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

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.