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

Abstract

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 β-anomer ⁴C₁ chair complex is observed, whereas for IdoA the data are consistent with the α-anomer ¹C₄ chair structure, as well as the corresponding β-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.