The protective effects of cyclodextrins against the oxidation of methyl orange by singlet oxygen.

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The degradation of Methyl Orange (MO) by singlet oxygen was inhibited by the addition of α-, β-, and γ-cyclodextrins (CD's) to an aqueous solution. The protective effect of the CD's against the MO oxidation decreased in this order; α->γ-\gtrsimβ-CD. From the relation between the rate constant of MO oxidation and the free MO concentration, evaluated by using the formation constants of CD's with MO, it was found that the MO included by α and γ-CD's is not decomposed by singlet oxygen, whereas the MO included by β-CD is oxidized by singlet oxygen. The inner cavity of α- or γ-CD well fit for the inclusion of one or two MO molecules respectively, and the access of singlet oxygen to the azo group of the included MO molecule is sterically inhibited. The finding that β-CD has smaller enthalpy and larger entropy changes relevant to the formation of the MO–CD complex than those of α- and γ-CD's suggested the less tight inclusion of one MO molecule in the cavity of β-CD. Such a loose inclusion of MO by β-CD resulted in the transient exposition of the azo group of the MO molecule to the bulk phase: the attack of the singlet oxygen on the azo group followed.
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