Dissociation of CP20 from iron(II)(cp20)3: A pulse radiolysis study

Article abstract of DOI:10.1002/ejic.200500821

Deferiprone (CP20), a hydroxypyridine-4-one, is used in iron-chelation therapy. For the iron(III)-/(II)(cp20)₃ couple, a standard electrode potential of -620 mV was measured by cyclic voltammetry [Templeton et al., Inorg. Chim. Acta 1996, 245, 199; Merkofer et al., Helv. Chim. Acta 2004, 87, 3021]. On the basis of this value and the overall stability constant for the equilibrium of iron(III) and three CP20 molecules, ligand dissociation ought to take place after reduction of the iron(III)(cp20)₃ complex. By pulse radiolysis, we show that hydrated electrons reduce the iron(III)(cp20) ₃ complex with a rate constant of k = (6.4 ± 0.3) × 10¹⁰ M^-1S^-1, and that the iron(II)(cp20) ₃ complex aquates rapidly. The dissociation of the first two CP20 molecules takes place within a few μs after the pulse. The dissociation rate constant for the last CP20 ligand is (8 ± 1) × 10³ S^-1. The observation of a dissociation illustrates that, under dilute conditions, iron(II) is not fully complexed by CP20 and could potentially participate in redox cycling to produce oxyradicals. The CO₂ ^- radical does not reduce iron(III)(cp20)₃, which indicates that this complex reacts, as expected, very slowly with inner-sphere reductants. Wiley-VCH Verlag GmbH & Co. KGaA, 2006).

Full text of DOI:10.1002/ejic.200500821

FULL PAPER
DOI: 10.1002/ejic.200500821
Dissociation of CP20 from Iron(II)(cp20)3: A Pulse Radiolysis Study
Martin Merkofer,[a] Anastasia Domazou,[a] Thomas Nauser,[a] and Willem H. Koppenol*[a]
Keywords: Iron / Chelates / Reduction / Electron transfer / Kinetics
Deferiprone (CP20), a hydroxypyridine-4-one, is used in
iron-chelation therapy. For the iron(III)-/(II)(cp20)3 couple, a
standard electrode potential of –620 mV was measured by
cyclic voltammetry [Templeton et al., Inorg. Chim. Acta 1996,
245, 199; Merkofer et al., Helv. Chim. Acta 2004, 87, 3021].
On the basis of this value and the overall stability constant
for the equilibrium of iron(III) and three CP20 molecules, li-
gand dissociation ought to take place after reduction of the
iron(III)(cp20)3 complex. By pulse radiolysis, we show that hy-
drated electrons reduce the iron(III)(cp20)3 complex with a
the first two CP20 molecules takes place within a few μs after
the pulse. The dissociation rate constant for the last CP20
ligand is (8 1)×103 s–1. The observation of a dissociation il-
lustrates that, under dilute conditions, iron(II) is not fully
complexed by CP20 and could potentially participate in re-
·–
dox cycling to produce oxyradicals. The CO2 radical does
not reduce iron(III)(cp20)3, which indicates that this complex
reacts, as expected, very slowly with inner-sphere reduc-
tants.
rate constant of k = (6.4 0.3)×1010
M
–1 s–1, and that the
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
iron(II)(cp20)3 complex aquates rapidly. The dissociation of
far removed from those of the standard state: the concen-
tration of iron–CP20 complexes is only in the micromolar
range, and the pH is much lower than 11. By cyclic voltam-
metry, we could show that, under dilute conditions, the elec-
trode potentials shift to more positive values, which may
occur as a result of dissociation of ligands from iron(ii).[8]
Indeed, binding data derived from electrode potentials indi-
cate that iron(ii) complexes of CP20 are not fully formed at
physiological pH and low iron concentration, even when
CP20 is present in fivefold excess. We show here by pulse
radiolysis that, indeed, the proposed ligand dissociation[8]
occurs after rapid reduction of iron(iii)(cp20)3 by hydrated
electrons.
