A heterobimetallic FeIIIMnII complex of an unsymmetrical dinucleating ligand: A structural and functional model complex for the active site of purple acid phosphatase of sweet potato

Article abstract of DOI:10.1002/ejic.201301375

The heterodinuclear mixed-valence complex [FeMn(ICIMP)(OAc)₂Cl] (1) {H₂ICIMP = 2-(N-carboxylmethyl)-[N-(N-methylimidazolyl-2-methyl) aminomethyl]-[6-(N-isopropylmethyl)-[N-(N-methylimidazolyl-2-methyl)] aminomethyl-4-methylphenol], an unsymmetrical N₄O₂ donor ligand} has been synthesized and fully characterized by several spectroscopic techniques as well as by X-ray crystallography. The crystal structure of the complex reveals that both metal centers in 1 are six-coordinate with the chloride ion occupying the sixth coordination site of the Mn^II ion. The phenoxide moiety of the ICIMP ligand and both acetate ligands bridge the two metal ions of the complex. M?ssbauer spectroscopy shows that the iron ion in 1 is high-spin Fe^III. Two quasi-reversible redox reactions for the complex, attributed to the Fe^IIIMn^II/Fe ^IIMn^II (at -0.67 V versus Fc/Fc⁺) and Fe ^IIIMn^II/Fe^IIIMn^III (at 0.84 V), were observed by means of cyclic voltammetry. Complex 1, with an Fe ^III-Mn^II distance of 3.58 ?, may serve as a model for the mixed-valence oxidation state of purple acid phosphatase from sweet potato. The capability of the complex to effect organophosphate hydrolysis (phosphatase activity) has been investigated at different pH levels (5.5-11) by using bis(2,4-dinitrophenyl)phosphate (BDNPP) as the substrate. Density functional theory calculations indicate that the substrate coordinates to the Mn^II ion. In the transition state, a hydroxide ion that bridges the two metal ions becomes terminally coordinated to the Fe^III ion and acts as a nucleophile, attacking the phosphorus center of BDNPP with the concomitant dissociation of the leaving group. Copyright

Full text of DOI:10.1002/ejic.201301375

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DOI:10.1002/ejic.201301375
A Heterobimetallic FeIIIMnII Complex of an
Unsymmetrical Dinucleating Ligand: A Structural and
Functional Model Complex for the Active Site of Purple
Acid Phosphatase of Sweet Potato
Biswanath Das,[a] Henrik Daver,[b] Amrendra Singh,[a]
Reena Singh,[a] Matti Haukka,[c] Serhiy Demeshko,[d]
Franc Meyer,[d] George Lisensky,[e] Martin Jarenmark,*[a]
Fahmi Himo,*[b] and Ebbe Nordlander*[a]
Keywords: Metalloenzymes / Mixed-valent compounds / Moessbauer spectroscopy / Density functional calculations /
Transition states / Hydrolysis
The heterodinuclear mixed-valence complex [FeMn(ICIMP)-
(OAc)2Cl] (1) {H2ICIMP = 2-(N-carboxylmethyl)-[N-(N-meth-
ylimidazolyl-2-methyl)aminomethyl]-[6-(N-isopropylmeth-
yl)-[N-(N-methylimidazolyl-2-methyl)]aminomethyl-4-meth-
ylphenol], an unsymmetrical N4O2 donor ligand} has been
synthesized and fully characterized by several spectroscopic
techniques as well as by X-ray crystallography. The crystal
structure of the complex reveals that both metal centers in 1
are six-coordinate with the chloride ion occupying the sixth
coordination site of the MnII ion. The phenoxide moiety of
the ICIMP ligand and both acetate ligands bridge the two
metal ions of the complex. Mössbauer spectroscopy shows
that the iron ion in 1 is high-spin FeIII. Two quasi-reversible
redox reactions for the complex, attributed to the FeIIIMnII/
FeIIMnII (at –0.67 V versus Fc/Fc+) and FeIIIMnII/FeIIIMnIII (at
0.84 V), were observed by means of cyclic voltammetry.
Complex 1, with an FeIII–MnII distance of 3.58 Å, may serve
as a model for the mixed-valence oxidation state of purple
acid phosphatase from sweet potato. The capability of the
complex to effect organophosphate hydrolysis (phosphatase
activity) has been investigated at different pH levels (5.5–
11) by using bis(2,4-dinitrophenyl)phosphate (BDNPP) as the
substrate. Density functional theory calculations indicate that
the substrate coordinates to the MnII ion. In the transition
state, a hydroxide ion that bridges the two metal ions be-
comes terminally coordinated to the FeIII ion and acts as a
nucleophile, attacking the phosphorus center of BDNPP with
the concomitant dissociation of the leaving group.
