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

The values of apparent molar volume V2,ϕ and apparent molar compressibility KS,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 (V2,m∘), standard partial molar isentropic compressibility (KS,2,m∘) and limiting enthalpy of dilution (ΔdilH°) of the amino acids in aqueous TMAO solutions. The standard partial molar volumes of transfer (ΔtrV2,m∘), isentropic compressibility of transfer (ΔtrKS,2,m∘), and enthalpy of dilution of transfer (ΔtrΔdilH°) of amino acids from water to aqueous TMAO solutions have been calculated from the measured quantities for these thermodynamic quantities. The linear correlation of V2,m∘ for a homologous series of amino acids has been utilized to calculate the contribution of the charged end groups (NH3+, COO−), CH2 groups, and the other alkyl chains of the amino acids to V2,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.

► Thermodynamics of interaction of amino acids with trimethy N-oxide (TMAO) studied.
► Partial molar properties in aqueous osmolyte provide interaction details.
► Volumes, compressibilites, enthalpies indicate predominant hydrophobic interactions.
► TMAO exerts its effect both by preferential hydration and hydrophobic interactions.
► Results suggest hydrophobic interactions lead to destabilization of the protein.