Carbonyl Inhibition of Biofuels Production from Biomass Hydrolysates: A Quantitative Structure-Activity Relationship (QSAR) Approach

Developing alternative biofuels from renewable biomass has great potential to reduce U.S. dependence on foreign oil while improving national energy security and addressing the environmental issues. However, one of the major bottlenecks impeding production of viable biofuels from renewable biomass is the lack of cost-effective processes for converting biomass into biofuels.

Biomass pretreatment is needed to break down the recalcitrant structure of the plant cell wall for subsequent enzymatic hydrolysis and fermentation. However, the pretreatment processes generate inhibitors from the degradation of cellulose, hemicellulose and lignin, many of which significantly reduce the microbial growth and fermentation productivity. The objective of this study was to identify the specific molecular descriptors that correlate molecular structure of carbonyl compounds to their inhibitory activity. Quantitative structure-activity relationship (QSAR) modeling was used in assessing the reactivity of carbonyls. Molecular descriptors that were studied in this work included hydrophobicity (Log P),  dipole moment, energy of the lowest unoccupied molecular orbital (ELUMO), energy of the highest occupied molecular orbital (EHOMO) and electrophilicity index (ω).

The quantitative information on carbonyl inhibition was evaluated based on the glucose initial consumption rate and the final alcohol yield. We identified that ELUMO is a very good global parameter to correlate the molecular structure of carbonyl compounds to their inhibitory effects.

We believe that the molecular structure and functional groups in carbonyls will most likely govern the reactivity of carbonyl compounds, and the reactivity of carbonyls will potentially dominate their inhibitory effects on microbial fermentation.