Physico-chemical studies on the charge-transfer complex formed between sulfadoxine and pyrimethamine with chloranilic acid.

Thermodynamic studies on the charge-transfer complexes of sulfadoxine and pyrimethamine with chloranilic acid in non-aqueous 1,4-dioxan were investigated by spectrophotometric method. The absorption maxima for sulfadoxine and pyrimethamine were observed at 500 and 520 nm, respectively. We observed a bathochromic shift in the transitions of these complexes with respect to chloranilic acid, which absorb at 420 nm. The enthalpy of complexation was found to be -3.044 kJ/mole for pyrimethamine and -5.934 kJ/mole for sulfadoxine. Equilibrium constant values were generally high, that is 261.202 l/mole and 344.29 l/mole for pyrimethamine and sulfadoxine, respectively, at 303 K. Free energy values were negative, of the order of -14.420 kJ/mole for pyrimethamine and -15.384 kJ/mole for sulfadoxine, indicating that complex formations were exothermic. These data suggest that the complexes are very stable. Arguments are presented to support the application of charge-transfer interactions in the quantitative analysis of these drugs.