Identification of the Principle of Taste Sensors to Detect Non-Charged Bitter Substances by 1H-NMR Measurement

Abstract

A taste sensor with lipid/polymer membranes is attracting attention as a method to evaluate taste objectively. However, due to the characteristic of detecting taste by changes in membrane potential, taste sensors cannot measure non-charged bitter substances. Many foods and medicines contain non-charged bitter substances, and it is necessary to quantify these tastes with sensors. Therefore, we have been developing taste sensors to detect bitter tastes caused by non-charged substances such as caffeine. In previous studies, a sensor for detecting bitterness caused by caffeine and theobromine, theophylline, was developed, using a membrane modified with hydroxybenzoic acid (HBA) as the sensing part. The sensor was designed to form intramolecular hydrogen bonds (H-bonds) between the hydroxy group and carboxy group of HBA and to successively cause the intermolecular H-bonds between HBA and caffeine molecules to be measured. However, whether this sensing principle is correct or not cannot be confirmed from the results of taste sensor measurements. Therefore, in this study, we explored the interaction between HBA and caffeine by 1H-nuclear magnetic resonance spectroscopy (NMR). By the 1H NMR detection, we confirmed that both the substances interact with each other. Furthermore, the nuclear Overhauser effect (NOE) of intermolecular spatial conformation in solution was measured, by which 2,6-dihydroxybenzoic acid (2,6-DHBA) preferably interacted with caffeine via the H-bonding and stacking configuration between aromatic rings. Identifying the binding form of 2,6-DHBA to caffeine was estimated to predict how the two substances interact.

Development of Taste Sensor with Lipid/Polymer Membranes for Detection of Umami Substances Using Surface Modification

“…Consequently, this returned H + leads to an increase in surface charge density, resulting in an increase in membrane potential. The binding of 2,6-DHBA with caffeine has been confirmed by 1 H-NMR measurement [54]. This novel sensor also has been studied to detect other non-charged bitter substances for medicine, such as theophylline [55].…”

“…Detection of Umami Substances Using Taste Sensors Treated with 2,6-DHBA and 2,6-DHTA First, we employed 2,6-DHBA and 2,6-DHTA as modifies to detect umami. These two materials were utilized to detect non-charged bitterness substances in previous studies [49,[53][54][55]. In contrast to the structure of 2,6-DHBA, which features only one carboxyl group on the benzene ring, 2,6-DHTA contains two carboxyl groups on the para-position of the benzene ring.…”

“…Within this modified membrane, 2,6-DHBA is ionized under a wide range of pH, forming intramolecular H-bond. The ionized 2,6-DHBA can interact with caffeine through its hydroxyl groups [54]. This interaction is achieved by breaking intramolecular Hbonds, forming intermolecular H-bonds with caffeine by taking back dissociated H + from the solution.…”

Research and development of taste sensors as a novel analytical tool

“…The results obtained from the taste sensor and 1 H NMR analysis for the five modifiers are presented in Table 3 . 61 ) It was observed that caffeine interacts with all three HBAs and resorcinol, but the sensor modified with 2,6-DHBA exhibited a substantial response. The membrane potential change was moderate with 2-HBA and almost negligible with 3,5-DHBA, as shown in Table 3 .…”

“…We conducted nuclear Overhauser effect spectroscopy (NOESY) measurement to identify the type of bond formed in the interaction. 61 ) We can predict that caffeine and 2,6-DHBA have a stacked structure as shown in Fig. 9 by taking into account two facts, i.e.…”