A
diphosphonucleotide phosphatase/phosphodiesterase
Introduction
from yellow lupin was lately reported to contain a PAP-like
FeMn center.[11] A new group of enzymes, very similar to
the RNR from C. trachomatis, has recently been discovered
and appears to require the same type of FeMn site.[5,12]
However, important structural differences indicate a dif-
ferent function, possibly relating to oxidase activity as a
part of lipid metabolism.[12]
Several classes of metalloenzymes have a dinuclear active
site in which the two transition-metal ions work coopera-
tively to effect various transformations.[1] At least two
enzymes, purple acid phosphatase (PAP) from sweet po-
tato[2–4] and ribonucleotide reductase (RNR) from Chlamy-
dia trachomatis,[5–9] contain a heterodinuclear FeMn site.[10]
Purple acid phosphatases are phosphoesterases with an
optimal activity at low to neutral pH and a characteristic
purple color that stems from a phenolate to FeIII charge-
transfer interaction.[13–15] There are several different iso-
forms isolated from both mammals and plants, and one iso-
form from sweet potato has an FeMn active site.[2] Owing
to a strong antiferromagnetic coupling, it is believed that
the two metals are bridged by an oxo ligand in the resting
state.[3] In the crystal structure available for this enzyme,[4]
inorganic phosphate, the product of phosphomonoester ac-
tivity, bridges the two metals in an unusual tripodal μ-η2-
η2-H2PO4 mode and one of the oxygen atoms of the phos-
phate has replaced the bridging oxo group, which suggests
[a] Inorganic Chemistry Research Group, Chemical Physics,
Center for Chemistry and Chemical Engineering,
Lund University, Box 124, 22100 Lund, Sweden
http://www.chemphys.lu.se/people/nordlander/
[b] Department of Organic Chemistry,
Arrhenius Laboratory, Stockholm University,
10691 Stockholm, Sweden
http://www.organ.su.se/himo/
[c] Department of Chemistry, University of Joensuu,
P. O. Box 111, 80101 Joensuu, Finland
[d] Institute of Inorganic Chemistry,
Georg August University Göttingen,
Tammanstrasse 4, 37077 Göttingen, Germany
[e] Department of Chemistry, Beloit College,
700 College St., Beloit, WI 53511, USA
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201301375.
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that the latter could be the active nucleophile during hydrol- the selective binding of FeIII, a hard acceptor, on the carb-
ysis. oxylate side to form a mononuclear synthon that might be
Ribonucleotide reductases are involved in the transfor- used in the systematic synthesis of heterodinuclear FeIIIMII
mation of ribonucleotides into deoxyribonucleotides.[16,17] complexes (ligand = IPCPMP, M = Zn, Co, Ni, Mn; ligand
Three different classes have been identified, among which = ICIMP, M = Zn).[24,25] All these complexes, except the
class I requires dioxygen. Class I RNRs are further divided FeMn derivative of IPCPMP, were crystallographically
into three subgroups depending on the type of reducing characterized, and they were all found to exhibit phospho-
equivalent and radical storage that is used. Class Ia and Ib esterase activity. The IPCPMP complexes in particular
use a dinuclear metal site to activate dioxygen and form a showed interesting differences in pH dependence for the
stable tyrosyl radical. The Ia enzymes utilize a diiron site, cleavage of the phosphodiester bis(2,4-dinitrophenol)phos-
whereas it has recently been demonstrated that the Ib en- phate (BDNPP). Herein, we report the synthesis, charac-
zymes contain analogous dimanganese sites.[17–19] The Ic terization, and phosphoesterase activity of an FeIIIMnII
enzymes have been shown to lack the tyrosine residue[5] and complex of the ligand ICIMP that is a structural and func-
instead have an FeMn site that both reduces dioxygen and tional model of the FeMn sweet-potato PAP and which also
acts as the radical storage site.[6–8,17] Class Ic appears to be might serve as a model complex for the newly identified
favored for pathogens and extremophiles, such as C. tra- class Ic RNR FeMn sites.
chomatis, which is responsible for sexually transmitted chla-
mydia infections.
