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 (ΔrHmθ), entropy change (ΔrSmθ) and free energy changes (ΔrGmθ) were also studied for the interactions, indicating that the formation of the metalloporphyrins–DMMP complex was an exothermic reaction.

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► The molecular interactions of metalloporphyrins (ZnTPP and CoTPP) with DMMP can be investigated by absorption/absorption difference spectroscopy.
► Both ZnTPP and CoTPP binded to DMMP via axial coordination, resulting in the formation of 1:1 metalloporphyrin–DMMP complexes.
► The thermodynamic parameters indicate that the formation of the metalloporphyrins–DMMP complex was an exothermic reaction.
► Based on the determination of dynamic parameters, the interactions of the two metalloporphyrins were different. ZnTPP showed stronger binding capacity than CoTPP.
► The structure–activity relationships between metalloporphyrins and DMMP has been discussed, which will be useful for revealing the mechanism in the detection process.