Exploring the reactivity of β-cyclodextrin-encapsulated anthraquinone-2,6-disulfonate

Abstract

The intracomplex reaction of anthraquinone-2,6-disulfonate (AQDS) in triplet excited state (3AQDS) with beta-cyclodextrin (CD) and the reaction of 3AQDS:CD complex with external quencher, N-Acetyl-l-Tyrosine (TyrO−), have been studied using laser flash photolysis, 1H NMR and 1H CIDNP. The two reactions were found to be competing with each other. The intracomplex hydrogen abstraction previously proposed for 3AQDS:CD complex may represent a two-step process, where at first stage an electron is transferred from the –OH group of CD to 3AQDS followed by a proton transfer and recombination of AQDS and CD radicals resulting in a stable AQDS-CD product. The rate constant of the intracomplex electron transfer from CD to 3AQDS was found to be 6 ± 1 × 106 s−1 and the rate of subsequent proton transfer is 1 ± 0.1 × 106 s−1. The rate constant of electron transfer between TyrO− and 3AQDS:CD complex (0.4 ± 0.1 × 109 M−1 s−1) drops approx. by half compared to that in the reaction between TyrO− and free 3AQDS (0.8 ± 0.1 × 109 M−1 s−1).