Thermodynamics of protein model compounds: apparent molar volumes and isobaric heat capacities of selected cyclic dipeptides and their transfer properties from water to aqueous urea solutions at T = 298.15 K

Abstract

The effects of added protein denaturant (urea) on the volumetric and thermochemical properties of several protein model compounds (cyclic dipeptides) have been investigated at T = 298.15 K and p = 0.1 MPa. Relative densities and specific heat capacities are reported for the cyclic dipeptides cyclo-glycylglycine, cyclo-alanylalanine and cyclo-sarcosylsarcosine in aqueous urea solutions in the concentration range 1 ≤ m(urea)/mol kg⁻¹ ≤ 11. The measurements were performed using a Sodev O2D vibrating tube densimeter and a Picker dynamic microcalorimeter. Apparent molar volumes and heat capacities have been calculated and their concentration dependences have been modeled to give partial molar properties at infinite dilution. The partial molar properties have been used to calculate thermodynamic parameters describing the transfer of the cyclic dipeptides from water to aqueous urea solutions. Estimates of transfer properties for the glycyl group and the alanyl side chain have been obtained using the principles of group additivity. The interaction of urea with the investigated protein model compounds is discussed in terms of the role of urea as a protein denaturant.