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.

Colorful surface architectures with three different types of dynamic covalent bonds: integration of anthocyanins, tritylium ions and flavins

“…This finding suggested that in the confined space of multicomponent surface architectures S1H2B6, boronic-ester formation occurs always with both catechols in PV B5, thus stabilizing the colorful but intrinsically disfavored tritylium cation in PV B5 (Fig. 23 Constraints by the confined space 14a or, less likely, 14d,e kinetic traps from strong binding could account for this relatively low η MAX . In solution, boronic-ester formation always occurred through the monoester *H2B5 (Fig.…”

Complex Functional Systems with Three Different Types of Dynamic Covalent Bonds

“…High activity and incomplete co-TSA were both consistent with the large size and the multivalencyo fB5 (Figure 1). [7] Polyphenon 60 (B6), ac ommercially available green-tea extract, inhibited TSA of B1 to S1H3 with an IC 50 = 5.1 mg mL À1 (Figure 3d;T able 1, entry 11). This intermediate activity was very meaningful considering that the weaker catechin (B4)a nd the stronger epigallocatechin gallate (B5)a re the main polyphenols present in green tea.…”

“…[7] Polyphenon 60 (B6), ac ommercially available green-tea extract, inhibited TSA of B1 to S1H3 with an IC 50 = 5.1 mg mL À1 (Figure 3d;T able 1, entry 11). [7] Taken together,t hese results demonstrate the existence and functional relevance of multicomponent architectures that operate with three different dynamic covalent bonds, namely disulfides,h ydrazones,a nd boronic esters.P ossible applications are diverse;directionality and compatibility with optoelectronic signal generation, transduction and detection, interfacing, and device fabrication appear particularly advantageous. Compatibility of dose-response curves for polyphenon 60 with enzymatic signal generation for polyphenol sensing applications has been demonstrated previously in adifferent context.…”

“…was general and occurred in quantitative yield ( Figure S1). Among other boronic acids tested, the ones in the ortho position to aldehydes [7,8] did not perform as well, presumably because of unfavorable topology and direct dependence of boronic ester exchange on hydrazone exchange. Among other boronic acids tested, the ones in the ortho position to aldehydes [7,8] did not perform as well, presumably because of unfavorable topology and direct dependence of boronic ester exchange on hydrazone exchange.…”

“…[1] In sharp contrast, only af ew pioneering examples exist for two types of dynamic covalent bonds that work together. [2][3][4][5][6] Additional promising alternatives include boronic ester, [7][8][9][10] thioester, [11] amide, [11,12] imine [8,[13][14][15] and hemiaminal exchange, [16] Michael adducts, [17] olefin metathesis, [15] and Cope rearrangements. [2][3][4][5][6] Additional promising alternatives include boronic ester, [7][8][9][10] thioester, [11] amide, [11,12] imine [8,[13][14][15] and hemiaminal exchange, [16] Michael adducts, [17] olefin metathesis, [15] and Cope rearrangements.…”