Conjugation of insulin-mimetic [ meso -tetrakis(4-sulfonatophenyl)porphyrinato]zinc(II) with chitosan in aqueous solution: kinetics, equilibrium and thermodynamics

Conjugation of insulin-mimetic [meso-tetrakis(4-sulfonatophenyl)porphyrinato]Zn(II), Zn(tpps), with chitosan was examined in aqueous solution at various contact time, Zn(tpps) concentrations and solution temperatures, respectively. The chitosan–Zn(tpps) conjugate was characterized by UV–Vis and Fourier transform infrared (FTIR) spectroscopic techniques. The electrostatic interaction between negatively charged sulfonate $$({\mathrm{SO}}_{3}^{-})$$ groups of Zn(tpps) and positively charged amino ( $${\mathrm{NH}}_{3}^{+}$$ ) groups of chitosan is responsible for the formation of chitosan–Zn(tpps) conjugate. Chitosan efficiently inhibits the demetallization and aggregation of Zn(tpps) in acidic aqueous solution. The conjugation kinetic data taken from various batch studies were examined by the pseudo-first-order, pseudo-second-order, Elovich kinetic, film diffusion and intra-particle diffusion models. The equilibrium conjugation isotherms were analyzed by Freundlich, Temkin and Langmuir isotherm models, respectively. The batch conjugation kinetic data were obeyed by the pseudo-second-order kinetic model rather than the pseudo-first-order and Elovich kinetic models. Equilibrium conjugation isotherms were explained well by the Langmuir isotherm model, and the highest extent of Zn(tpps) bound to chitosan was obtained to be 151.52 µmol/g at 45 °C. The activation energy (Ea: 10.21 kJ/mol), changes in Gibb’s free energy (∆G: –26.38 to –28.12 kJ/mol), enthalpy (∆H: 8.74 kJ/mol) and entropy (∆S: 115.94 J/mol K) suggest that the chitosan–Zn(tpps) conjugation is an endothermic spontaneous process. The in vitro release of Zn(tpps) from chitosan–Zn(tpps) conjugate was investigated in phosphate buffer solution (pH 7.4) at 37 °C. The kinetics of Zn(tpps) release followed the first-order kinetics with a rate constant 0.0025 h‒1. The chitosan–Zn(tpps) conjugate may be used as an oral therapeutics for diabetes mellitus.