Effect of osmolyte or GdnHCl on volumetric properties of aqueous solutions containing cyclic dipeptides

To understand the biomolecular interactions between osmolytes or guanidine hydrochloride (GdnHCl) with proteins, we have measured the densities (ρ) of a homologous series of cyclic dipeptides (CDs) in water and in aqueous solutions of osmolytes or GdnHCl through the high precision vibrating tube digital densitometer up to saturated state, as a function of solute concentrations at 25 °C under atmospheric pressure. The materials investigated in the present study included the CDs of cyclo(Gly-Gly), cyclo(Ala-Gly), cyclo(Ala-Ala), cyclo(Leu-Ala) and cyclo(Val-Val), the osmolytes of trimethylamine N-oxide (TMAO), sarcosine, betaine, proline and sucrose, and the denaturant of GdnHCl. In this series of measurements, the aqueous samples were prepared with various concentrations of the CDs, up to saturated conditions, and as a function of osmolyte or GdnHCl concentrations. The density increments resulting from the addition of the osmolyte or GdnHCl and different model compounds of CDs were investigated, respectively. An empirical linear combination equation with an augmented term to account the interactions between CDs and osmolyte or GdnHCl was used to quantitatively correlate the experimental densities over the entire concentration ranges. The addition of this augmented term significantly improved the correlation of the density data for those systems which were not accurately represented by the linear combination equation.