The effects of ionic strength and temperature on the dissociation constants of adefovir and cidofovir used as antiviral drugs

The effects of ionic strength and temperature on the dissociation constants of adefovir (PMEA) and cidofovir (HPMPC) used as antiviral drugs were studied at 298 K, 308 K, and 318 K in aqueous media and at different ionic strength backgrounds of NaCl potentiometrically. The dissociation constants of the ligands were determined via the calculation of the titration data with the SUPERQUAD computer program. The thermodynamic parameters ( ∆G , ∆H , and ∆S) for all species were calculated. The dissociation order of nitrogen and oxygen atoms in the ligands according to proton affinities values were obtained using PM6 semiempirical methods. Moreover, pKa values of the ligands were determined at 0.00, 0.10, 0.15, 0.20, and 0.5 mol dm ^−3 ionic strength (NaCl) at 298 K. Consequently, when the ionic strength and temperature in the titration cells were increased, the obtained dissociation constants of PMEA (pKa3 , pKa4 , and p Ka5) and HPMPC (p Ka2 and pKa3) decreased.