Introduction
Iron is essential to life. It is used, transported and stored
by specific proteins such as haem proteins, transferrin and
ferritin. When, for various reasons, iron is released from
these proteins or excessively absorbed from food, it can par-
ticipate in redox cycling that generates toxic oxyradicals.[1,2]
The bidentate chelating agent CP20 (1,2-dimethyl-3-hy-
droxypyrid-4(1H)-one, also known as deferiprone or L1,
Scheme 1) is effective at facilitating iron removal from iron-
overload patients when administered orally.[3] It forms five-
membered chelate rings in which the metal is coordinated
by two vicinal oxygen atoms. This chelating agent is an al-
ternative to the well-established desferrioxamine, which re-
quires intravenous administration. Another orally active
drug, tridentate ICL670 (4-[3,5-bis(2-hydroxyphenyl)-1H-
1,2,4-triazol-1-yl]benzoic acid), is under development.[4] It
is generally assumed that these chelating agents are not
toxic when the standard electrode potentials of the iron(iii)/
iron(ii) couples are such that they cannot be reduced by
ascorbate or oxidised by hydrogen peroxide, that is, below
E°Ј(Asc·–,H+/HAsc–) = +282 mV[5] or above E°Ј(H2O2,H+/
HO·,H2O) = +370 mV, respectively. The latter value is
based on Gibbs energies of formation of the hydroxyl radi-
cal and hydrogen peroxide determined by Schwarz and
Dodson[6] and Kern,[7] respectively. This is the case for the
electrode potential of the iron(iii)-/iron(ii)(cp20)3 couple,
–620 mV at pH = 11. However, the in vivo conditions are
Scheme 1.
Results
Distribution Diagram of the Iron(II)–(CP20) and Iron(III)–
(CP20) Complexes
Speciation diagrams for the iron(ii)–(CP20) and iron(iii)–
(CP20) complexes at 20 μm iron, 100 μm CP20 and pH =
7.2 are shown in Figure 1A and Figure 1B, respectively. As
the binding constants β1 and β2 are not known for iron(ii),
we assumed that the ratios β3/β2 and β3/β1 for iron(ii) are
[a] Laboratorium für Anorganische Chemie, Departement Chemie
und Angewandte Biowissenschaften, ETH Hönggerberg,
8093 Zürich, Switzerland
E-mail: koppenol@inorg.chem.ethz.ch
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M. Merkofer, A. Domazou, T. Nauser, W. H. Koppenol
FULL PAPER
similar to those of iron(iii). The data for Figure 1A and
Figure 1B were obtained from the literature.[8] While
iron(iii) tightly binds three CP20 ligands at pH = 7 and
higher, iron(ii) does not. In fact, not even the 1:1 complex
of iron(ii) and CP20 is fully formed at pH = 7.
Figure 2. Reduction of FeIII(cp20)3 (100 μm) with hydrated elec-
trons (14 μm) at 450 nm. Reaction conditions: phosphate buffer
(5 mm) with tert-butyl alcohol (0.1 m) at pH = 7.2 and room tem-
perature. Following rapid reduction, a slower process is observed
that is ascribed to ligand dissociation. The reduction with hydrated
electrons proceeds quantitatively.
Figure 1. Iron(ii) (A) and iron(iii) (B) speciation in the presence of
CP20. [CP20] = 100 μm, [Fe2+] and [Fe3+] = 20 μm. While iron(iii)
tightly binds three CP20 ligands at pH = 7 and higher, iron(ii) does
not. The pKa value used for CP20 is 9.76.[20]
Figure 3. Reduction of FeIII(cp20)3 (100 μm) with hydrated elec-
trons (12 μm) at 450 nm and decay of the hydrated electrons at
600 nm. Reaction conditions: phosphate buffer (5 mm) with tert-
butyl alcohol (0.1 m) at pH = 7.2 and room temperature. The decay
of the hydrated electrons at 600 nm shows the same time-depen-
dence as the fast decay at 450 nm.
·–
Reduction of the Iron(III)(cp20)3 Complex by e– and CO2
aq
Rapid reduction of iron(iii)(cp20)3 (100 μm) by hydrated
electrons (14 μm) leads to decreases in absorption at
450 nm. The fastest is finished within one μs and the slow-
est within one ms (Figure 2 and Figure 3). Reduction pro-
ceeds quantitatively (Figure 2), based on an extinction coef-
ficient of 4600 m–1 cm–1 for the iron(iii)(cp20)3 complex at
450 nm.[8] These processes are not caused by the presence
of tert-butyl alcohol (data not shown). The rate of decay of
hydrated electrons measured at 600 nm is identical to the
rate of the fast process at 450 nm (Figure 3). This rate is
first-order in iron(iii)(cp20)3 (Figure 4), from which a rate
[FeII(cp20)]+ Ǟ Fe2+ + CP20–
(1)
Mechanistically, the observed first-order reaction might
also be represented by Equation (2) at constant pH.
