A KINETIC-STUDY OF THE REACTION BETWEEN NORADRENALINE AND IRON(III) -AN EXAMPLE OF PARALLEL INNER-SPHERE AND OUTER-SPHERE ELECTRON-TRANSFER

In anaerobic acid solution noradrenaline [norepinephrine, 4-(2-amino-1 -hydroxyethyl)benzene-1,2-diol, H2LH+ tin which the phenolic protons a re written on the left of L)] reacts with iron(In) [in the form of Fe( OH)(2+)] to yield iron(II) and the semiquinone form of noradrenaline w hich is in turn oxidised rapidly by more iron(III) to 'noradrenoquinon e'. This reaction proceeds both directly(i.e. via an 'outer-sphere' re action) and after prior formation of the complex Fe(LH)(2+) which then decomposes via intramolecular electron transfer. [The observed rate o f formation of the complex (monitored at 714 nm) is faster than the ra te of its decomposition by a factor of about 200.] The quinone then cy clises by an intramolecular Michael addition giving the (UV transparen t) leuconoradrenochrome (indoline-3,5,6-triol), which is able to react with iron(III) at high pH to give noradrenochrome (3,5-dihydro-3,6-di hydroxy-2H-indol-5-one). At lower pH values the presence of chloride i ons shows a marked effect on the rate of complex formation because the species FeCl2+ is also able to react with noradrenaline to form the c omplex although chloride is not involved in the reverse reaction. The stability constant for the formation of FeCl2+ (K-1(Cl)) was found to be 35, i.e. log K-1(Cl) = 1.54 (identical to the value obtained from p revious work with dopamine). The ring-closure reaction was studied by following the rate of quinone decomposition monitored at 380 nm, and a mechanism for this cyclisation is proposed. The following rate consta nts have been evaluated: (i) for the reversible formation of the iron- noradrenaline complex [via Fe(OH)(2+) + H2LH+] = 21 +/- 2 dm(3) mol(-1 ) s(-1), from which the stability constant of the Fe(LH)(2+) k(1) = 21 70 +/- 20 dm(3) mol(-1) s(-1) and k(1)(M) = 21.2), (ii) rate of format ion of the complex via FeCl2+ + H2LH+, complex has been calculated (lo g K-1(M) = 21.2(, (ii) rate of formation of the complex via FeCl2+ + H 2LH+, k(Cl) = 48 +/- 3 dm(3) mol(-1) s(-1), (iii) rate of decompositio n of the complex Fe(HLH)(3+), k(2) = 2.6 +/- 0.1 s(-1) [protonation co nstant for Fe(LH)(2+), K-M(H) = 34 +/- 1 dm(3) mol(-1)], (iv) rate of outer-sphere redox reaction, k(2)' = 100 +/- 2 dm(3) mol(-1) s(-1), (v ) rate of indole formation (ring-closure reaction), k(cyc) = 1400 +/- 20 s(-1) (for quinone) and k(cyc)(H) =(2.0 +/- 0.1) x 10(5) s(-1) (for protonated quinone). All measurements were carried out at 25.0 degree s C in solutions of ionic strength 0.10 mol dm(-3) (KNO3 serving as in ert electrolyte).