Solubility and Thermodynamics of Dissolution of 13,17-Di-N-(2-aminoethyl) amide of Deuteroporphyrin-IX in Aqueous HCl and Tetraoxalate Buffer at 288-328 K

This paper describes the thermodynamic study of aqueous solutions of 13,17-di-N-(2-aminoethyl)amide of deuteroporphyrin-IX( DDA) which is considered as a potential red natural dye and a perspective substrate for creating new water-soluble sensitizers for photodynamic therapy (PDT). Solubility of the macrocycle in highly aqueous solutions of hydrochloric acid and standard tetraoxalate buffer has been determined in the temperature range from T-288 K to T-328 K. Solubility values are found to decrease with the temperature in both cases due to the weakening of porphyrin basicity at elevated temperatures. Free energies, enthalpies and entropies of solution have been computed from solubility data via the Clark and Glue equation. Enthalpies and entropies of solution are negative and constant for both cases in the temperature range studied. The negative enthalpic term favors DDA dissolution, but large and negative entropies of solution induce the overall unfavorable free energy change.