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 (4)C(1) chair complex is observed, whereas for IdoA the data are consistent with the alpha-anomer (1)C(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).