Solubility of d‑Histidine in Aqueous Cosolvent Mixtures of N,N‑Dimethylformamide, Ethanol, Dimethyl Sulfoxide, and N‑Methyl-2-pyrrolidone: Determination, Preferential Solvation, and Solvent Effect

This work mainly focused on d-histidine solubility in N,N-dimethylformamide (DMF) + water, ethanol + water, dimethyl sulfoxide (DMSO) + water, and N-methyl-2-pyrrolidone (NMP) + water mixtures. All experiments were performed at temperatures between T = 293.15 and T = 333.15 K at intervals of 5 K by a shake-flask method. The largest solubility of d-histidine was found in the neat solvents of NMP (DMF or DMSO) for the NMP (DMF or DMSO) mixtures and water for the ethanol mixture. The values of mole fraction solubility were well-correlated using the Jouyban–Acree model, which obtained relative average deviation values lower than 2.78% and root-mean-square deviation values lower than 27.43 × 10^–4. We used the inverse Kirkwood–Buff integral method to quantitatively evaluate the local mole fraction of DMF (ethanol, DMSO, or NMP) and water nearby d-histidine. Within the water-rich compositions, d-histidine was preferentially solvated by water for the DMSO and NMP mixtures and by ethanol for the ethanol mixture. Within the other compositions for the three mixtures, d-histidine was preferentially solvated by DMF or NMP in the DMF or NMP + water mixtures and by water in the ethanol + water mixture. Nevertheless, for the aqueous solution of DMSO (1) + water (2) with entire compositions, d-histidine was not preferentially solvated by DMSO or water. Moreover, linear solvation energy relationships were implemented on experimental data to obtain insight into solvent dependence of solubility variation in mixed solvents. It was observed that the basicity of DMSO (NMP and DMF) + water mixtures and the polarity-polarizability of the ethanol + water mixture mainly control the variation in solubility.