Results and Discussion
Sequence alignment has identified several proteins with
strong similarities to the class Ic RNRs but which lack an
important residue in the radical-transfer pathway essential
for RNR activity. One of these proteins, Rv0233, has been
Synthesis
In an attempt to prepare a suitable model for the sweet-
isolated and structurally characterized (Figure 1) and con- potato purple acid phosphatase active site, the heterodi-
tains one Fe and one Mn but shows no RNR activity.[12] nuclear complex [FeMn(ICIMP)(OAc)2Cl] (1) was synthe-
The dinuclear metal site and other structural features of this sized by the sequential addition of FeCl3 and Mn(OAc)2,
protein are very similar to diiron bacterial multicomponent and the weak base tributylamine, to a solution of the ligand
monooxygenases (BMMs), except that one iron ion is re- in methanol. Complex 1 (Figures 2d and 3) was isolated in
placed by a manganese ion, and the presence of a tyrosine– pure form from a methanol/2-propanol solution and char-
valine crosslink.[12] Two metal ions are coordinated by a acterized by spectroscopic methods. A 0.12 mm solution of
fatty acid ligand through a μ-1,3-carboxylate bridging complex 1 in methanol exhibits an absorption band around
mode. These facts suggest that this group of enzymes might 509 nm (ε = 2480 m–1 cm–1), which can be assigned to a
be involved in some oxidase activity in lipid metabolism.
transition that involves charge transfer from the bridging
phenolate to the ferric ion [ligand-to-metal charge transfer
(LMCT)].[26] Such a charge-transfer transition is the origin
of the purple color of the PAP enzymes; in sweet potato,
the PAP exhibits a tyrosine-to-FeIII charge-transfer transi-
tion at 560 nm (ε = 3200 m–1 cm–1).
The ESI mass spectrum of a solution of 1 in methanol
(Figure S1 in the Supporting Information) shows a promi-
nent peak envelope at 735.3 amu with a characteristic iso-
tope pattern that corresponds to the molecular ion [FeM-
n(ICIMP)(OAc)2HCl(H2O)]+, and the formula is corro-
borated by elemental analysis (see the Experimental Sec-
Figure 1. The FeMn active site of Rv0233 protein with a fatty acid
ligand coordinated to the metal centers. Adapted from the litera-
ture.[11]
There are only a few well-characterized heterodinuclear tion). UV/Vis spectra recorded at varying pH (pH = 6–11)
FeMn complexes that model the active sites of the above- show a clear blueshift of the maximum absorption peak as
mentioned enzymes (see below). This is likely due to the the pH is raised from 6 to 9; this shift might be connected
relatively late discovery of heterodinuclear metalloenzymes, to the formation of a hydroxido-bridged species (see the
in particular FeMn enzymes, and the difficulties in selec- Computational Studies below). The absorption maximum
tively binding the two metals at specific sites while avoiding could only be seen as a broad shoulder at pHՆ10 (Fig-
mixtures and homodinuclear side products. We have de- ure S4 in the Supporting Information). This might be due
signed two unsymmetric dinucleating ligands that contain a to the instability of the complex at high pH, a phenomenon
phenolate group as the bridging moiety and either pyridyl that is indirectly demonstrated in reactivity studies of the
(IPCPMP)[20,21] or imidazole (ICIMP)[22,23] groups. In these complex (see below).
ligands, the two pockets of the ligand differ both in terms
The IR spectrum of the complex shows characteristic
of the number of donor moieties and their identities. Speci- bands of the coordinated acetate group for the antisymmet-
fically, one side of the phenolate contains a carboxylate ric stretching (1653 and 1609 cm–1) and for the symmetric
group, a relatively hard donor, and the other side contains stretching (1468 and 1393 cm–1) vibrations in KBr. The dif-
a non-coordinating isopropyl group. This setup allows for ference between the symmetric and antisymmetric stretches
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oxylate moiety,[27] and the broadened peaks at 1653 and
1393 cm–1 (Δν = 260 cm–1) are in accordance with what is
expected for a monodentate carboxylate.[24,28]
Crystal and Molecular Structure of [FeMn(ICIMP)(OAc)2-
Cl] (1)
Crystals of 1 suitable for X-ray analysis were grown from
a methanol/2-propanol solution, and the crystal structure
was determined. The molecular structure of 1 is shown in
Figures 2 (d) and 3. The crystallographic data as well as
selected distances and angles are given in Tables 1 and 2.
