Modeling Boronic Acid Based Fluorescent Saccharide Sensors: Computational Investigation of d-Fructose Binding to Dimethylaminomethylphenylboronic Acid

Design and Synthesis of Novel Di-Boronic Acid-Based Chemical Glucose Sensors

Chemical-based fluorescent sensors with the capability of long-term stability and low cost are promising agents in clinical diagnosis and medical research. Measuring glucose levels inside cells and their surroundings provides insight into cellular metabolic homeostasis and may be employed as an indicator for potential pathological conditions. Anthracene-based diboronic acid (BA) derivatives offer a reversible and covalent binding mechanism for glucose recognition, which enables robust and continuous glucose monitoring. To improve its poor solubility and biological applicability, a diboronic acid chemical structure design was explored. To date, several anthracene-based ortho-amino methylphenyl boronic acid glucose-sensors have been developed. Most recently, the structure of Mc-CDBA (((((2-(methoxycarbonyl) anthracene-9,10-diyl) bis (methylene)) bis(methylazanediyl)) bis(methylene)) bis(4-cyano-2,1-phenylene)) diboronic acid was disclosed. Mc-CDBA exhibits suitable water-solubility and sensitivity toward glucose, with limited modification sites and suitability to extra-cellular applications. Here, we present a palette of Mc-CDBA derivatives: carboxylic (BA), amid (BA 5) and acryl (BA 21)-based Mc-CDBA sensors for extra- and intracellular glucose monitoring, respectively. The developed chemical glucose sensors were designed to obtain a final product with fewer synthetic steps, allowing easier scale-up capacity. Moreover, we showed that ortho-amino site modifications do not interfere with the sensor activity, allowing alternative water solubility solutions without chemically modifying the chromophore/aromatic subunits within the molecule. Among these probes, we also developed an extracellular hydrogel-embedded sensor (BA 21) to monitor extracellular glucose levels under persistent solution flow, a feature that is lacking in other glucose sensors. The synthesized derivatives could serve as diverse fluorescent sensors for glucose monitoring in medical applications.

Progress in the preparation and evaluation of glucose-sensitive microneedle systems and their blood glucose regulation

Glucose-sensitive microneedle systems (GSMSs) as an intelligent strategy for treating diabetes can well solve the problems of puncture pain, hypoglycemia, skin damage, and complications caused by the subcutaneous injection of insulin. According to the various functions of each component, herein, therapeutic GSMSs are reviewed based on three parts (glucose-sensitive models, diabetes medications, and microneedle body). Moreover, the characteristics, benefits, and drawbacks of three types of typical glucose-sensitive models (phenylboronic acid based polymer, glucose oxidase, and concanavalin A) and their drug delivery models are reviewed. In particular, phenylboronic acid-based GSMSs can provide a long-acting drug dose and controlled release rate for the treatment of diabetes. Moreover, their painless, minimally invasive puncture also greatly improves patient compliance, treatment safety, and potential application prospects.

Probing the Role of the Bridging Nitrogen in the Signaling Mechanism of an Anthracene-Boronic Acid Sugar Sensor and a Different Version of the PET-Based Mechanism

The N-quaternized derivative 5 of the James-Shinkai anthracene-boronic acid fluorescence sugar sensor 1 was prepared to probe the role of the bridging nitrogen in the signaling mechanism of 1. Both 5 and 1 contain positively charged bridging groups NMe⁺ or NH⁺, respectively, but 5 lacks the ability to form the intramolecular ammonium-boronate doubly ionic hydrogen bond present in 1. Receptors 1 and 5 display opposite fluorescence vs pH profiles with a small turn-on effect of the sugar binding to the zwitterion of 5 in contrast to a large effect observed with 1. It is concluded that the ammonium-boronate hydrogen bond is essential for the signaling mechanism of 1. Its possible function is enabling the PET quenching effect by shifting the NH⁺ proton toward boronate anion inside the hydrogen bond, the degree of which is modulated by the ester formation with diols affecting the basicity of boronate anion. This mechanism agrees with observed signaling selectivity of 1 toward a series of di- and polyols of variable structures as well as with the behavior of 1 in buffered D₂O and methanol solvents at controlled pH and provides an addition to the established "loose bolt" mechanism signaling mode essential for receptors with nonpolar fluorophores.

The mechanisms of boronate ester formation and fluorescent turn-on in ortho-aminomethylphenylboronic acids

ortho-Aminomethylphenylboronic acids are used in receptors for carbohydrates and various other compounds containing vicinal diols. The presence of the o-aminomethyl group enhances the affinity towards diols at neutral pH, and the manner in which this group plays this role has been a topic of debate. Further, the aminomethyl group is believed to be involved in the turn-on of the emission properties of appended fluorophores upon diol binding. In this treatise, a uniform picture emerges for the role of this group: it primarily acts as an electron-withdrawing group that lowers the pKa of the neighbouring boronic acid thereby facilitating diol binding at neutral pH. The amine appears to play no role in the modulation of the fluorescence of appended fluorophores in the protic-solvent-inserted form of the boronic acid/boronate ester. Instead, fluorescence turn-on can be consistently tied to vibrational-coupled excited-state relaxation (a loose-bolt effect). Overall, this Review unifies and discusses the existing data as of 2019 whilst also highlighting why o-aminomethyl groups are so widely used, and the role they play in carbohydrate sensing using phenylboronic acids.