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in CO2 seemed an ideal combination of green solvent with green catalyst. During the early 1990s, a number of enzymes were evaluated in carbon dioxide, primarily in support of es- terification reactions [233]. For the most part, activities were very low, much lower than for the same reaction conducted in a conventional organic solvent. In addi- tion, rates in CO2 were substantially lower than rates in other compressible fluids (ethane, propane, fluoro- form). In some key publications, Russell et al. outlined the reason for CO2’s low activity—apparently carbon dioxide reacts with primary amine residues (primarily from lysine) to form carbamic acid and/or ammonium carbamates [234]. This derivatization was observed experimentally and is apparently responsible for the reduced activity of many enzymes in CO2 (note that not all enzymes suffer from this reduced activity, con- sistent with the fact that enzymes exhibit a range of protein sequences, secondary and tertiary structures). Carbamate formation is reversible, as removal of the enzyme from CO2, followed by examination of the rate in either water or another organic solvent reveals no change in inherent activity. Even bubbling of gaseous carbon dioxide through a suspension of enzyme in or- ganic solvent can produce the reversible drop in ac- tivity. Consequently, interest in enzymatic chemistry using enzyme powder in CO2 diminished greatly. At this same time, advancements in the design of CO2-philic surfactants allowed for the possibility of performing enzymatic chemistry in the aqueous core of micelles formed in carbon dioxide, a situation that would eliminate the problems due to carbamate for- mation (polar solvents destabilize the carbamates). In- deed, work by Randolph and Johnston [235], as well as Beckman et al. [236], showed that one could solu- bilize an enzyme in the core of a micelle, and then re- cover the protein via depressurization. However, CO2 dissolves in water and forms carbonic acid and not surprisingly the pH within the micelles was shown to be <3.0. While Johnston showed that one could buffer such a system to a pH from 5.0 to 6.0 [31], the ionic strength required was far higher than would normally be recommended for use with an active enzyme. Thus, realization of the full ‘green’ potential of enzyme–CO2 systems was again blocked by technical realities. Other issues to note regarding use of enzymes in CO2 include the need by the enzyme for a certain amount of bound water and the equilibrium nature of many of the reactions. Although CO2 is usually con- sidered a non-polar solvent, it will solubilize ≈2500 ppm water at moderate pressures (100 bar, room tem- perature). Because enzymes will not function in or- ganic media if stripped of all of their water, care must be taken to prevent CO2 from dehydrating the enzyme. In addition, many of the enzymatic reactions that one might wish to perform in CO2 are governed by equi- librium and hence, one must examine means by which to remove the byproduct or product from the neigh- borhood of the enzyme. A final obstacle to use of enzymes in supercritical fluids lies in the poor solubility of many of the polar substrates that one might wish to transform. For ex- ample, while many of the literature studies performed during the early 1990s examined esterifications, the starting material (carboxylic acid) was usually not par- ticularly soluble in CO2 (hardly surprising given what is known about CO2 ). The previous paragraphs make plain the technical hurdles that would need to be overcome to render en- zymatic chemistry in CO2 generally practical and use- ful. Either enzymes must be identified (or developed through a directed evolution-like process) that do not form carbamates with CO2 (or where carbamate for- mation does not impede activity) or a way must be found to buffer a CO2/water mixture without resorting to an ionic strength that will harm the enzyme. Con- versely, identification of enzymes that thrive at low pH or high ionic strength would also be worthwhile in this regard. If one could overcome the problems described above, then one could evaluate a number of issues regarding the use of enzymes in compressible fluids. For example, work by Russell [237] using fluoro- form showed that pressure (through its effect on fluid properties) could be used to tune enzyme activity and also, to a certain extent, selectivity for a given reaction path. However, given the preference for CO2 versus other compressible fluids, until the problems regarding CO2 and enzymes are dealt with, enzymatic chemistry in compressible fluids will likely continue at only a very low level of research activity. 4.2. Diels-Alder chemistry The Diels-Alder reaction is employed on a large scale industrially to help to purify cyclopentadiene, E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 169PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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