The two metal sites can be easily distinguished by compar-
ing bond lengths to coordinated heteroatoms, since they are
significantly shorter for the Fe site, owing to the smaller
ionic radius of the FeIII ion. The two metals have N2O4 (Fe)
and ClN2O3 (Mn) coordination environments in distorted-
octahedral geometries with average bond lengths of 2.04
and 2.26 Å, respectively. The metals are unsymmetrically
bridged by the phenolate oxygen of the ligand (O1) and two
syn,syn-μ1,3-acetate groups (O3,O4 and O2,O5) with differ-
ences between the Mn–O and Fe–O bonds of 0.141 and
0.177 Å for each acetate, respectively, and, significantly,
0.262 Å for the phenolate bridge. The long Mn–O1 distance
[2.227(3) Å] is likely due to the bridging phenoxo ligand
being located trans to the negatively charged chloride ion
[Mn1–Cl1 2.464(2) Å]. It should, however, be noted that
Carboni et al.[29] have observed a similar Mn–O (bridging
phenoxo) bond length [2.188(2) Å] when a neutral water
molecule is in a trans position to the bridging phenolate
(see Figure 2, c); it may thus be that the differences in the
M–Ophenolate distances primarily reflect differences in ionic
radii. The bonds from the metal to the tertiary amines are
longer than the corresponding bonds to the heterocyclic
nitrogen atoms for each metal, which is in agreement with
what has been previously observed for similar com-
plexes.[24,25,29] In particular, one may note the long distance
Figure 2. Schematic depiction of structurally characterized
FeIIIMnII complexes (a) [FeMn(BPBPMP)(μ-OAc)2]+,[30b] (b)
[FeMn(BIMP)(μ-OAc)2]2+ [30a]
,
(c) [FeMn(L–Bn)(μ-OAc)2]2+ [29]
,
and the present complex (d) [FeMn(ICIMP)(OAc)2Cl]. The pyridyl
analogue of [FeMn(BIMP)(μ-OAc)2]2+
–
[FeMn(BPMP)-
(μ-OAc)2]2+ – has been reported, but only the reduced FeIIMnII
complex [FeMn(BPMP)(μ-OAc)2]+ has been structurally charac-
terized.[31]
.
Table 1. Crystal data.
1
Formula
Mr
C31H46ClFeMnN6O8
776.98
T [K]
λ [Å]
Crystal system
Space group
a [Å]
100(2)
0.71073
orthorhombic
P212121
10.7678(5)
10.8869(4)
30.9782(2)
3631.5(3)
4
1.421
0.876
45000
6404
b [Å]
c [Å]
V [Å3]
Z
ρcalcd. [Mgm3]
μ(Mo-Kα) [mm–1]
Number of reflections
Unique reflections
GoF (F2)
Rint
Figure 3. ORTEP representation of the crystal structure of
[FeMn(ICIMP)(OAc)2Cl] (1) with thermal ellipsoids at the 40%
probability level. Solvent molecules have been omitted for clarity.
1.061
0.1075
0.0550
0.1129
has been used extensively to evaluate the coordination
mode of carboxylate groups.[24] The separation between the
two sharp peaks at 1609 and 1468 cm–1 (Δν = 141 cm–1
R1[a] [IՆ2σ(I)]
wR2[b] [IՆ2σ(I)]
Ͻ200 cm–1) suggests a bidentate bridging mode of a carb- [a] R1 = Σ||Fo| – |Fc||/Σ|Fo|. [b] wR2 = {Σ[w(F2o – Fc2)2]/Σ[w(F2o)2]}1/2
.
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to the tertiary amine from the Mn ion of complex 1 [Mn– methyl)amino]methyl}-4-methylphenol] (see Figure 2).[29,30]
N3 2.445(4) Å].
This indicates that changes in the coordination sphere at
the divalent metal can influence the bond lengths at the
FeIII site, in particular for groups that bridge the metals
through a one-atom bridge. This might be of importance
for the mechanism of the phosphoester hydrolyses that this
type of complex often catalyzes.
Table 2. Selected measured and calculated distances [Å] and
angles[a] for 1.
Crystal structure
DFT calculations
Fe1–O5
1.958(4)
1.96
2.03
1.99
1.96
2.19
2.35
2.15
2.15
2.22
2.40
2.59
2.43
3.68
100.7
160.7
164.4
86.1
76.0
168.1
102.1
158.9
73.8
105.5
The fact that a negatively charged group (chloride in this
case) coordinated at the divalent metal may cause changes
at the FeIII site, relative to neutral donors at the same site,
is striking since the phosphodiester or -monoester sub-
strates often used are also negatively charged species. The
metal–metal distance is also longer in 1 [3.578(1) Å] relative
to those for the BIMP [3.472(1) Å],[30a] BPBPMP
[3.510(9) Å][30b] and L–Bn (3.47 Å) complexes. A FeZn
complex of ICIMP,[25] in which the Zn is five-coordinate
with a square-pyramidal geometry and lacks a ligand trans
to the phenolate, has a significantly shorter metal–metal
distance of 3.339 Å, supporting the idea that the coordina-
tion environment around the divalent metal exerts an influ-
ence on the distance to and coordination environment of
the FeIII ion.
