Kinetics, Equilibrium, and Thermodynamics for Conjugation of Chitosan with Insulin-Mimetic [ meso-Tetrakis(4-sulfonatophenyl)porphyrinato]oxovanadate(IV)(4-) in an Aqueous Solution

Abstract

This study investigated the conjugation of chitosan with the insulin-mimetic [meso-tetrakis(4-sulfonatophenyl)porphyrinato]oxovanadate(IV)(4-), VO(tpps), in an aqueous medium as a function of conjugation time, VO(tpps) concentrations, and temperatures. To validate the synthesis of chitosan-VO(tpps) conjugate, UV-visible and Fourier transform infrared spectrophotometric techniques were utilized. Conjugate formation is ascribed to the electrostatic interaction between the NH₃⁺ units of chitosan and the SO₃^- units of VO(tpps). Chitosan enhances the stability of VO(tpps) in an aqueous medium (pH 2.5). VO(tpps) conjugation with chitosan was best explained by pseudo-second-order kinetic and Langmuir isotherm models based on kinetic and isotherm studies. The Langmuir equation determined that the maximal ability of VO(tpps) conjugated with each gram of chitosan was 39.22 μmol at a solution temperature of 45 °C. Activation energy and thermodynamic studies (E_a: 8.78 kJ/mol, ΔG: -24.52 to -27.55 kJ/mol, ΔS: 204.22 J/(mol K), and ΔH: 37.30 kJ/mol) reveal that conjugation is endothermic and physical in nature. The discharge of VO(tpps) from conjugate was analyzed in freshly prepared 0.1 mol/L phosphate buffer (pH 7.4) at 37 °C. The release of VO(tpps) from the conjugate is a two-phase process best explained by the Higuchi model, according to a kinetic analysis of the release data. Taking into consideration all experimental findings, it is proposed that chitosan can be used to formulate both solid and liquid insulin-mimetic chitosan-VO(tpps) conjugates.

Figure 1

Structure of chitosan (a) and [meso-tetrakis(4-sulfonatophenyl)porphyrinato]oxovanadate (IV) (4−), VO(tpps) (b).

Figure 2

UV–vis absorption spectra of 0.5% (w/v) chitosan (a), 2 μmol/L VO(tpps) (b), and a chitosan-VO(tpps) conjugate (c) in an aqueous environment (pH 2.5).

Figure 3

Chitosan (a), VOtpps (b), and chitosan-VOtpps complex (c) FTIR spectra noted in KBr.

Figure 4

Proposed structure of the chitosan-VO(tpps) conjugate.

Figure 5

(a) Kinetic UV–vis absorption spectra of chitosan-VO(tpps) conjugate in an aqueous environment (pH 2.5) at various times. The conjugate absorption spectra were noted at 0, 6, 12, 24, 48, 72, 96, and 120 h. The VO(tpps) concentration in chitosan solution (0.5% (w/v); pH 2.5) was 2.5 μmol/L. (b) Kinetic trace at 413 nm of the chitosan-VO(tpps) conjugate. The data are extracted from Figure 5a.

Figure 6

Influence of contact time on kinetics and quantity of VO(tpps) bound to chitosan in aqueous solution at the temperature of 30 °C (volume of solution: 40 mL, chitosan: 0.5% w/v, [VO(tpps)]₀: 4 μmol/L). The solid line was generated numerically using pseudo-second-order kinetics eq 2, and the values of k₂ and q_e(cal) are stated in Table 1.

Figure 7

VO(tpps) conjugation with chitosan in aqueous solution at 30 °C (volume of solution: 40 mL; chitosan: 0.5% w/v; [VO(tpps)]₀: △: 2 μmol/L; ▲: 4 μmol/L; □: 6 μmol/L; ■: 8 μmol/L; and ○: 10 μmol/L. The solid line was calculated numerically using pseudo-second-order kinetic eq 2 and the values of k₂ and q_e(cal) from Table 1.

Figure 8

Influence of solution temperature on the kinetics and quantity of VO(tpps) bound to chitosan in an aqueous environment (volume of solution: 40 mL; chitosan: 0.05% w/v; [VO(tpps)]₀: 4 μmol/L; solution temperature: △, 30 °C; ▲, 35 °C; □, 40 °C; ■: 45 °C). The solid line was calculated numerically using pseudo-second-order kinetic eq 2 and the values of k₂ and q_e(cal) from Table 1.

Figure 9

Characteristic charts of ln(1 – F) versus t (a) and q_t versus t^0.5 (b) for chitosan-VO(tpps) conjugation in the aqueous environment at five different concentrations of VO(tpps) (solution volume: 40 mL; chitosan: 0.5% w/v; [VO(tpps)]₀: △: 2 μmol/L; ▲: 4 μmol/L; □: 6 μmol/L; ■: 8 μmol/L; ○: 10 μmol/L).

Figure 10

Conjugation isotherms of chitosan-VO(tpps) conjugate in aqueous medium at various temperatures (volume of solution: 40 mL; [VO(tpps)]₀: 2–10 μmol/L; chitosan concentration: 0.5% w/v; solution temperature: ○: 30 °C; ●: 35 °C; △: 40 °C; ▲: 45 °C). The Langmuir isotherm eq 12 is used to generate all solid lines numerically.

Figure 11

(a) Usual UV–vis spectra of VO(tpps) liberated from chitosan-VO(tpps) conjugate at different intervals of time in 0.1 mol/L phosphate buffer (pH 7.4) at 37 °C; (b) in vitro discharge profile of VO(tpps) from chitosan-VO(tpps) conjugate in 0.1 mol/L phosphate buffer (pH 7.4) at 37 °C; (c) plot of log [residual VO(tpps) (%)] vs time (data are taken from Figure 11b); and (d) plot of VO(tpps) release (%) vs square root of time (data are taken from Figure 11b).