A detailed kinetic study has been carried out of the reaction of dopamine, 2-(3,4-dihydroxyphenyl)ethylamine, with dioxygen over the pH range 7-9 where it reacts spontaneously without the necessity of metal ion catalysis. The reaction was found to be accurately first-order in [O,] and in[dopamine] and first-order in [H '3-l and, furthermore, stoichiometric amounts of H, O, were shown to be produced. The other product of oxidation is, initially, the pink dopaminochrome which, however, is not stable and reacts further (without the consumption of dioxygen) to form the insoluble polymeric material known as 'melanine'. The rate-determining step is assumed to be hydrogen atom abstraction from the monodeprotonated species by 0, (as with many other catecholamines, dopamine is stable towards oxidation in acidic media in the complete absence of metal ions) with a second-order rate constant of k, = 0.47 & 0.05 dm3 mot-' s-l at 25 "C in a solution of ionic strength 0.1 mol-' dm-3 (KCI).Dopamine is 2-(3,4-dihydroxyphenyl)ethylamine, I, and is referred to as H2LH+ in this paper where the phenolic protons HO I are written to the left of L. When a neutral solution of dopamine is exposed to air, after a while it turns pink owing to oxidation to dopaminochrome, 11, even in the absence of metal ions. Eventually, the pink colour disappears to be replaced by a precipitate of the polymeric material, melanine. Addition of a small amount of acid inhibits this oxidation, unless metal ions such as Fe3 -t , Cu2 + or V02 + are present. Paper 4/05 175K

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The shift of the visible d-d absorption bands of [Cu(acac) (tmen)] -+ (acac = acetylacetonate, tmen = N,/V,N',N'-tetramethylethylenediamine) in various solvents resulting from the co-ordination of solvent molecules and anionic species X has been investigated. The results have been used to establish donor numbers (DN,,,) of anions dissolved in 1.2-dichloroethane (dce) on the same scale as that already established for solvents. Apparent donor numbers for anions dissolved in various solvents other than dce (DNx,miv) are also given and used to show that the donor properties of anions are directly related to the acceptor number (AN,,) of the solvent. This arises from competition between solvent and acceptor ion for the anion leading to a steady decrease in the effective donor number of the anion as AN,,iv increases, culminating in an effective donor number of zero for all anions in the strong acceptor solvent water (ion pairing is at a minimum in water). The values of the anion donor numbers are related to some experimental results such as spectral, equilibrium, kinetic and thermodynamic data for selected systems. This method of establishing donor numbers for anions is

The reactions of dopamine (1-amino-2-(3,4-dihydroxyphenyl)-ethane, DA), 5-hydroxydopamine (5-OHDA), and 6-hydroxydopamine (6-OHDA), with molecular oxygen-with and without the addition of catalytic amounts of iron(III) and other metal ions-have been studied and the implication of these results with respect to the chemistry involved in the progress of Parkinson's disease is discussed. In the presence of O2 DA reacts spontaneously without the necessity of metal-ion catalysis under the production of stoichiometric amounts of H2O2, to form initially pink dopaminochrome, which is not stable and reacts further (without the consumption of dioxygen) to form the insoluble polymeric material known as 'melanine'. DA reacts with iron(III) yielding an intermediate 1:1 complex, which decomposes releasing Fe(II) and the semiquinone, which reacts further under involvement of both Fe(III) and dioxygen. 6-OHDA reacts without showing the necessity of such an intermediate, and it is shown to be able to release iron as Fe(II) from ferritine. On the other hand, it is shown (in vitro) that Fe(II) reacts in a Fenton type reaction with DA and the present H2O2 producing 5-OHDA and especially 6-OHDA. Based on these mutual interacting reactions a mechanism for the initiation and progress of Parkinson's disease is suggested. The catalytic effects of some other transition-metal ions are presented and an explanation for the peculiarly toxic effects of manganese(II) is put forward. Finally, a possible reason for the effect that nicotine has in the mitigation of Parkinson's disease is discussed.

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The so-called reactive oxygen species (ROS) are defined as oxygen-containing species that are more reactive than O(2) itself, which include hydrogen peroxide and superoxide. Although these are quite stable, they may be converted in the presence of transition metal ions, such as Fe(II), to the highly reactive oxygen species (hROS). hROS may exist as free hydroxyl radicals (HO·), as bound ("crypto") radicals or as Fe(IV)-oxo (ferryl) species and the somewhat less reactive, non-radical species, singlet oxygen. This review outlines the processes by which hROS may be formed, their damaging potential, and the evidence that they might have signaling functions. Since our understanding of the formation and actions of hROS depends on reliable procedures for their detection, particular attention is given to procedures for hROS detection and quantitation and their applicability to in vivo studies.