Several crystal structures of purple acid phosphatases
from plants are available,[4,32–37] of which one is from sweet
potato and contains an FeMn center in the active site.[4]
The metal–metal distances for these enzymes are 3.1–3.3 Å
{i.e., shorter than the Fe–Mn distance in complex 1
[3.578(1) Å]}. However, the Fe–Mn distance in Rv0233 is
3.6 Å[12] (i.e., similar to that in 1). The bridges between the
Fe1–O1
Fe1–O4
Fe1–O6
Fe1–N6
Fe1–N4
Mn1–O2
Mn1–O3
Mn1–N1
1.965(4)
1.997(4)
2.004(4)
2.111(4)
2.223(5)
2.135(4)
2.138(4)
2.183(5)
Mn1–O1
Mn1–N3
2.227(3)
2.445(4)
2.464(3)
Mn1–Cl1
Fe1–Mn1
O5–Fe1–O1
O1–Fe1–O6
O5–Fe1–N4
O6–Fe1–N4
N6–Fe1–N4
O2–Mn1–N1
O2–Mn1–N3
O3–Mn1–N3
N1–Mn1–N3
O3–Mn1–Cl1
3.578(2)[b]
100.93(15)
164.40(16)
167.09(16)
76.01(16)
79.77(17)
172.49(16)
102.28(16)
166.22(17)
75.78(16)
97.70(12)
[a] The angles shown are the ones that deviate most from that ex-
pected for octahedral complexes. [b] Determined using Diamond
v3.1.
Very few cases of crystallographically well-characterized metals in 1 and Rv0233 are also similar to two syn,syn-μ-
monophenoxo-bridged FeIIIMnII complexes similar to 1 1,3-carboxylates and a single-atom oxygen bridge, although
have been published.[10,29,30] The overall structures are the this is derived from different functionalities, water in the
same but the donor moieties in the phenolate-based ligands enzyme, and phenolate in 1 (see Figure 1).
are different (Figure 2). With the exception of the complex
Because of the numerous stereogenic centers present in
[FeMn(BIMP)(μ-OAc)2]2+ [30a]
,
all complexes possess an complex 1, several different stereoisomers are possible. The
asymmetric ligand environment. In particular, [FeMn(L– complex, however, crystallized as a single stereoisomer with
Bn)(μ-OAc)2]2+[29] and the present complex share the fea- a Flack parameter of 0.02(2)[38] (space group P212121; see
ture of different denticities in the two pockets of the dinu- Table 1). The conformation adopted by the ligand is very
cleating ligand with non-coordinating benzyl (L–Bn) or iso- similar to that found for other heterodinuclear complexes
propyl (ICIMP) side arms.
of similar phenolate bridging ligands[24,25] with the two het-
A comparison of the average bond lengths for the six eroaromatic rings on either side of the phenolate plane. The
donor moieties on each metal that are chemically identical phenol ring plane (C10–C12, C14–C16) is tilted 55° relative
or equivalent (tertiary amines, methyl imidazole or pyridyl, to the plane defined by the two metals and the phenolate
and the bridging phenolate), shows that the distances to the oxygen (Mn, O1, Fe), a feature that is also found in other
FeIII ion are somewhat shorter for 1 [Fe–Xn(avg.) = 2.05 Å] complexes of ICIMP and IPCPMP, in which the offset an-
than in the corresponding BIMP (2.10 Å), BPBPMP gle varied between 51 and 53°.[24,25]
(2.08 Å), and L–Bn (2.06 Å) complexes, but longer to the
Mössbauer Spectroscopy
MnII ion (2.23 vs. 2.14, 2.20, and 2.19 Å, respectively). In
particular, the bonds to the bridging phenolate are elon-
The 57Fe Mössbauer spectrum of 1 was analyzed using a
gated at Mn and shortened at Fe for 1 [Mn–O1 2.227(3) Å Lorentzian line doublet and confirms unambiguously the
vs. Fe–O1 1.965(4) Å] relative to the BIMP [2.092(3) vs. high-spin FeIII configuration (Figure S3 in the Supporting
2.035(3) Å], BPBPMP [2.155(2) vs. 2.010(2) Å], and L–Bn Information), which shows a single quadruple doublet with
[2.188(2) vs. 1.937(2) Å] complexes [HBIMP = 2,6-bis- δ = 0.49 mms–1, ΔEQ = 0.71 mms–1, and linewidth Γ =
({bis[(1-methylimidazol-2-yl)methyl]amino}methyl)-4- 0.68 mms–1. The rather large linewidth might be rational-
methylphenol; H2BPBPMP = 2-bis{[(2-pyridylmethyl)- ized in view of the presence of iron(III) ions in slightly dif-
aminomethyl]-6-[(2-hydroxybenzyl)(2-pyridylmethyl)]- ferent environments in the microcrystalline material (per-
aminomethyl}-4-methylphenol; HL–Bn = 2-{[benzyl- haps resulting from slightly different torsion angles of the
(pyridin-2-ylmethyl)amino]methyl}-6-{[bis(pyridin-2-yl- bridging acetate[39]).
