Whole cell biocatalysis in the presence of supercritical and compressed solvents.

摘要

Supercritical fluids have successfully mimicked organic solvents as media for non-traditional biocatalysis using enzyme preparations. However, the use of compressed fluid technology for whole cell biocatalysis, including extractive fermentations, has lagged the technologies developed in organic solvents. This is largely due to the choice of compressed solvent, supercritical carbon dioxide, which has been demonstrated to inhibit cellular metabolism and growth. This work investigates the potential of compressed hydrocarbons for the in situ extraction of volatile fermentation products.; The anaerobic, thermophilic bacterium, Clostridium thermocellum , is of interest since it can convert biomass feedstocks directly to ethanol. However, C. thermocellum is limited by low ethanol concentrations; hence, a method of in situ product removal may be necessary to increase productivity. Furthermore, organic acid formation tends to reduce ethanol yields. This work demonstrates for the first time, fermentation using whole cells in the presence of compressed hydrocarbons. This success suggests the feasibility of compressed solvents for extractive fermentation to remove ethanol and reduce product inhibition.; The ability of a traditional measure of solvent biocompatibility, the octanol-water partition coefficient (log P), to describe the reduced metabolic activity of C. thermocellum in the presence of compressed solvents was examined. The log P was extended to elevated temperatures and pressures for the purpose of correlating metabolic activities. Using the Group Contribution Associating-Equation of State, the log P values for various compressed solvents were calculated. Results indicate that log P is not capable of describing biocompatibility with compressed solvents or with the phase change of the extractive solvent.; Organic solvents have been used to induce shifts in product selectivity with whole cells and enzymes. This work shows product selectivity shifts in resting cells due to hydrostatic pressure or the presence of compressed solvents. Decreased lactate formation was hypothesized to be due to the inhibition of membrane transport proteins. Furthermore, acetate inhibition by hydrostatic pressure may be caused by changes in the membrane structure or the thermodynamic sensitivity of end-product formation. This work suggests the potential for the manipulation of product selectivity of non-barophilic microorganisms using hydrostatic and hyperbaric pressure to enhance bioconversion rates and optimize desirable products.