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

The interaction of monosulfonate tetraphenyl porphyrin (H(2)TPPS(1)) 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 H(2)TPPS(1) resulted from the formation of a dye-esterase complex. According to the modified Stern-Volmer equation, the effective quenching constants (K(a)) between H(2)TPPS(1) and plant-esterase at four different temperatures (297 K, 300 K, 303 K, and 306 K) were obtained to be 14.132×10(5), 5.734×10(5), 2.907×10(5), and 2.291×10(5) 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 H(2)TPPS(1) to plant-esterase primarily took place in the active site. The binding distance (r) was obtained to be 5.99 nm according to F?rster 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.