Vibrational signatures of metal-chelated monosaccharide epimers: gas-phase infrared spectroscopy of Rb+-tagged glucuronic and iduronic acid

Gas-phase binding of the alkali metal Rb+ to two monosaccharide isomers, glucuronic acid (GlcA) and iduronic acid (IdoA), is investigated by infrared photodissociation spectroscopy. The infrared spectra display striking differences, exemplified by the clear absence of a band at 3625 cm(-1) in the case of IdoA + Rb+. Comparison of the experimental spectra to computed spectra of DFT-optimized structures suggests that Rb+-tagged GlcA and IdoA each adopt their own distinctive complexation pattern. For GlcA, mainly the beta-anomer C-4(1) chair complex is observed, whereas for IdoA the data are consistent with the alpha-anomer C-1(4) chair structure, as well as the corresponding beta-anomer. The differences in the Rb+ binding motif rationalize the disparities in the infrared multiple-photon dissociation (IR-MPD) spectra. Whereas Rb+ binding to GlcA leaves the intramolecular hydrogen-bonding network between the OH groups intact, this network is disrupted for IdoA. The lack of stronger hydrogen-bonding for IdoA + Rb+ thus correlates well with the absence of the red-shifted OH stretch band at 3625 cm(-1).