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Electrochemical Studies
The redox properties of complex 1 were examined by
cyclic voltammetry (Figure S2 in the Supporting Infor-
mation). A semireversible reduction peak is observed at
–0.67 V versus Fc/Fc+ with similar heights for the forward
and reverse reactions. The peak-current dependence on
the square root of the scan rate indicates that this couple,
presumably FeIIIMnII/FeIIMnII, is chemically reversible.
Since ΔEp is about twice that of the reversible ferrocene
couple, the electron-transfer rate is slower than required for
Nernstian conditions. An additional reduction at –1.43 V,
presumably on account of the ligand, and an oxidation at
0.84 V, presumably on account of FeIIIMnII/FeIIIMnIII, are
less reversible. In comparison, [FeMn(μ-BPBPMP)(μ-
OAc)2]+, shows an FeIIIMnII/FeIIMnII reduction peak at
–0.87 and an FeIIIMnII/FeIIIMnIII oxidation peak at 0.44 V
versus Fc/Fc+,[30b] thus indicating that the ICIMP ligand
makes both the MnII and the FeII states more favorable.
Figure 5. Dependence of the initial rates on pH for the hydrolysis of
BDNPP: (-᭿-) catalyzed by complex 1; (-᭹-) uncatalyzed reaction.
Phosphate Ester Hydrolysis
bridges, formation of terminal and/or bridging metal–
hydroxo bonds, and so on.
The phosphatase-like activity[40] of complex 1 was deter-
mined by means of hydrolysis of the model substrate
bis(2,4-dinitrophenyl)phosphate (BDNPP)[6,40] under an ex-
cess amount of substrate concentration with the pH value
varied from 5.5 to 11 in buffered water/acetonitrile (1:1
v/v) solution.
The hydrolysis reaction (Figure 4) was monitored by fol-
lowing the increase in the absorbance at 400 nm, at which
the product 2,4-dinitrophenolate chromophore absorbs
strongly. The observed rate constants of BDNPP hydroly-
sis[42–44] are of the same order of magnitude as those found
for similar systems under identical conditions.[24,30b]
The bell-shaped pH dependence has been observed pre-
viously for both the PAP enzymes and model complexes
thereof. For these, however, the optimum occurs at con-
siderably lower pH (6–7).[13,24,30b,44,46–49] Whereas there is a
slight rate enhancement for 1 at pH 6.5 relative to the mini-
mum activity at pH 7.0, it is only approximately 20% of the
maximum activity at pH 9.5. The heterodinuclear FeM (M
= Mn, Co, Ni) complexes of the ligand IPCPMP, which is
identical to ICIMP except for its 2-pyridyl instead of imid-
azole groups, also show increased activity at high pH, in
contrast to similar complexes reported for the BPBPMP li-
gand by Neves and co-workers, in which activity steadily
decreases at a pH higher than 7.[30b,44,47–49] It was proposed
that the decrease is due to competitive binding of a hydrox-
ide to the divalent metal, thereby preventing substrate coor-
dination. The major difference between these ligands is that
ICIMP and IPCPMP have one coordinating group less than
the BPBPMP ligand, and this might yield an extra coordi-
nation site available for the substrate on the divalent metal
even when a terminal hydroxide is formed. The saturation
Figure 4. The overall BDNPP hydrolysis reaction.
