A tong-like fluorescence sensor for metal ions: perfect conformational switch of hinge sugar by pyrene stacking

Abstract

Carbohydrates are among the potential materials for molecular devices, since they are abundant natural resources. However, their rigidity has restricted their use for movable devices. Hinge sugars, 2,4-diamino-2,4-dideoxy-xylopyranosides, shed light on the use of carbohydrates as movable components, as demonstrated by the motion by which all four equatorial substituents can change to an axial orientation in synchronization with a chelation-driven ⁴C₁–¹C₄ ring flip. In this study, we synthesized a tong-like metal ion sensor, 1,3-di-O-pyrenylmethylated hinge sugar (1), and its model compound, methyl 2,4-di-O-pyrenecarbonyl-xylopyranoside (2), to extend the abilities of hinge sugars as molecular components. From observations of the solvent-dependent conformational and fluorescent behavior of 2, we found that the pyrene stacking assists the ¹C₄ formation of xylopyranoside by 1.7 kcal mol⁻¹. We also found that compound 1 produced excimer fluorescence by chelation to Pt²⁺, Zn²⁺, Cd²⁺, Mg²⁺ or Mn²⁺, and unexpectedly by addition of acids. ¹H NMR measurements ascribed this behavior to the ⁴C₁–¹C₄ ring flip of hinge sugar in response to chelation or protonation at N2, and revealed rapid and perfect ¹C₄ formation in the case of Zn²⁺. These findings will extend the scope of hinge sugars as movable components.