Hydrazinoanthrylboronic acids as exciton‐coupled circular dichroism (ECCD) probes for multivalent catechols, particularly epigallocatechin gallate

Abstract

We report the use of exciton-coupled circular dichroism (ECCD) spectroscopy in multianalyte sensing systems in complex matrices. To prepare ECCD sensors, anionic anthrylhydazides are reacted with formylphenylboronic acids to give hydrazinoanthrylboronic acids that in turn are reacted with multivalent catechols in aqueous solution. The ECCD signal between the anthracene chromophores in the resulting boronate ester products depends strongly on the structure of the boronic acid sensor and the polyphenol analyte. This dependence of the ECCD signal on analyte structure is interesting for sensing applications. Best ECCD response is found for epigallocatechin gallate (EGCG), a key polyphenol in green tea. Weakly bell-shaped pH profiles, sensitivity to ionic strength and decreasing ECCD with decreasing solvent polarity imply that the CD active product is stabilized by hydrophobic interactions between the anthracene chromophores and by formation of the conjugate bases of the boronic esters. Analyte screening reveals selectivity for divalent catechols, with effective concentrations down to EC(50) = 5 microM for EGCG. Monovalent or achiral catechols such as (+)-catechin, protocatechuate or homoprotocatechuate are not detected. However, the latter two become detectable when attached to a chiral, divalent 1,2-cyclohexylamine scaffold. Application of this simple, user-friendly ECCD system to polyphenol sensing in various green tea extracts delivers easily accessible and reproducible values in the expected range.