The bell-shaped pH dependence (Figure 5) suggests that behavior observed with an increase of the substrate concen-
at least two protonation equilibria are relevant to cataly- tration modeled with the Michaelis–Menten equation gives
sis.[45] The highest reactivity is reached at pH 9.5 and the Km = 3.84 mm and Vmax = 8.69ϫ10–8 ms–1 at pH 9.0 [2-(N-
increase of the reaction rate up to this point suggests that cyclohexylamino)ethanesulfonic acid (CHES) buffer]. This
a metal-coordinated hydroxide is required for the hydrolysis. saturation behavior might be attributed to the occupancy
The decrease in the reaction rate after pH 9.5 might be of all possible binding sites of the catalyst when a certain
due to instability of the metal complex at higher pH (Fig- concentration of substrate is reached, and it suggests the
ure S4 in the Supporting Information); the formation of a existence of a pre-equilibrium before the nucleophilic attack
brown precipitate is observed for solutions with high pH (Figure S5 in the Supporting Information).[24]
(Ͼ9.5). Several trials were undertaken with pH-dependent
The identity of the nucleophile at higher pH cannot be
UV/Vis spectroscopic measurements to find isosbestic unambiguously determined by the present experiments but
points, which might corroborate a single-step conversion. it is most likely either a hydroxide coordinated to the di-
However, the absence of clear isosbestic points in the UV/ valent metal or a non-coordinated hydroxide that makes an
Vis spectra was probably due to the involvement of consec- exogenous attack on the substrate, as proposed pre-
utive reactions such as cleavage of one and/or both acetate viously.[24]
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Computational Studies
Quantum chemical calculations were carried out using
density functional theory (DFT) to gain further insight into
the electronic structure of complex 1 and to investigate the
general reaction mechanism of the BDNPP hydrolysis.
The optimized geometry of 1 is shown in Figure 6, in
which important distances and calculated spin densities are
also indicated. The calculated structure is in very good
agreement with the experimental one (see Figure 2 and
Table 2). In accordance with preliminary SQUID magnetic
measurements,thecalculationsgiveasmallnegativevalueforJ
(–15.1 cm–1), thus indicating weak antiferromagnetic cou-
pling between the FeIII and MnII centers.
Figure 6. Computationally optimized geometry of 1. Distances [Å]
(see Table 2) and Mulliken spin densities are indicated.
Turning to the mechanistic study of BDNPP hydrolysis,
it was assumed that in the active catalyst one of the acetates
of 1 is replaced by a hydroxide and the BDNPP substrate
(S) replaces the chloride ion.
An investigation of the structure of the catalyst–substrate
complex was initially undertaken. The lowest-energy geom-
etry of this catalyst–substrate complex (C·S) is shown in
Figure 7. The calculations indicate that the substrate coor-
dinates in a monodentate fashion to the Mn ion through a
phosphate oxygen atom. It is interesting to note that the
hydroxide bridge causes the Fe–Mn distance to be signifi-
cantly shorter than in 1 (by 0.6 Å).
Next the geometry of the transition state for nucleophilic
attack of the hydroxide on the phosphorus center of the
substrate was optimized. It was found that in the lowest-
energy transition state (TS in Figure 7), the hydroxide ion
becomes terminally coordinated to the FeIII ion and attacks
the phosphorus in an SN2 fashion (i.e., resembling an Ia
Figure 7. Optimized geometries of BDNPP hydrolysis. Distances
[Å] and Mulliken spin densities are indicated.
mechanism). An oxygen of a nitro moiety of the leaving metallic distance increases to 3.50 Å at the TS. The transi-
group simultaneously coordinates to the MnII center. As tion state is calculated to be 15.2 kcalmol–1 higher in energy
the hydroxide no longer bridges the two metals, the inter- than the C·S complex.
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As the reaction proceeds, both the resulting phosphate demonstrated by the rate enhancement of hydrolysis of the
group (P) and the 2,4-dinitrophenolate leaving group (LG) substrate BDNPP (a DNA model).
anion remain bound to the catalyst (C·P·LG in Figure 7).
Computational modeling of the hydrolysis reaction indi-
The energy of this catalyst–product complex is calculated cates that the substrate coordinates in a monodentate fash-
to be as much as 16.5 kcalmol–1 lower than C·S. The leav- ion to the Mn ion. In the transition state, a bridging
ing group now coordinates to the Mn center with the phen- hydroxide becomes terminally bound to the iron ion,
olic oxygen instead of the nitro moiety. The phosphate thereby attacking the phosphorus center of BDNPP with a
product P bridges the two metal ions, and the metal–metal concomitant dissociation of the leaving group.
distance increases to 3.75 Å.
