Shoichi Nishitani

Understanding the Molecular Structure of the Sialic Acid-Phenylboronic Acid Complex by using a Combined NMR Spectroscopy and DFT Study: Toward Sialic Acid Detection at Cell Membranes

Abstract The origin of the unusually high stability of the sialic acid (SA) and phenylboronic acid (PBA) complex was investigated by a combined nuclear magnetic resonance (NMR) spectroscopy and density functional theory (DFT) study. SA is a glycan‐terminating monosaccharide, and its importance as a clinical target has long been recognized. Inspired by the fact that the binding properties of SA–PBA complexation are anomalously high relative to those of typical monosaccharides, great effort has been made to build a clinical platform with the use of PBA as a SA‐selective receptor. Although a number of applications have been reported in recent years, the ability of PBA to recognize SA‐terminating surface glycans selectively is still unclear, because high‐affinity SA–PBA complexation might not occur in a physiological environment. In particular, different forms of SA (α‐ and β‐pyranose) were not considered in detail. To answer this question, the combined NMR spectroscopy/DFT study revealed that the advantageous binding properties of the SA–PBA complex arise from ester bonding involving the α‐carboxylate moieties (C1 and C2) of β‐SA but not α‐SA. Moreover, the facts that the C2 atom is blocked by a glycoside bond in a physiological environment and that α‐SA basically exists on membrane‐bound glycans in a physiological environment lead to the conclusion that PBA cannot selectively recognize the SA unit to discriminate specific types of cells. Our results have a significant impact on the field of SA‐based cell recognition.

Potentiometric Adsorption Isotherm Analysis of a Molecularly Imprinted Polymer Interface for Small-Biomolecule Recognition

In this paper, we report a direct and quantitative analytical method of small-biomolecule recognition with a molecularly imprinted polymer (MIP) interface, taking advantage of the potentiometric principle of a field-effect transistor (FET) sensor, which enables the direct detection of ionic charges without using labeling materials such as fluorescent dyes. The interaction of low-molecular-weight oligosaccharides such as paromomycin and kanamycin with the MIP interface including phenylboronic acid (PBA) was directly and quantitatively analyzed from the electrical signals of an MIP-coated FET sensor. In particular, the change in the potential response of the FET sensor was derived on the basis of the multi-Langmuir adsorption isotherm equations, considering the change in the molecular charges of PBA caused by the adsorption equilibrium of the analytes with the vinyl PBA-copolymerized MIP membrane. Thus, the potentiometric adsorption isotherm analysis can elucidate the formation of selective binding sites at the MIP interface. The electrochemical analysis of the functional biointerface used in this study supports the design and construction of sensors for small biomarkers.