Abstract
The reaction between the thionine (Th) and the ribose was observed spectrophotometrically and changes in absorbance of Th were recorded at variable concentration of dye, reductant and pH. A pseudo first order rate of reaction was found to establish the reduction kinetics of the dye, studied at a pH range of 0.34 to 12.8. Absorption spectrum of Th at different pH, with ribose showed a pH (12.8) dependent introversion. The reduction most probably took place with enediol intermediate of the sugar at high pH. A full geometry optimization of predominant species of Th namely, mono-deprotonated, di-deprotonated Th, and LTh (leuco thionine) respectively, at low and high pH, was performed at B3LYP level of theory. The data obtained from the energy minimization were in excellent agreement with other experimental and theoretical observations. The calculated enthalpies of formation for both reduction reactions (mono-deprotonated Th+H+→leucothionine and di-deprotonated Th+2H+→leucothionine) provided evidences for maximum reduction of the dye at high pH.
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