(507b) Understanding Glycolaldehyde Formation from Various Monosaccharides

To understand the mechanism(s) of glycolaldehyde formation from hemicellulose, we have probed the causes of experimental differences among the pyrolyses of glucose, mannose, glucuronic acid, fucose, xylose and arabinose. These monosaccharides have differences in functional groups that alter the pathway to produce the desired product glycolaldehyde. At carbon-5, glucose and mannose have H and a hydroxymethyl group; glucuronic acid has an H and a carboxyl group; fucose has an H and a methyl substituent, and xylose and arabinose have two Hs.

Samples of D-glucose, D-mannose, D-glucuronic acid, D-xylose, ¹³C1-labelled D-xylose, D-arabinose, and L-fucose were flash-pyrolyzed (Pyroprobe 5200, CDS) at 250°C, 300°C, and 350°C. Product gases were analyzed by two-dimensional gas chromatography with time-of-flight mass spectrometry (Pegasus 4D, LECO). The yields of glycolaldehyde were lower from the pyrolysis of hexoses than from pentoses. Among the hexoses, a negligible amount of glycolaldehyde was observed in fucose pyrolysis. Among pentoses, a higher amount of glycolaldehyde was obtained in pyrolysis of arabinose than that of xylose.

The pyrolysis of ¹³C1-labeled xylose resulted in negligible amounts of labelled glycolaldehyde. With the C5 site blocked for glycolaldehyde formation in hexoses, pyrolysis of ¹³C xylose implies negligible formation of glycolaldehyde from C1-C2 fragments and other minor paths in hexose pyrolysis. The pathways involving C4-C5 fragments of xylose have been deemed primary to formation of glycolaldehyde in previous studies [1], [2].

Quantum chemistry calculations (Gaussian 16, B3LYP/6-31G(d,p)) were performed to understand the disparity in yields of glycolaldehyde from the hexoses. Glucose and glucuronic acid only show differences in substituents at C5, but the experiments yielded higher amount of glycolaldehyde from glucose than from glucuronic acid. A study on the charge distribution of fucose found the methyl substituent was unusually negative with respect to C5. The substituents altered the overall charge distribution in the molecule. This result suggested involvement of charge distribution in altering pathways adopted for glycolaldehyde among the hexoses. Reasons will be examined why the mechanism(s) worked for the pentoses and not hexoses.

References:

[1] Q. Lu, H. Tian, B. Hu, X. Jiang, C. Dong, and Y. Yang, “Pyrolysis mechanism of holocellulose-based monosaccharides : The formation of hydroxyacetaldehyde,” J. Anal. Appl. Pyrolysis, vol. 120, pp. 15–26, 2016.

[2] Jain, A., Bose, A., Westmoreland, P.R,, “Comparing Pyrolysis of D-Arabinose and L-Fucose to D-Xylose,” AIChE Annual Meeting, Orlando FL, November 10-15, 2019, Paper 35d.