Comparative catalytic activity of hemin and hematin in the breakdown of methyllinoleate hydroperoxide and peroxidation of methyllinoleate in methanol.

The breakdown of methyllinoleate hydroperoxide (LOOH) and peroxidation of methyllinoleate (LH) catalyzed by hemin and hematin were studied in 98% methanol. Spectrophotometry was used to follow the breakdown of LOOH. The peroxidation process of LH was monitored by the oxygen consumption. The pKa for the conversion of hemin to hematin was determined as 7.3 in 98% methanol. The catalytic rates for both processes were found to reach a maximum at pH 8 where about 80% of hemin was in the hematin form. This could be accounted for by the acceleration of the breakdown of LOOH due to binding of hydroxide or methoxide ion to the hemin iron. The reduction in the catalytic activity at a higher pH, where 100% of hemin was in the hematin form, however, suggested that H+ was also necessary. We propose a new scheme which shows the role of H+ and OH- (or CH3O-) together with ferryl iron in the catalytic process. The breakdown of hemin itself was also observed and its catalytic cycle number was estimated as 9. The gradual decomposition of hemin suggests involvement of a Fenton-type mechanism as a minor catalytic process.