Bean Chemsketch Simulation Studies

4.4.4 Markin Bean ChemSketch® Simulation Studies

Marvin Bean ChemSketch® is usually employed to study the pKa of drugs and chemical compounds to determine the ionizable functional group within the compound (Uemera et al., 2009; Manchester et al., 2010). In this study, the Marvin Bean ChemSketch® software was applied for the first time to study the microspecies distribution of TSPP, acetate and citrate at pH range 0-14. As mentioned earlier, the observed optical changes were most likely due to conformational changes in the porphyrin macrocycle which resulted from the protonation and deprotonation processes. The TSPP porphyrin can be easily protonated and deprotonated with the presence of ions when the medium pH was close to its pKa. By studying

This implied that the amount of TSPP with a charge of 2 in the solution was decreased, while the amount of TSPP with a charge of –4 was increased. Hence, it can be inferred that upon the addition of acetate or citrate ion, the pH of the TSPP solution was increased. Consequently, the –2 charged TSPP was converted to one with a charge of –4 which resulted in the blue shift and the decrease of the Soret band. This inference was well supported by the acetate and citrate ions microspecies distributions predicted by Marvin Bean ChemSketch®. Figure 4.22 showed the predicted microspecies distribution of acetate at pH 0-14. The pKa value of acetate ion calculated by Marvin Bean ChemSketch® is 4.75. In the pH 4 TSPP solution, the negatively charged citrate ion was dominant. This –1 charged species reacts with the free H+ ions in the pH 4 TSPP solution to form CH3COOH (aq) (Equation 4.1). When more the acetate was added into the solution, the free H+ ions consumption correspondingly increased. The decrease of H+ ions increased the pH of the solution which consequently led to the deprotonation of TSPP. Eventually, the changes