Carboxylate-substituted radicals from phenylselenide derivatives. Designs on models for coenzyme B12-dependent enzyme-catalyzed rearrangements.
Laser flash photolysis (266 nm) of alpha- and beta-phenylselenyl esters, carboxylic acids, and carboxylates in aqueous acetonitrile media gave the corresponding radicals by homolytic cleavage of the phenylselenyl groups. In the beta-substituted systems, acid and carboxylate radicals reacted in intramolecular reporter reactions with approximately equal rate constants. For the alpha-substituted systems, an ester- and carboxylic acid-substituted radical reacted in an intramolecular reporter reaction with the same rate constants, but the analogous alpha-carboxylate radical, a radical anion, reacted an order of magnitude less rapidly and with an activation energy that is 3 kcal/mol greater than that found for analogues. A kinetic titration of the equilibrating alpha-acid and alpha-carboxylate radicals gave pKa = 4.6. The results indicate that alpha-ester and alpha-carboxylic acid radicals are unlikely to be appropriate models for alpha-carboxylate radicals, the intermediates formed in a large subset of coenzyme B12-dependent enzyme-catalyzed reactions.
Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607, USA.
Neil
N
Miranda
Pierre
P
Daublain
John H
JH
Horner
Martin
M
Newcomb
D002264:Carboxylic Acids / Q000138:chemical synthesis / Q000737:chemistry*; D003038:Cobamides / Q000737:chemistry* / Q000378:metabolism; D003500:Cyclization; D005609:Free Radicals / Q000737:chemistry; D007700:Kinetics; D008765:Methylmalonyl-CoA Mutase / Q000737:chemistry / Q000378:metabolism; D016566:Organoselenium Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D010782:Photolysis
D002264:Carboxylic Acids; D003038:Cobamides; D005609:Free Radicals; D016566:Organoselenium Compounds; D008765:Methylmalonyl-CoA Mutase; C000913:cobamamide