Molecular interactions of dimethyl methylphosphonate (DMMP) with metalloporphyrins: determination of the binding mechanism by spectroscopic methods.

The molecular interactions of 5,10,15,20-tetraphenylporphine zinc (ZnTPP) and 5,10,15,20-tetraphenyl-21H,23H-porphine cobalt(II) (CoTPP) with dimethyl methylphosphonate (DMMP) have been investigated by absorption/absorption difference spectroscopy. The interactions between the metalloporphyrins and DMMP change the absorbance characteristics of the porphyrins resulted from the formation of the metalloporphyrin-DMMP complexes. According to the Benesi-Hildebrand (B-H) equation, the equilibrium constants and stoichiometries of the binding systems at four different temperatures (288, 293, 298 and 303 K) were obtained. Experimental results showed that both ZnTPP and CoTPP bind to DMMP via axial coordination, resulting in the formation of 1:1 metalloporphyrin-DMMP complexes. However, it was found that ZnTPP showed stronger binding capacity with the equilibrium constant (K) of 83.864 M(-1) at room temperature, while CoTPP exhibited weaker binding with K of 24.904 M(-1). The thermodynamic parameters, enthalpy change (Δ(r)H(m)(θ)), entropy change (Δ(r)S(m)(θ)) and free energy changes (Δ(r)G(m)(θ)) were also studied for the interactions, indicating that the formation of the metalloporphyrins-DMMP complex was an exothermic reaction.