calorimetric studies

Abstract

The values of apparent molar volume V 2 ,ϕ and apparent molar compressibility K S, 2 ,ϕ of glycine, l-alanine, dl-α-amino-n-butyric acid, l-valine, and l-leucine have been determined in the aqueous solution of 1 mol · kg −1 and 2 mol · kg −1 trimethylamine N-oxide (TMAO) solutions by density and sound velocity measurements. Isothermal titration calorimetry has been employed to determine the values of heats of dilution q of the aqueous solutions of these amino acids in TMAO at temperatures from T = 288.15 K to T = 308.15 K. These data have been used to calculate values of the infinite dilution standard partial molar volume ( V 2 , m ∘ ), standard partial molar isentropic compressibility ( K S , 2 , m ∘ ) and limiting enthalpy of dilution (Δ dilH °) of the amino acids in aqueous TMAO solutions. The standard partial molar volumes of transfer ( Δ tr V 2 , m ∘ ), isentropic compressibility of transfer ( Δ tr K S , 2 , m ∘ ), and enthalpy of dilution of transfer (Δ tr Δ dilH °) of amino acids from water to aqueous TMAO solutions have been calculated from the measured quantities for these thermodynamic quantities. The linear correlation of V 2 , m ∘ for a homologous series of amino acids has been utilized to calculate the contribution of the charged end groups ( NH 3 + , COO −), CH 2 groups, and the other alkyl chains of the amino acids to V 2 , m ∘ . The results for the partial molar properties of transfer from water to aqueous TMAO solutions have been interpreted in terms of ion–ion, ion–polar, hydrophilic–hydrophobic, and hydrophobic–hydrophobic group interactions. The volume, compressibility, and enthalpy of dilution of transfer data suggest that hydrophobic interactions predominant in this system. It is inferred that the effect of TMAO on protein stability need not be necessarily due to preferential hydration, it can also interact with the hydrophobic groups of the constituents of protein affecting its stability.