While recently visiting an academic lab, we had some interesting conversations surrounding enzyme catalysis, proteins and carbohydrate chemistry under microwave irradiation. For each of these areas, high temperatures was the central theme as a drawback to utilizing this technology for these approaches, so it got me to searching a bit. It is true, a substantial amount written points to this drawback. Unfortunately this would paint a pretty ugly picture if one were reading this and hasn’t thought in depth about the possibilities. For example, most of the bad press for enzyme reactions have been studied in aqueous environments — and we are now learning that depending on the enzyme (or an enzyme stabilized on a surface) one can achieve higher thermal stability in organic solvents. That said, the rates of reaction in general are supposed to be slower. For sugar chemistry, the story is much the same — ” I can only heat my molecule to 80 or 90C before I get degradation….well, take a look at all of those reactions done at room temperature and add to that some of the solvents that are used with low boiling points. There is a large and accelerated window between RT and 90C…a huge amount of energy. I would argue that something taking place at room temperature overnight would be an excellent candidate for some heating that you can’t normally do at reflux — say methylene chloride (not a good microwave absorber). Sparing most of the detail, I have seen a number of these reactions accelerated from 12-24 hrs, even days to an hour or two under the right microwave conditions…we simply need some thinking out of the box in these areas to get a more in-depth view, even methylene chloride at 90C works under the right choice of microwave conditions.

To generate some thinking around this area I found a nice publication (Journal of Molecular Catalysis B: Enzymatic 2007) on a lipase immobilized on a polyacylic resin for the resolution of (R,S)-2-octanol through a transesterification. Long story short….they found a significant rate acceleration through microwave irradiation over conventional heating. In the paper, solvent, temperature, catalytic rate and decomposition were studied with some interesting results.

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Enzymatic resolution with immobilized enzyme

Without going through the entire paper (solvent, enzyme stability, conversion are all in there), the microwave acceleration over conventional heating is shown in the graph below: at the 50% conversion rate the microwave conditions gave a 3 hr time point compared with 12 h conventionally (the octanol 50 mmol, enzyme 60 mg in n-heptane 50 mL with stirring at 40C).

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MW – circles and conventional in squares

To keep you a little more enticed — I have also included the table on solvents studied and the activity of the enzymes for the the reaction (seems like the enzyme in a microwave environment does its’ work in a non-polar media as compared with a more polar environment, but not everything holds true)– the rest you will have to work at — there is more (in addition to rate enhancement the ee%s went up as well.

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Solvent and catalytic activity

Hopefully this will get everyone to think a little bit more on their protein, enzyme and carbohydrate chemistry and see where there might be some rate enhancements. Happy reading!

 

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