Publications Details

Fundamental Properties of Confined Enzymes

Rempe, Susan R.; Vanegas, Juan

We recently developed an enzymatically active, ultra-thin, nano-stabilized liquid membrane for CO₂ separation from a mixture of gases, which was recognized by an international R&D 100 Award in 2015. The separation membrane is an approximately 18-nm thick water layer stabilized by capillary condensation within a hydrophilic mesoporous silica film and embedded with the metallo-enzyme carbonic anhydrase. The enzyme speeds CO₂ uptake and release from the membrane by catalyzing the rapid inter-conversion of carbon dioxide and water to bicarbonate and a proton. The membrane separates CO₂ from 1:1 gas mixtures at a rate of 2600 GPU with CO₂/N₂ and CO₂/H₂ selectivities exceeding 788 and 1500, the highest combined flux and selectivity yet reported. That membrane performance exceeds, for the first time, the U.S. Department of Energy standards for CO₂ capture technology. CO₂ flux depends sensitively on nanopore surface chemistry in the active region. To understand that dependence, we applied molecular simulations to interrogate enzyme behavior in the presence of varied surface chemistries. The results indicate that a polar surface chemistry within the membrane nanopores prevents aggregation of enzymes that would otherwise occur in both bulk liquid solution and non-polar nanopores. Additionally, the enzyme active site maintains a stable structure, even when the overall protein structure deforms within the nanopores. In summary, confinement in the ultra-thin layer of water within mesoporous silica nanopores facilitates a 15x higher enzyme concentration than in bulk conditions, without affecting the structure of the enzyme active site, when the nanopore surfaces are covered with polar functional groups. Thus, confinement of the carbonic enzymes in the membrane water-filled nanopores facilitates higher rates of CO₂ uptake and release than achievable in bulk solutions.