Jewett, SL, Eggling, S, and Geller, L (1997). J. Inorg.
Biochem. 66: 166-173.
Novel Method to Examine the Formation of Unstable 2:1 and
3:1 Complexes of Catecholamines and Iron(III)
ABSTRACT
Fe(CAT)2- + CAT2- <===========>
Fe(CAT)33-
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 25o C starting with iron(II). No spectro-photometrically
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 and 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 conversionwhich
occurs for the catecholamines from pH 6.5 to 8.5 and for catechol from
7 to 9. Kinetic 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 were too fast to follow by the conventional
techniques used in this work.