Novel method to examine the formation of unstable 2:1 and 3:1 complexes of catecholamines and iron(III)

• Published in: Journal of inorganic biochemistry 1997-05, Vol.66 (3), p.165-173
• Main Authors: Jewett, Sandra L., Eggling, Sue, Geller, Louis
• Format: Article
• Language: English
• ISSN: 0162-0134; 1873-3344
• DOI: 10.1016/S0162-0134(96)00198-5

The formation of the 2:1 and 3:1 complexes of the catecholamines, epinephrine and norephinephrine, as well as the less easily oxidized catechol, and iron(III) was studied in a physiological pH range in air-saturated buffers at 25°C starting with iron(II). No spectrophotometrically observable complexes are formed in deoxygenated solutions. In addition, there is no spectral evidence for oxidation of the catecholamines during the measurements reported. For every different condition desired, fresh buffer was used along with fresh aliquots of iron and catechol(amines) from acidified stock solutions. Although this is not a traditional method, it works well for these catecholamine systems, and allows characterization of stoichiometry, equilibria, and kinetics of these relatively unstable iron(III) complexes starting with iron(II). This methodology was used to determine the equilibrium constant for the 2:1 to 3:1 conversion (as Fe(CAT) 2 − + CAT 2− ↔ Fe(CAT) 3 3−) which occurs for the catecholamines from pH 6.5 to 8.5 and for catechol from 7 to 9. Kinetics studies indicated that the formation of the 2:1 complexes is first order in both iron (II) and catechol(amine) and has a pH dependence of 1/[H +] 2. These data are interpreted in terms of a mechanism involving the formation of a 1:1 iron(II)-catechol(amine) dianion complex followed by the rate-determining step of oxidation by dioxygen to give the iron(III) complex. This step is assumed to be followed by the rapid addition of a second (and presumably third) catechol(amine) to give the 2:1 complex (and 3:1 complex). The formation of the 3:1 complexes was too fast to follow by the conventional techniques used in this work.