Unprotected Galactosamine as a Dynamic Key for a Cyclochiral Lock.

Caterina Fraschetti,Matthias C Letzel,Jochen Mattay,Maria Elisa Crestoni,Barbara Chiavarino,Marlene Paletta,Antonello Filippi

doi:10.1021/jasms.0c00437

Abstract

The discrimination of d-galactosamine (G), representative of the amino-sugar class of compounds, has been probed through nano-ESI-FT-ICR mass spectrometry by isolating the relevant [C·H·G]+ proton-bound complexes with the enantiomers of the cyclochiral resorcin[4]arene C and allowing them to react toward three primary amines (B = EtNH2, iPrNH2, and (R)- and (S)-sBuNH2). The system under investigation presents several features that help to unveil the behavior of unprotected G in such a supramolecular architecture: (i) the hydrophobic derivatization of the C convex side forces the polar guest G to be coordinated by the cyclochiral concave region; (ii) protonated d-galactosamine exists as an anomeric mixture, dynamically interconverting throughout the experimental time-window; and (iii) different basicities of B allow the experiment to subtly tune the reactivity of the [C·H·G]+ complexes. Three [C·H·G]+ aggregate-types were found to exist, differing in both their origin and reactivity. The most reactive adducts ([C·H·G]ESI+), generated in the electrospray environment, undergo a G-to-B ligand exchange in competition with a partial isomerization to the unreactive [C·H·G]GAS+-type complexes. Finally, the poorly reactive [C·H·G]SOL+ aggregates are formed in solution over an hours-long time scale. A cyclochirality effect on the reactivity was found to depend on the considered [C·H·G]+ aggregate-type.

Highlights

Over the last decades, supramolecular chemistry played a central role in a number of fields due to its impressive versatility in virtually any applied science
Different efforts have very recently been devoted to discriminating between saccharides; pentoses have been extracted over hexose[17] and their homo- and heterodimers were recognized through self-assembled double helical foldamers,[18] non-all-equatorial saccharides were selected by “synthetic lectins”,19 oligosaccharides were distinguished by probing their length and building block compositions through specific nanoparticles,[20] and β-glucoside was used as a reference carbohydrate for a series of synthetic receptors.[21]
The t0h kinetic plots obtained in the reactions toward B = EtNH2 and iPrNH2 reach plateau (Supporting Information Figures S1−S4) whose values clearly decrease as the proton affinity of B (PAB) increases, eventually becoming undetectable when B = sBuNH2 (Figures S5 and S6; PAB40 in kilojoules per mole: EtNH2 = 912; iPrNH2 = 923.8; and sBuNH2 = 929.7)

Summary

Introduction

Supramolecular chemistry played a central role in a number of fields due to its impressive versatility in virtually any applied science. Among many other bioactive molecules, sugars and their derivatives constitute a class of compounds whose discrimination and sensing are challenging, especially in water where host−guest interactions compete with solvation phenomena. Different efforts have very recently been devoted to discriminating between saccharides; pentoses have been extracted over hexose[17] and their homo- and heterodimers were recognized through self-assembled double helical foldamers,[18] non-all-equatorial saccharides were selected by “synthetic lectins”,19 oligosaccharides were distinguished by probing their length and building block compositions through specific nanoparticles,[20] and β-glucoside was used as a reference carbohydrate for a series of synthetic receptors.[21] Our research efforts, aimed at exploiting a highly directional noncovalent network to select saccharides, have been applied to their amino derivatives. An amphiphilic oligomer was found to form very stable complexes with D-

Results

Discussion

Conclusion