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.

Isomer activation controls stereospecificity of class I fructose-1,6-bisphosphate aldolases

“…Binding of the cis-D-G3P configuration is also consistent with computational and experimental studies on the solution stability of its glycoaldehyde analogue (32). The most stable glycoaldehyde complexes have cis-configurations with respect to the water molecules that solvate glycolaldehyde.…”

Structural Characterization of β-Glycolaldehyde Dimer

“…authors gave the ratio of hydrated aldehyde to aldehyde monomer to dimer forms in water solution as 4 : 0.25 : 1. [13] They confirmed that enediol form of glycolaldehyde exists only in alkaline solutions and furthermore established that only the Z form of enediolate participates in exchange with the aldo form. Another study of monomer and oligomer equilibria used NMR spectra to determine concentrations of different species in glycolaldehyde water solutions and calculated reaction free energies and barriers for interconversion of different species.…”