Understanding the Aldo-Enediolate Tautomerism of Glycolaldehyde in Basic Aqueous Solutions

Abstract

The biochemically important interconversion process between aldoses and ketoses is assumed to take place via 1,2-enediol or 1,2-enediolate intermediates, but such intermediates have never been isolated. The current work was undertaken in an attempt to detect the presence of the 1,2-enediol structure of glycolaldehyde in alkaline medium, actually a 1,2-enediolate, and to try to clarify the scarce data existing about both the formation of deprotonated enediol and the aldo-enediolate equilibrium. The Raman spectra of neutral and basic solutions were recorded as a function of time for eleven days. Several bands associated with the presence of the enediolate were observed in alkaline medium. Glycolaldehyde exists as three different structures in aqueous solution at neutral pH, that is, hydrated aldehydes, aldehydes and dimers, with a respective ratio of approximately 4:0.25:1. Additionally, the formation of Z-enediolate forms takes place at basic pH, together with an increase in the concentration of aldehyde species, such as 2-oxoethan-1-olate, and a decrease in the concentrations of the hydrated aldehyde and dimeric forms. The theoretical ratio of ≈1.5:1 for aldehyde:Z-enediolate reproduces the experimental Raman spectrum in basic medium, with an additional contribution of the previously mentioned ratio between the hydrated aldehyde and dimeric forms. Finally, Raman spectroscopy allowed us to monitor the enolization of this carbohydrate model and conclude that aldo-enediol tautomerism - formally aldo-enediolate - happens when a suitable amount of basic species is added.LMA thanks the MICINN for a PhD grant (No. BES-2010-031225). MMQM thanks the Universidad de Jaén for a predoctoral fellowship. This work has been supported by the CTQ2012-35513-C02- 02 (MINECO), P08-FQM-04096 (CICE, Junta de Andalucía) and S2013/MIT-2841 (Fotocarbon, Comunidad Autûnoma de Madrid) Projects. Computer, storage and other resources from CTI (CSIC) are gratefully acknowledged. Gratitude is also due to the University of Jaén for continuing financial support and to its CICT for instrumental facilities. The authors also thank D. Francisco Hermoso Torres for his help in the laboratoryPeer Reviewe