H+ + [FeII(cp20)]+ Ǟ [FeII(Hcp20)]2+ Ǟ Fe2+ + Hcp20
(2)
constant of (6.4 0.3)×1010
m
–1 s–1 is calculated. The rate
of decay of the hydrated electrons measured at 600 nm indi-
cates that the reduction is over within one μs. The change However, control experiments at pH = 8.6 showed that the
in absorbance at 450 nm during 1 and 3 μs is interpreted as rate of reaction is independent of pH; therefore, reaction 2
a result of the sequential dissociation of two CP20 ligands. is not relevant.
The slow absorbance change after that process, which fits
Identical results to those with tert-butyl alcohol were ob-
to a single exponential, is attributed to the dissociation of tained when methanol was used as a hydroxyl-radical scav-
the last ligand, as described in Equation (1). The reaction enger. Interestingly, dioxocarbonate(·1–), a strong reductant
is strictly first-order, with a reaction rate constant of E° (CO2/CO2·–) = –1.9 V,[9] did not reduce iron(iii)(cp20)3
(8 1)×103 s–1.
to any detectable degree.
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Eur. J. Inorg. Chem. 2006, 671–675

Dissociation of CP20 from Iron(ii)(cp20)3: A Pulse Radiolysis Study
FULL PAPER
the iron(iii) complex. At high pH values, this process is
slower. Hegetschweiler and coworkers[10] reported that the
iron(ii)–ICL670 complex behaves similarly.
Figure 4. First-order rate constants at various concentrations of
FeIII(cp20)3. The reactions with hydrated electrons (5 μm) were car-
ried out at pH = 7.2 and room temperature. The reduction of
iron(iii)(cp20)3 is linearly dependent on the complex concentration.
The bimolecular rate constant is k = (6.4 0.3)×1010 –1 s–1.
m
Figure 6. Time-dependent spectra measured by pulse radiolysis
(diamonds, squares and triangles) of the reaction of iron(iii)
(33 μm) and CP20 (100 μm) with hydrated electrons (3.4 μm) at pH
= 7.2 and room temperature. The small crosses indicate the differ-
ence spectrum of the iron(ii)- and the iron(iii)(cp20) complex mea-
sured by stopped-flow spectroscopy (see text). Per redox equivalent,
the final difference spectrum from the pulse radiolysis is equal to
that obtained by the stopped-flow technique.
Oxidation of Iron(II) by Hydrogen Peroxide in the Presence
of CP20
The absorbance change of this oxidation, per reducing
equivalent, should equal that for the reduction of iron(iii)-
(cp20)3 by hydrated electrons. To calculate a difference
spectrum for the iron(ii)- and the iron(iii)(cp20) complexes,
we mixed a solution of iron(ii) (20 μm) and CP20 (100 μm)
with hydrogen peroxide (200 μm) at pH = 7.2 and 15 °C
(Figure 5). At this temperature, the reaction becomes
slower, and the signal-to-noise ratio of the time-dependent
spectrum improves. Between 350 and 550 nm, the absorp-
tion increase per hydrogen peroxide matches the absorption
decrease per hydrated electron in this range, as shown in
Figure 6. The hydroxyl radical or oxoiron(iv) formed in the
Fenton reaction is scavenged by the Tris buffer. The re-
sulting Tris radical is not expected to re-reduce iron(iii)-
(cp20)3. Because the iron(ii)(cp20) complex can reduce
water, it is impossible to prepare solutions of the pure com-
plex: Under anaerobic conditions, in gas-tight syringes as
well as in a flask purged with argon, the colour of the
iron(ii)(cp20) solution changes from light green to light
orange within several minutes, indicating the formation of
Discussion
As revealed by the speciation diagrams and as substanti-
ated by kinetics experiments, CP20 stabilises iron(iii), but
not iron(ii), very well. The first-order rate constant for the
dissociation of the last CP20 molecule from iron(ii) is of the
same order of magnitude, 104 s–1, as that for the dissoci-
ation of a pyridine ligand from a ferriprotoporphyrin in
chloroform.[11] CP20 is a bidentate ligand with two hard,
oxygen coordination atoms, whereas pyridine is a mono-
