ABSTRACT
Ureido-heterocycles exhibiting different triple- and quadruple H-bonding patterns are useful building blocks in the construction of supramolecular polymers, self-healing materials, stimuli-responsive devices, catalysts and sensors. The heterocyclic group may provide hydrogen bond donor/acceptor sites to supplement those in the urea core, and they can also bind metals and can be modified by pH, redox reactions or irradiation. In the present review, the main structural features of these derivatives are discussed, including the effect of tautomerization and conformational isomerism on self-assembly and complex formation. Some examples of their use as building blocks in different molecular architectures and supramolecular polymers, with special emphasis on biomedical applications, are presented. The role of the heterocyclic functionality in catalytic and sensory applications is also outlined.
ABSTRACT
Heme iron and nonheme dimanganese catalases protect biological systems against oxidative damage caused by hydrogen peroxide. Rubrerythrins are ferritine-like nonheme diiron proteins, which are structurally and mechanistically distinct from the heme-type catalase but similar to a dimanganese KatB enzyme. In order to gain more insight into the mechanism of this curious enzyme reaction, non-heme structural and functional models were carried out by the use of mononuclear [FeII(L1-4)(solvent)3](ClO4)2 (1-4) (L1 = 1,3-bis(2-pyridyl-imino)isoindoline, L2 = 1,3-bis(4'-methyl-2-pyridyl-imino)isoindoline, L3 = 1,3-bis(4'-Chloro-2-pyridyl-imino)isoindoline, L4 = 1,3-bis(5'-chloro-2-pyridyl-imino)isoindoline) complexes as catalysts, where the possible reactive intermediates, diiron-perroxo [FeIII2(µ-O)(µ-1,2-O2)(L1-L4)2(Solv)2]2+ (5-8) complexes are known and well-characterized. All the complexes displayed catalase-like activity, which provided clear evidence for the formation of diiron-peroxo species during the catalytic cycle. We also found that the fine-tuning of iron redox states is a critical issue, both the formation rate and the reactivity of the diiron-peroxo species showed linear correlation with the FeIII/FeII redox potentials. Their stability and reactivity towards H2O2 was also investigated and based on kinetic and mechanistic studies a plausible mechanism, including a rate-determining hydrogen atom transfer between the H2O2 and diiron-peroxo species, was proposed. The present results provide one of the first examples of a nonheme diiron-peroxo complex, which shows a catalase-like reaction.
