Galactosamine in the gas phase. Unveiling features of the conformers existing in a mixture of the interconverting anomers

2-Aminosugars are monosaccharide derivatives present in a modified form in several biopolymers (e.g. chitin is an important component of the cell walls of many living organisms). Their further importance lies in the very simple but unique structure that can assume an archetypal behavior by experiencing different conformations. The conformation of neutral aminosugar arises from the balance of different factors, including the hyperconjugative and the electrostatic effects, as well as the peculiar hydrogen bonding network available to the hydroxyls substituents and the amino group. In the present work the IRMPD spectrum of protonated galactosamine (GalN·H+), that is a C4 epimer of glucosamine, has been measured to determine for the first time the IR features of protonated - and -anomers present in a methanolic mixture, on the ground of their different absorption bands. In order to distinguish the signals coming from different anomers, the spectrum of GalN·H+ extracted from the same methanol solution of GalN·HCl has been measured at different solution ages, with the purpose of estimating possible modifications in the spectrum profile, which can be safely attributed to the time-evolution of the anomeric composition of the mixture. The experimental data have been compared with a systematic conformational search (B3LYP/6-311++G(d,p)) which clearly indicated the identity of the most populated conformers for both the anomers, which are characterized by a clockwise arrangement of the N2H3+-O3H-O4H-O6H hydrogen bond chain and by a 4C1 chair. The gas phase / distribution has been determined, and then compared with the condensed phase distribution measured by a NMR analysis carried out on increasing age solutions. The results indicate a very similar position of the equilibrium measured in the gas phase and in condensed phase, but very different anomerization rates. The orbital occupancies of the structures matching the experiment have been calculated through the natural bond method, and the obtained results firmly indicates that in the  and  anomers of protonated GalN·H+ an opposite effect is active: endo anomeric effect is predominant in the first meanwhile in the latter is more important the exo one. Further computational work is in progress to exactly estimate the specific contribute of the electrostatic and hyperconjugative effect to differentially stabilize the anomers of protonated galactosamine.