dentate ligand with a soft donor atom that stabilises
iron(ii), which leads to similar stability constants and sim-
ilar dissociation rate constants. To the best of our knowl-
edge, no other dissociation rates of ligands similar to hy-
droxypyridinones have been reported in the literature. The
solvent-exchange rates for water (k = 4.4×106 s–1),[12] meth-
anol (k = 5×104 s–1)[13] and acetonitrile (k = 6.6×
105 s–1)[14] on iron(ii) are higher. Given the speciation dia-
gram (Figure 1A), the observed dissociation process in-
volves all three ligands. After dissociation of CP20 from
iron(ii), monodentate ligands, such as water, bind to the me-
tal ion and exchange much faster than CP20. Consequently,
the probability that hydrogen peroxide coordinates to the
metal centre and undergoes a Fenton reaction is in-
creased.[15]
It is to be emphasised that the dissociation rate constant
is independent of pH, and, therefore, does not result from
protonation of the coordinated CP20 ligands. The spectral
changes underline that the observed process is also not a
dissociation of CP20 from iron(iii), because this would re-
sult in a red shift, rather than the observed blue shift, of
the spectrum.[8]
Figure 5. Time-dependent spectrum of the reaction of iron(ii)
(20 μm) and CP20 (100 μm) with hydrogen peroxide (200 μm) at pH
= 7.2 and 15 °C. Formation of the iron(iii)(cp20)3 complex.
Eur. J. Inorg. Chem. 2006, 671–675
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673

M. Merkofer, A. Domazou, T. Nauser, W. H. Koppenol
FULL PAPER
The reduction of the iron(iii)(cp20)3 complex by hydrated plexes with CP20 and ICL670 are near –150 mV under
electrons is most likely a diffusion-controlled process. Re- physiological conditions, and that superoxide is unable to
duction is possible with hydrated electrons, but not with reduce these complexes when they are fully formed. For
dioxocarbonate(·1–) radicals, although from a comparison complexes with less than three CP20 or two ICL670 mole-
of the electrode potential of the iron(iii)-/iron(ii)(cp20)3 cules, reduction is thermodynamically feasible.[8] Electrode
couple (–0.62 V)[8] with that of the carbon dioxide/ potentials of other reducing agents will be discussed in a
dioxocarbonate(·1–) couple (–1.90 V), it is clear that re- future communication.
duction is thermodynamically very favourable. This result
shows that it is difficult to reduce the iron(iii)(cp20)3 com-
plex with an inner-sphere reductant, because accessibility of
Experimental Section
the metal centre is sterically hindered by the large CP20
All chemicals used were of analytical reagent grade or higher.
ligands. In a pulse radiolysis study of 2-pyridone, Moorthy
Fe(NO3)3·9H2O was the source of FeIII. Hydrogen peroxide stock
et al.[16] could reduce the ligand with hydrated electrons, but
solutions were prepared from Merck Analyzed Reagent 30% hy-
drogen peroxide. Water was purified with a Millipore Milli-Q unit
fed with deionised water.
not with dioxocarbonate(·1–) radicals. They concluded that
an electrode potential smaller than –1.90 V is required to
reduce 2-pyridone. On the basis of these and our observa-
tions, we conclude that the iron(iii) is not directly accessible
and is reduced via the ligand.
Reoxidation of iron(ii) by hydrogen peroxide formed dur-
ing the irradiation was not detected. This is understandable:
For pulse radiolysis, a Febetron 705 (Titan Systems Corp., San Le-
andro, CA, USA) 2.3-MeV accelerator with a pulse width (full
width at half maximum) of Ͻ50 ns was used as the radiation
source. The optical system consisted of a 75W Xe arc lamp (Hama-
matsu, Schüpfen, Switzerland),
a 1-cm quartz cell (Hellma
if 10 μm iron(ii) is produced, 2.5 μm hydrogen peroxide is
also formed. Given a reaction rate constant of 104 m–1 s–1
for the Fenton reaction, the pseudo-first-order rate constant
would be ca. 10–2 s–1.
