Reactions of biologically important molecules

Sun et al. [34] reported that the rate of hydrolysis of the biomolecule ATP under MW irradiation was 25 times faster than under classical heating at similar temperatures. However, the same research group [35] later observed that, with more accurate temperature control, the hydrolysis rates were in fact almost identical. 

There have also been reports [36, 37] that racemization of amino acids occurs more rapidly using MW heating than conventional heating at the same temperature. Chen et al. [36] observed that racemization of amino acids in acetic acid the presence of benzaldehyde was accelerated by MW heating. Lubec et al. [37] reported that some D-proline and ris-4-hydroxy-D-proline were found in samples of infant milk formula when they were heated in a MW oven. On the other hand, conventionally heated samples did not contain these unnatural D-amino acids. This report caused concern, and received media attention because D-proline is neurotoxic and suggested that MW heating of some foods could have deleterious effects on their nutritional value and the health of the consumer. 

However, Marchelli et al. [38], were unable to detect any D-proline when milk formula containing 1-proline was heated in open vessels in a MW oven. Westaway and Gedye [30] showed that very small amounts of D-proline (0.1-0.2%) were produced when an aqueous solution of L-proline was heated under reflux in a MW oven for 

15 min. Although D-proline was not detected when the same solution was heated under reflux conventionally for the same time, approximately 1% of D-proline was produced after 24 h of conventional reflux. MW did not have a significant effect on the rate of racemization of L-proline in acetic acid in the presence of benzaldehyde [30], 

The small increase in racemization rate observed when an aqueous solution of L-pro-line was heated under reflux on a MW oven at atmospheric pressure could be attributed to localized superheating or a generalized superheating of the solvent. It is known that water superheats by 4—10 °C when boiled in a MW oven [39, 40]. 

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We have shown how the band structure of photoexcited semiconductor particles makes them effective oxidation catalysts. Because of the heterogeneous nature of the photoactivation, selective chemistry can ensue from preferential adsorption, from directed reactivity between adsorbed reactive intermediates, and from the restriction of ECE processes to one electron routes. The extension of these experiments to catalyze chemical reductions and to address heterogeneous redox reactions of biologically important molecules should be straightforward. In fact, the use of surface-modified powders coated with chiral polymers has recently been reputed to cause asymmetric induction at prochiral redox centers. As more semiconductor powders become routinely available, the importance of these photocatalysts to organic chemistry is bound to increase. 

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