Nitrogen-15 NMR Spectroscopy of Sugar Sensor with B–N Interaction as a Key Regulator of Colorimetric Signals

Abstract

Nitrogen-15 NMR spectroscopy showed strongly upfield values of the chemical shift for one of the azo nitrogen atoms of a boronic acid appended azo dye; this indicated the formation of a boron-nitrogen (B-N) dative bond. The B-N dative bond was cleaved by sugar addition.Boronic acids are widely used for sugar sensors because they react with diol moieties of sugars to form boronate esters. 1 Sugar sensors based on boronic acids require the occurrence of signal changes upon sugar binding. James et al. have developed a novel fluorescent sensor that contains a nitrogen atom at a position adjacent to the borzonic acid group. 2 The sensor compound shows a significant increase in fluorescent intensity upon sugar binding. They proposed that the fluorescent intensity of the sugar sensor was controlled by a boron-nitrogen (B-N) interaction. Since then, the B-N motif has been widely used not only in fluorescence sensors but also in electrochemical and colorimetric sensors. 3 In previous studies, our group has synthesized a colorimetric sugar sensor based on the B-N interaction between boronic acid and azo groups. 4 Although B-N interactions have been recognized as the key regulators of many sensor signals, there are few ways to investigate B-N interactions in solution state, except for 11 B NMR spectroscopy. 5 Unfortunately, 11 B NMR is not able to provide conclusive evidence for the B-N interaction because 11 B chemical shifts only suggest a difference in the hybridization states of boron atoms. When a boron atom is tetrahedral, its chemical shift is upfield from that of the trigonal planar geometry, where pure sp 3 and sp 2 are approximately 0 and 30 ppm, respectively. 5 In many cases, adding sugar induces a change in 11 B NMR spectra of sugar sensors, and this change is interpreted as a change in the B-N interaction. However, it is very difficult to identify the state of the B-N interaction from the limited information about boron hybridization because not only the adjacent nitrogen but also hydroxides, solvent molecules, and sugars interact with the boronic acid group, and all of these interactions are reflected in 11 B chemical shifts. Consequently, B-N interactions of sugar sensors have been investigated and debated for a long time. 5,6 In order to gain insight into B-N interactions of sugar sensors, we utilize 15 N NMR spectroscopy because the formations of coordination bonds are sensitively reflected in the 15 N chemical shifts. 7 We synthesized a 15 N-labelled boronic acid-appended azo dye ( 15 N-BA, Figure 1 (a)) which shows the same character as the colorimetric sugar sensors reported previously. 4 We measured its 15 N NMR spectra and conducted density function theory (DFT) calculations to Figure 1. (a) Structures of 15 N-BA and BA, (b) An energy minimized structure of BA containing a hydrogen bond with a water molecule. Hydrogen bonds are shown in light green.Figure 2. 15 N NMR spectra of 15 N-BA (20 mM) under various conditions: (a) in D2O, (b) in a 1.0 M NaOD D2O solution, (c-e) in a mixed solvent (100 ...

Colorimetric Sugar Sensing Using Boronic Acid-Substituted Azobenzenes

“…We introduced a boronic acid group to the o -position of the azo group. Some o -boronic acid substituted azobenzenes were successfully synthesized with diazo-coupling reactions [ 65 , 66 ]. An azo dye, 6 ( Figure 5 ), which has a basic skeleton of a series of o -boronic acid substituted azobenzenes, shows an absorption maximum at 505 nm in aqueous MeOH, which is significantly red-shifted compared to that of 4-aminoazobenzene (365 nm) [ 67 ].…”

“…In order to gain insight into the B–N interaction, we used 15 N NMR spectroscopy, because the formations of coordination bonds are sensitively reflected in the 15 N chemical shifts [ 70 , 71 ]. We synthesized a 15 N-labelled azo dye ( 8 , Figure 8 ), which corresponds to dye 6 [ 66 ].…”

“… ( a ) UV-visible absorption spectra of dye 6 (10 μM) in different pH solutions (pH 7.0, 10.0, 10.5, 11.0, 11.5, 12.0, 12.5 and 13.0), measured in a methanol/water mixture (1/1, v / v ) containing 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES, 5.0 mM); ( b ) the UV-visible absorption spectra of dye 6 (10 μM) in the presence and absence of D-fructose (0, 1, 2, 5, 10, 20, 50 and 100 mM), measured in a methanol/water mixture (1/1, v / v ) containing N -cyclohexyl-2-aminoethanesulfonic acid (CHES, 5.0 mM), pH 10.0. Reprinted with permission from [ 66 ]. Copyright 2010 The Chemical Society of Japan.…”

“…The 11 B-frequency (0 ppm) is Et 2 O·BF 3 in toluene- d 8 . Reprinted with permission from [ 66 ]. Copyright 2010 The Chemical Society of Japan.…”

o-Azophenylboronic Acid-Based Colorimetric Sensors for d-Fructose: o-Azophenylboronic Acids with Inserted Protic Solvent Are the Key Species for a Large Color Change

“…These sensors exhibit a marked color change caused by the rehybridization of the boron center from sp 2 (boronic acid) to sp 3 (boronate ester) when reacted with sugars. In particular, Egawa et al reported o -azophenylboronic acids ( azoB s) with excellent features for the colorimetric detection of sugars, one of which shows a color change from red (λ max = 521 nm) to yellow (λ max = 398 nm) in response to d -fructose. , Some azoB -based applications have already been established, such as dye-modified polymers, , imprinted hydrogels, and functionalization of Au superparticles . In these reports, the drastic color change upon reaction with d -fructose was attributed to the direct N–B dative bond between the β-N atom of the azo group (Chart ) and the boron center of boronic acid.…”