Hyper-Raman spectroscopic detection of trace glyceraldehyde enediolate in aqueous glycerol solutions.

Hyper-Raman (HR) spectroscopy with 532 nm excitation was applied to glycerol as a model sugar. The HR spectrum of a 50 mM aqueous solution at pH 11.3 exhibited two prominent bands at 1577 and 1537 cm-1, standing in sharp contrast to both the IR and 532 nm Raman spectra of bulk glycerol. Based on examinations of laser power and concentration dependencies, isotope effects, and theoretical calculations, the two HR bands were assigned to the stretching vibration of the O-C=C-C linkage and the mixed COH bending/HCH wagging mode of glyceraldehyde enediolate, a photoproduct generated via laser-induced radical reactions. Despite its low concentration (≤60 nM), this species dominates the HR spectrum, most likely owing to a double (one- and two-photon) resonance enhancement effect. This study demonstrates the unique capability of HR spectroscopy for the detection of trace intermediates, highlighting its potential for studying chemical reactions involving sugars.