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.
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