Guidelines of Designing Zwitterions for One-Pot Bioethanol Production from Biomass

Abstract

Although ionic liquids are attractive pretreatment solvents, their high toxicity inhibits enzyme and yeast activity. The present study focused on low-toxicity zwitterions to produce bioethanol in one pot by successive pretreatment, hydrolysis, and fermentation in a single container (one-pot bioethanol production). However, an optimal zwitterion design has not yet been developed. We investigated the pretreatment ability, inhibitory effect on cellulase activity, and toxicity to yeast of zwitterions with varying structures (cations, anions, and spacers) for the first time. This study provides guidelines for designing zwitterionic structures for one-pot bioethanol production. Among the zwitterions studied, C₁imC₅C, consisting of an imidazolium cation, a carboxylate anion, and 5-carbon-length spacer, showed the highest pretreatment ability, followed by C₁imC₃C (an analogue with 3-carbon-length spacer). Cation structures affected both delignification and decrystallization abilities, whereas anion structures and spacer lengths primarily affected delignification and decrystallization abilities, respectively. C₁imC₅C showed a high inhibition of enzyme activity. None of the zwitterions showed strong toxicity toward yeast. Overall, C₁imC₃C is determined as the promising prototype for zwitterion design in one-pot bioethanol production from the viewpoints of pretreatment, enzymatic hydrolysis, and fermentation. In the future, this skeletal structure could be used as a prototype to better design zwitterions suitable for one-pot ethanol production.