Stability and Properties of Ultraviolet Filter Avobenzone under Its Diketo/Enol Tautomerization Induced by Molecular Encapsulation with β-Cyclodextrin

Inclusion complexation of the sunscreen ingredient avobenzone (AVB) with β-cyclodextrin (β-CD) was investigated to improve its aqueous solubility and photostability; another ultraviolet (UV) filter, oxybenzone (OXB), and the phytochemical antioxidant curcumin (CUR) served as a comparison. In this study, the 1-octanol/water partition coefficients, acid dissociation constants, phase-solubility diagrams with β-CD, and ultraviolet–visible (UV–vis) spectral changes induced by UVA1 (365 nm) irradiation were evaluated. β-CD at concentrations 50–100 times that of AVB most effectively protected the photostability of AVB. Additionally, an UVA1-insensitive species with a diketo tautomer, which has an UVC-absorbing band and the potential to cause photodegradation, was stored in the inclusion complex. Acetonitrile–water mixtures at various volume ratios were screened to mimic the internal cavity of β-CD for the AVB tautomeric species using nuclear magnetic resonance (NMR) spectral integrals for the components. The results indicated that β-CD provides a hydrophobic environment similar to that of a 40–50% acetonitrile aqueous solution and enhances the photostability of AVB. However, excess β-CD induced a hyperchromic effect on the diketo tautomer. Aggregation of the AVB/β-CD inclusion complexes at β-CD concentrations of ≥2 mM enhances UVC band absorption. To avoid excess β-CD, a molar ratio of 50–100 of β-CD to AVB is recommended as the optimal composition. This study newly exhibited that the cavity of β-CD mitigates the reactivity of UVA1 toward AVB by inducing the diketo tautomer form of AVB within the cavity.