Dopamin, oxidativer Stress und Protein‐Chinonmodifikationen bei Parkinson und anderen neurodegenerativen Erkrankungen

Hydrogen Atom Transfer from HOO. to ortho-Quinones Explains the Antioxidant Activity of Polydopamine

Melanins are stable and non-toxic biomaterials with a great potential as chemopreventive agents for diseases connected with oxidative stress, but the mechanism of their antioxidant action is unclear. Herein, we show that polydopamine (PDA), a well-known synthetic melanin, becomes an excellent trap for alkylperoxyl radicals (ROO. , typically formed during autoxidation of lipid substrates) in the presence of hydroperoxyl radicals (HOO. ). The key reaction explaining this peculiar antioxidant activity is the reduction of the ortho-quinone moieties present in PDA by the reaction with HOO. . This reaction occurs via a H-atom transfer mechanism, as demonstrated by the large kinetic solvent effect of the reaction of a model quinone (3,5-di-tert-butyl-1,2-benzoquinone) with HOO. (k=1.5×107 and 1.1×105  M-1  s-1 in PhCl and MeCN). The chemistry disclosed herein is an important step to rationalize the redox-mediated bioactivity of melanins and of quinones.

Observation and Kinetic Characterization of Transient Schiff Base Intermediates by CEST NMR Spectroscopy

In aqueous solution, many biochemical reaction pathways involve reaction of an aldehyde with an amine, which progresses through generally unstable, hydrated and dehydrated, Schiff base intermediates that often are unobservable by conventional NMR. There are 4 states in the relevant equilibrium: 1) gem-diol, 2) aldehyde, 3) hemiaminal, and 4) Schiff base. For the reaction between protein amino groups and DOPAL, a highly toxic metabolite of dopamine, the ¹ H resonances of both the hemiaminal and the dehydrated Schiff base can be observed by CEST NMR, even when their populations fall below 0.1 %. CEST NMR reveals the quantitative exchange kinetics between reactants and Schiff base intermediates, explaining why the Schiff base NMR signals are rarely observed. The reactivity of DOPAL with N^α -amino groups is greater than with lysine N^ϵ -amines and, in the presence of O₂ , both types of Schiff base DOPAL-peptide intermediates rapidly react with free DOPAL to irreversibly form dicatechol pyrrole adducts.