Spectroscopic analysis direct intermediate detection

In contrast to haem proteins there is no direct spectroscopic evidence for FeIV=0 involvement in catalysis by non-haem iron enzymes. Both the absence of a highly coloured prosthetic group and the short lifetimes of the proposed intermediates make the task of detection difficult. However, analysis of possible reaction pathways and the nature of the products formed has provided some indirect evidence for FeIV=0 formation, both in binuclear and mononuclear non-haem iron enzymes. 

Oxidative decarboxylation of 3,4-dihydroxymandelic acid (DHMA) to 3,4-dihydroxybenzaldehyde (DHBald) is also catalyzed by tyrosinase [245-251]. At first a direct oxidative decarboxylation to quinone methide was proposed [245]. Later, the reaction through the quinone intermediate was proposed based on the spectroscopic detection of the intermediate and the kinetic analysis as shown in Scheme 13 [246, 248]. The mechanism was supported by the fact that nearly 90% deuterium retains in the formation of 3,4-dihydroxybenzaldehyde from deuterated 3,4-dihydroxymandelic acid [251]. 

---