Interaction of monosulfonate tetraphenyl porphyrin (H2TPPS1) with plant-esterase: Determination of the binding mechanism by spectroscopic methods

The interaction of monosulfonate tetraphenyl porphyrin (H2TPPS1) with plant-esterase was investigated using fluorescence and UV–vis absorption spectroscopy. Fluorescence quenching, from which the binding parameters were evaluated, revealed that the quenching of the esterase by H2TPPS1 resulted from the formation of a dye–esterase complex. According to the modified Stern–Volmer equation, the effective quenching constants (Ka) between H2TPPS1 and plant-esterase at four different temperatures (297 K, 300 K, 303 K, and 306 K) were obtained to be 14.132 × 105, 5.734 × 105, 2.907 × 105, and 2.291 × 105 M−1, respectively. The thermodynamic parameters, enthalpy change (ΔH) and entropy change (ΔS) for the reaction were calculated to be −181.67 kJ M−1 and −0.49 kJ M−1 K−1, indicating that van der Waals force and hydrogen bonds were the dominant intermolecular force in stabilizing the complex. Site marker competitive experiments showed that the binding of H2TPPS1 to plant-esterase primarily took place in the active site. The binding distance (r) was obtained to be 5.99 nm according to Förster theory of non-radioactive energy transfer. The conformation of plant-esterase was investigated by synchronous fluorescence and UV–vis absorption spectroscopy, and the results confirmed some micro-environmental and conformational changes of plant-esterase molecules.