Sensitive NMR method for detecting carbohydrate influx into competing chemocatalytic pathways

Abstract

Reaction pathways are often tracked with stable isotopes in order to determine the provenance of products in the pathway and to deduce mechanistic information. NMR spectroscopy can provide direct insight into the specific labelling position of the stable isotope. We suggest a simple assay that allows rapid quantitative measurements of isotope distributions in biomass-derived products using commercially available carbohydrate substrates and routine instrumentation. In the assay, biomass-derived products in post reaction material are quantitatively reduced with NaBH₄ to install hydrogens at each carbon site in the product. In this manner, the detection of ¹³C and ¹²C sites becomes possible in multiplets of the sensitive 2D ¹H–¹H TOCSY experiment. The approach detects the usage of competing upstream reactions from isotope patterns in chemically identical reaction products. Changing influx into Sn-Beta-catalysed carbohydrate conversion reactions in the absence and in the presence of K⁺ was quantitatively assessed, showing how the presence of K⁺ alters the intial reactions towards methyl lactate.