CHLOROPEROXIDASE-CATALYZED BENZYLIC HYDROXYLATION

Chloroperoxidase oxidizes p-methylanisole and p-ethylanisole to 4-meth oxybenzyl alcohol and 1-(4'-methoxyphenyl)ethanol, respectively. It in effectively oxidizes toluene to benzyl alcohol but does not appear to oxidize toluene substituted with strong electron-withdrawing groups. O -Demethylation is also observed. The enzyme is sensitive to substituen ts at other than the para position and does not detectably catalyze be nzylic hydroxylation of p-methylanisole if it bears additional methyl or methoxy groups. An exception is 1,2-(methylenedioxy)-4-methylbenzen e, which is oxidized to both 3,4-(methylenedioxy)benzyl alcohol and 2- hydroxy-4-methylphenol. Studies with (H2O2)-O-18 indicate that all the oxygen incorporated into the product in the oxidation of p-methylanis ole to 4-methoxybenzyl alcohol derives from the peroxide. The mono- an d dideuterated methyl analogues of p-methylanisole are oxidized with a pparent intramolecular isotope effects of 3.51 and 3.34, respectively. Abstraction of a hydrogen from a carbon bearing a hydroxyl group comp etes effectively with benzylic oxidation because 2-[1,1-H-2(2)]phenyle thanol is oxidized to 2-[1-H-2]- rather than 2-[1,2-H-2(2)]phenylaceta ldehyde. Aldehyde formation therefore involves abstraction of the carb inol hydrogen rather than hydrogen migration to a benzylic carbocation intermediate. Chloroperoxidase resembles cytochrome P450 in that it c atalyzes benzylic hydroxylation reactions but it has a more limited su bstrate specificity. (C) 1995 Academic Press, Inc.