Energy-efficient bioethanol production from plant biomass is in high demand, and one of the most promising procedures reported to date is one-pot ethanol production, that is, the production of ethanol from biomass in the same reaction pot, such as industrial first-generation bioethanol. This process requires cellulose solvents whose toxicity toward fermentative microorganisms is extremely low. Herein, we have developed a low-toxic zwitterionic cellulose solvent known as 4-(1-(2-(2-methoxyethoxy)­ethyl)­imidazol-3-io)­butyrate (OE2imC3C). OE2imC3C is the only reported solvent that satisfies the following properties: being liquid at mild temperature and having good cellulose dissolution ability and low toxicity, even when including other types of solvents. We here investigated the relationship between the chemical structures and properties by synthesizing 22 zwitterions. Long alkyl- or oligoether chains attached to the cation (cation tails) were necessary to be a liquid. The zwitterions, except for that with an octyl tail, exhibited biocompatibility. Interestingly, the spacers of the zwitterions, alkyl chains between the cations and anions, were expected to be inert, but affected the toxicity. The molecular mechanisms were investigated using molecular dynamics simulations. The zwitterions exhibiting low toxicity scarcely inserted their cation tails into cell membrane and thus did not rupture the cell membrane. Ionic liquids, which have free cations and anions, induced molecular-level disruption of the cell membrane, suggesting that the zwitterion structure is a critical factor for low toxicity. The spacers, which were expected to be inert, shifted the solvent cluster structures in the bulk phase and induced molecular-level disruption of the cell membrane. The requirements for low-toxic cellulose solvents are zwitterionic structures, carboxylate anions, long polar cation tails, and in some cases, short spacers.

The physicochemical properties of room temperature ionic liquids (RTILs) consisting of bis(trifluoromethanesulfonyl)amide (TFSA−) combined with 1‐hexyl‐1‐methylpyrrolidinium (Pyr1,6+), 1‐(butoxymethyl)‐1‐methylpyrrolidinium (Pyr1,1O4+), 1‐(4‐methoxybutyl)‐1‐methyl pyrrolidinium (Pyr1,4O1+), and 1‐((2‐methoxyethoxy)methyl)‐1‐methylpyrrolidinium (Pyr1,1O2O1+) were investigated using both experimental and computational approaches. Pyr1,1O2O1TFSA, which contains two ether oxygen atoms, showed the lowest viscosity, and the relationship between its physicochemical properties and the position and number of the ether oxygen atoms was discussed by a careful comparison with Pyr1,1O4TFSA and Pyr1,4O1TFSA. Ab initio calculations revealed the conformational flexibility of the side chain containing the ether oxygen atoms. In addition, molecular dynamics (MD) calculations suggested that the ion distributions have a significant impact on the transport properties. Furthermore, the coordination environments of the Li ions in the RTILs were evaluated using Raman spectroscopy, which was supported by MD calculations using 1000 ion pairs. The presented results will be valuable for the design of functionalized RTILs for various applications.

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