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

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).