Abstract
Different mechanisms for the emission turn-on of ortho-aminomethylphenylboronic acids with appended fluorophores in response to saccharide binding in aqueous media have been postulated, such as photoinduced electron transfer (PET), "pKa switch", and disaggregation. However, none of the hypotheses is consistent with all the data for boronic acid-based sensors. To create a unifying theory that can explain the data, we performed a series of experiments to explore the origin of the emission turn-on with several boronic-acid based sensors upon binding fructose. First, we showed that the receptors and their complexes with fructose are solvent-inserted, with no B-N interactions. Second, we verified that the sensors are not aggregated. Third, in pure methanol, that exchanges -B(OH)2 to -B(OMe)2 groups, we found no fluorescence response upon binding fructose. We propose this occurs via lessening of internal conversion mechanisms. To investigate this proposal further, we performed a solvent isotope effect study. The fluorescence of the probes in D2O (-B(OH)2 → -B(OD)2) does not change upon fructose binding. It is well accepted that -OD oscillators are less efficient energy acceptors due to their lower frequency vibrational modes. Thus, our studies reveal that modulating the -B(OH)2-induced internal conversion (an example of a "loose bolt effect") explains how potentially all ortho-aminomethylphenylboronic acid-based fluorescence sensors signal the presence of sugars.
Highlights
Boronic acids groups are the most common functionalities introduced into unnatural receptors to ensure some degree of vicinal diol, catechol, or α-hydroxy carboxylate binding in water.[1,2] they are the most-commonly used functional groups in fluorescent saccharide sensing.[3]
A photoinduced electron transfer (PET) mechanism was proposed based on a modulation of the strength of a Lewisacid/base interaction between the boronic acid/boronate ester with the nitrogen lone pair on the o-aminomethyl group (Scheme 1A).[4,5,11]
Functional N-substituted-1,8-naphthalimide derivatives have been successfully employed as DNA targeting, anticancer, and cellular imaging agents.[24,25]. Due to their excellent photophysical and photochemical properties, N-substituted-1,8-naphthalimides are widely used as D-π-A chromophores in the design of fluorescent probes.[26]
Summary
Boronic acids groups are the most common functionalities introduced into unnatural receptors to ensure some degree of vicinal diol, catechol, or α-hydroxy carboxylate binding in water.[1,2] they are the most-commonly used functional groups in fluorescent saccharide sensing.[3]. The 11B NMR studies on 2 revealed a 2 ppm change, indicating binding occurs between probe 2 and fructose in methanol (Figure 3B) This is the same phenomenon we previously reported for compound 1, which indicated that 1 is not aggregated in methanol. While water and alcohols (R−OH) are effective at this form of quenching,[32] our studies reveal that B−OH groups can play exactly the same role
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