Table 4. Parameters Describing the Potential Antiradical Ability of Catecholaminesa.
| BDEO–Hb | DHTc | HOMOd | E°′(H2O)e | E°′(MeCN)f | |
|---|---|---|---|---|---|
| L-DOPA | 79.1 (81.3) | –0.50 | –8.61 | 0.30834–0.44g | 1.52 |
| DA | 78.4 (80.8) | –0.69 | –8.53 | 0.370h,35–0.4234 | 1.29 |
| NOR | 78.6 (81.3) | 0.41 | –8.76 | 0.38436 | 1.52 |
| ADR | 78.9 (81.0) | 0 | –8.68 | 0.37236 | 1.50 |
O–H bond dissociation enthalpies (BDEO–H, in kilocalories per mole), HOMO energies (in electronvolts), energy difference between the phenoxyl radical and its parent catecholamine DHT (in kilocalories per mole), two-electron reduction potentials in water [E°′(H2O) vs NHE, in volts)], and oxidation potentials in acetonitrile [E°′(MeCN) vs HNE, in volts)].
Data calculated in benzene (and methanol, in italics).11 All listed values represent the weakest O–H bond in the para position, but exceptionally, for DA in benzene the O–H bond in the para hydroxyl is stronger (+0.9 kcal/mol) than the O–H bond in the meta position. In the same work, the BDEO–H calculated for unsubstituted catechol was 79.8 kcal/mol in benzene and 82.2 kcal/mol in methanol.
With respect to adrenaline, for which DHT = 398.1 kcal mol–1. Data from ref (9c).
Calculated by Ohkubo et al.9c for catecholamines as ammonium cations. HOMO energies for neutral compounds are reported by Dimić et al.:11 −0.290 eV (for DA), −0.294 eV (for ADR and NOR), and −0.293 eV (L-DOPA).
For the two-electron, two-proton (−2e/–2H+) oxidation potential. All values measured or recalculated vs NHE, unless otherwise stated.
In acetonitrile, measured vs Ag/AgNO3 (0.01M)9b and recalculated for NHE.
Calculated for two-proton, two-electron reduction of L-DOPA to dopaquinone at pH 7.4 from the equation E°′pH = E° – 0.059 × pH, where E° is a formal redox potential (0.745 V vs NHE at pH 0).37 The same calculations for pH 5.5 gave an E°′5.5 of 0.42 V.
At pH 7.0, in agreement with values of 0.405 V38 and 0.40 V.39 The redox potential for nondeprotonated DA is 0.752 V40 or 0.801 V41 at pH 0, 0.612 V41 at pH 3.2, and 0.56 V at pH 4.5.42 The standard potential value (E°) for two-electron, two-proton (−2e/–2H+) reduction of DA quinone to DA was described as40E°′ = −47.93 × pH + 558.4 mV (vs Ag/AgCl, 3 M KCl) giving, after recalculation into NHE, E° = 0.75 V at pH 0, E°′5.5 = 0.491 V, and E°′7.4 = 0.40 V.