GmbH & Co KG, Müllheim, Germany) and an Acton SP300 mon-
ochromator (Roper Scientific, Ottobrunn, Germany). For signal
detection, there were two options: (1) a R928 photomultiplier
(Hamamatsu, Japan) with a DHPCA-200 amplifier (Femto Mes-
stechnik GmbH, Berlin, Germany) and a DL7100 digital storage
oscilloscope (Yokogawa Electric Corporation, Tokyo, Japan) for ki-
netics traces or (2) a Princeton Instruments PI-MAX 512T gateable
ICCD-camera (Roper Scientific) for time-resolved spectra. The
dose per pulse used was 7–65 Gy, which corresponds to hydrated
electron concentrations of 1.8–17 μm.[19] Dosimetry was carried out
with thiocyanate.
We have shown that reduction by hydrated electrons is
followed by ligand dissociation. One might now ask what
would happen with more physiologically relevant reduc-
tants. El-Jammal and Templeton[17] proposed on the basis
of their electrochemical measurements that reduction of
iron(iii) takes place after dissociation of the complex. We
assess here the thermodynamics of two scenarios: first re-
duction followed by dissociation, and then the opposite.
The standard electrode potential of the iron(iii)-/iron(ii)-
(cp20)3 couple is –0.62 V, and reduction by an electron
donor with an electrode potential near 0 V “costs” ca.
58 kJmol–1. This process is then followed by favourable li-
gand dissociation. The dissociation of one CP20 ligand
The primary products from the radiolysis of water are H+aq, e–
,
aq
HO·, H·, H2 and H2O2. We used tert-butyl alcohol (0.1 m) or meth-
anol (0.1 m) as scavengers for HO· and H· radicals to produce the
fairly unreactive 2-hydroxy-2-methylpropyl- and hydroxymethyl
radicals.[19] Solutions for experiments with hydrated electrons con-
tained phosphate buffer (5 mm) and tert-butyl alcohol (0.1 m) or
methanol (0.1 m), and were evacuated and saturated with argon at
from the iron(iii)(cp20)3 complex requires less energy, about least five times.
53 kJmol–1, based on the difference between β3 and β2. In
The dioxocarbonate(·1–) radical was generated by reactions repre-
dilute solutions, this Gibbs energy decreases considerably,
which makes the “ligand dissociation first” scenario more
favourable. Therefore, for biologically relevant conditions,
we agree with the proposal of El-Jammal and Templeton.[17]
Iron(ii) complexes of other hydroxypyridinones and of
ICL670 are also unlikely to be formed, given the similarly
low electrode potentials.[8] Previously, a chelating agent with
an electrode potential for its iron(iii)/iron(ii) complex of
ca. –470 mV or lower, such as that of iron(iii)-/iron(ii)
desferrioxamine,[18] was considered nontoxic. This hexaden-
tate ligand binds iron(ii) well at high dilution, in contrast
to bi- and tridentate chelating agents. Iron complexes with
bidentate chelating agents are more sensitive to dissociation
upon dilution, which causes their electrode potentials to
shift to more positive values.[8] Considering that the “free”
iron concentration in vivo is about 10 times lower than that
used here, one can conclude that CP20 does not bind
iron(ii) under physiological conditions. Recently, we showed
sented by Equation (3) and Equation (4). These solutions were sat-
urated with dinitrogen monoxide (24 mm) and contained formate
(50 mm, pH = 8.6).
e– + N2O Ǟ HO· + HO– + N2
(3)
(4)
aq
·–
HCOO– + HO· Ǟ CO2 + H2O
The kinetics of the reduction of the iron(iii)(cp20)3 complex and
the ligand dissociation were recorded at 450 nm. The decay of the
hydrated electrons was followed at 600 nm.
Stopped-flow experiments were obtained with an OLIS stopped-
flow instrument (Bogart, GA, USA) equipped with an OLIS RSM
1000 rapid-scanning monochromator set to collect up to 1000 spec-
tra per second. Solutions for the stopped-flow experiments were
prepared in Tris buffer (0.1 m), and evacuated and saturated with
argon at least five times before addition of iron(ii). Hydrochloric
acid was used to adjust the pH. The hydrogen peroxide solution
that the electrode potentials of the iron(iii)/iron(ii) com- was purged with argon for at least 30 min.
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Dissociation of CP20 from Iron(ii)(cp20)3: A Pulse Radiolysis Study
FULL PAPER
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Acknowledgments
This work was supported by the ETH Zürich, Swiss National Sci-
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Received: September 15, 2005
Published Online: December 20, 2005
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