Recognition of α-hydroxycarboxylates by N-heterocyclic boronic acids

Abstract

Quinolineboronic and isoquinolineboronic acids were suggested earlier as receptors for sugars with fluorescence signaling properties. Here we report a detailed study of the acid–base, optical, tautomeric and recognition properties of 3-quinolineboronic (3-QBA) and 5-isoquinolineboronic acids (5-IQBA) using a set of α-hydroxyacids of different structures as well as fructose as target compounds and demonstrate that these receptors possess optimum binding properties towards α-hydroxyacids rather than for usual sugars. Particularly strong binding with stability constants reaching 10⁴–10⁵ M⁻¹ at optimum pH between 4 and 5 is observed for tartaric and citric acids. Also, very large affinity was found for the enolic form of oxalacetic acid. The tautomeric equilibrium between molecular and zwitterionic forms of formally neutral acids is shifted to the zwitterion for 3-QBA but to the molecular form for 5-IQBA. The fluorescence intensity of both acids is reduced by complexation with α-hydroxyacids but enhanced by complexation with fructose. These opposite trends are related to different types of binding: α-hydroxycarboxylate anions bind to a fully protonated form of boronic acid containing a neutral trigonal –B(OH)₂ group, but fructose binds to the zwitterion or anion of boronic acid containing an anionic tetrahedral –B(OH)₃⁻ group restoring the quenching effect of this group.