The aggregation of ␣-synuclein has been implicated as a critical step in the development of Parkinson's disease. Parkinson's disease is a progressive neurodegenerative disorder caused by the loss of dopaminergic neurons from the substantia nigra; currently, no cure exists. Baicalein is a flavonoid with antioxidant properties; upon oxidation, it forms several products including quinones. We show here that low micromolar concentrations of baicalein, and especially its oxidized forms, inhibit the formation of ␣-synuclein fibrils. In addition, existing fibrils of ␣-synuclein are disaggregated by baicalein. The product of the inhibition reaction is predominantly a soluble oligomer of ␣-synuclein, in which the protein molecules have been covalently modified by baicalein quinone to form a Schiff base with a lysine side chain in ␣-synuclein. The binding of baicalein was abolished by conversion of the Tyr residues into Phe, demonstrating that Tyr is involved in the interaction of ␣-synuclein with baicalein.

The binding stoichiometry between Cu(II) and the full-length beta-amyloid Abeta(1-42) and the oxidation state of copper in the resultant complex were determined by electrospray ionization-Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) and cyclic voltammetry. The same approach was extended to the copper complexes of Abeta(1-16) and Abeta(1-28). A stoichiometric ratio of 1:1 was directly observed, and the oxidation state of copper was deduced to be 2+ for all of the complexes, and residues tyrosine-10 and methionine-35 are not oxidized in the Abeta(1-42)-Cu(II) complex. The stoichiometric ratio remains the same in the presence of more than a 10-fold excess of Cu(II). Redox potentials of the sole tyrosine residue and the Cu(II) center were determined to be ca. 0.75 and 0.08 V vs Ag/AgCl [or 0.95 and 0.28 V vs normal hydrogen electrode (NHE)], respectively. More importantly, for the first time, the Abeta-Cu(I) complex has been generated electrochemically and was found to catalyze the reduction of oxygen to produce hydrogen peroxide. The voltammetric behaviors of the three Abeta segments suggest that diffusion of oxygen to the metal center can be affected by the length and hydrophobicity of the Abeta peptide. The determination and assignment of the redox potentials clarify some misconceptions in the redox reactions involving Abeta and provide new insight into the possible roles of redox metal ions in the Alzheimer's disease (AD) pathogenesis. In cellular environments, the reduction potential of the Abeta-Cu(II) complex is sufficiently high to react with antioxidants (e.g., ascorbic acid) and cellular redox buffers (e.g., glutathione), and the Abeta-Cu(I) complex produced could subsequently reduce oxygen to form hydrogen peroxide via a catalytic cycle. Using voltammetry, the Abeta-Cu(II) complex formed in solution was found to be readily reduced by ascorbic acid. Hydrogen peroxide produced, in addition to its role in damaging DNA, protein, and lipid molecules, can also be involved in the further consumption of antioxidants, causing their depletion in neurons and eventually damaging the neuronal defense system. Another possibility is that Abeta-Cu(II) could react with species involved in the cascade of electron transfer events of mitochondria and might potentially sidetrack the electron transfer processes in the respiratory chain, leading to mitochondrial dysfunction.

Scanning electrochemical microscopy is a scanning probe technique that is based on faradaic current changes as a small electrode is moved across the surface of a sample. The images obtained depend on the sample topography and surface reactivity. The response of the scanning electrochemical microscope is sensitive to the presence of conducting and electroactive species, which makes it useful for imaging heterogeneous surfaces. The principles and instrumentation used to obtain images and surface reaction-kinetic information are discussed, and examples of applications to the study of electrodes, minerals, and biological samples are given.

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