Biocatalytic Oxidation of Biobased Furan Aldehydes: Comparison of Toxicity and Inhibition of Furans toward a Whole-Cell Biocatalyst

Furan carboxylic acids are promising and renewable building blocks in polymer and pharmaceutical industries. In this work, biocatalytic synthesis of furan carboxylic acids was performed from biobased furan aldehydes using recombinant Escherichia coli expressing 3-succinoylsemialdehyde-pyridine dehydrogenase (SAPDH). Particularly, the inhibition and toxicity of furan aldehydes and their carboxylic acid derivatives toward this whole-cell biocatalyst were evaluated. It was found that this biocatalyst displayed the highest tolerance level (200 mM) toward 5-hydroxymethylfurfural (HMF) in the oxidation of furan aldehydes, lower toward 5-methoxymethylfurfural (MMF, 150 mM), and the lowest (75 mM) toward furfural. This may be explained by the less detrimental effects of high concentrations of HMF on the biocatalyst which might be closely related to the low hydrophobicity of this substance. In addition, the presence of furan carboxylic acids resulted in the substantial decrease in both catalytic activities and viability of the cells. The inhibition and toxicity of furan carboxylic acids toward the cells were greatly relieved upon neutralization with a base. Based on the results obtained above, a combined reaction engineering strategy (fed-batch operation coupled with pH controlling) was designed for efficient synthesis of 5-methoxymethyl-2-furancarboxylic acid (MMFCA) from MMF. In addition, the gram-scale preparation of MMFCA was implemented. The desired product was obtained in an isolated yield of 89% and a purity of >98%. Its space-time yield was up to 1.2 g/L h.