To close the catalytic cycle, a new substrate and a water
Experimental Section
molecule should enter, the phosphate and the leaving group
should dissociate, and the water should be deprotonated
and bind to the catalyst as a hydroxide.[50] These events are
difficult to model in detail using the methods employed in
the present study. Instead, an approximate approach may
Materials and Methods: All reagents were obtained from commer-
cial sources and used as received. Solvents were purified by stan-
dard methods before use. UV/Vis spectra and the kinetics experi-
ments were carried out with a CARY 300-Bio UV/Visible spectro-
be used. The net hydrolysis reaction (S + H2OǞP + LG) photometer instrument equipped with 1.0 cm matched quartz cells
and operating in the range 190–800 nm. IR spectra were measured
as KBr pellets with a Nicolet Avatar 360 FTIR spectrometer.
given in Figure 4 was calculated to be exergonic by
5.6 kcalmol–1. Therefore, the energy difference between
C·P·LG in one cycle and C·S in the next is 16.5–5.6 =
Mössbauer spectra were recorded at 80 K with a 57Co source in a
10.9 kcalmol–1, as shown in Figure 8. According to this, the Rh matrix with an alternating constant-acceleration Wissel Möss-
bauer spectrometer operated in the transmission mode and
equipped with a Janis closed-cycle helium cryostat or with a Möss-
bauer-Spectromag cryostat. Details concerning the Mössbauer
measurements can be found in the Supporting Information.
rate-determining free-energy barrier becomes the difference
between C·P·LG in one cycle and TS in the next (i.e., 10.9
+ 15.2 = 26.1 kcalmol–1). This value is comparable to the
free-energy barrier calculated very recently for the hydroly-
sis of the same BDNPP substrate using a biomimetic dizinc
catalyst.[51]
Syntheses: The ICIMP ligand was synthesized according to a pre-
viously published procedure.[23] The complex [FeMn(ICIMP)-
(OAc)2Cl] (1) was prepared by mixing H2ICIMP·HCl (31.5 mg,
0.06415 mm) in methanol (25 mL), and FeCl3 (10.4 mg,
0.0645 mm). After 2 h of continuous stirring, Mn(OAc)2·4H2O
(15.7 mg, 0.0645 mm) was added to the mixture followed by the
addition of NBu3 (15.3 μL, 1 equiv.). This changed the color from
pink-purple to reddish purple. The solution was filtered, 2-prop-
anol (5 mL) was added, and the resultant solution was left to evap-
orate slowly at room temperature. Recrystallization of the precipi-
tate from 1:1 MeOH/2-propanol produced reddish-purple single
crystals of [FeMn(ICIMP)(OAc)2Cl] suitable for X-ray crystallo-
graphic analysis, yield 58% (27 mg, 0.037 mmol). ESI-MS: calcd.
for [FeMn(ICIMP)(OAc)2HCl(H2O)] [M + H]+ 735.15; found
735.3. C29H49Cl5FeMnN6O9 [FeMn(ICIMP)(OAc)2Cl·4H2O]:
calcd. C 42.63, H 5.88, N 10.65; found C 42.96, H 5.63, N 10.20.
IR (KBr): ν = 3423 (s), 2961 (w), 1609 (s), 1502 (s), 1468 (s), 1393
˜
Figure 8. Calculated free-energy profile for BDNPP hydrolysis.
(m), 1284 (m), 1160 (m), 1083 (m), 1021 (w), 920 (w), 863 (m), 808
(w), 740 (w), 655 (m) cm–1. UV/Vis (MeCN): λ = 509 (br. m), 285
(s) nm.
Two alternative mechanisms were also probed computa-
tionally. In the first, both acetates in the crystal structure
were substituted for hydroxides, whereas in the second the
X-ray Structure Determination: A crystal of 1 was immersed in
hydroxide acted as a base rather than a nucleophile, thereby cryo-oil, mounted in a Nylon loop, and measured at a temperature
of 100 K. The X-ray diffraction data was collected with a Nonius
KappaCCD diffractometer using Mo-Kα radiation (λ = 0.71073 Å).
The Denzo-Scalepack[52] program package was used for cell refine-
ment and data reduction. The structure was solved by direct meth-
ods using the SIR2004[53] program with the WinGX[54] graphical
activating an external water molecule that in turn per-
formed the nucleophilic attack. Both of these mechanisms
yielded higher free-energy barriers relative to above (see the
Supporting Information for details).
user interface.
A
semiempirical absorption correction
(SADABS)[55] was applied to the data. Structural refinement was
carried out using SHELXL-97.[56] The crystal of 1 was only weakly
diffracting. Furthermore, one of the methyl groups (C28) was
slightly disordered. No disorder model was used in the final refine-
ment, but the carbon atom C28 was restrained so that its Uij com-
ponents approximate isotropic behavior. The idealized position of
the OH hydrogen atom was estimated with the HYDROGEN[57]
program, and the hydrogen atom was constrained to ride on its
Conclusion
The heterodinuclear complex [FeIIIMnII(ICIMP)(OAc)2-
Cl] (1) is a structural and functional model for the dinuclear
active site of purple acid phosphatase (PAP) from sweet po-
tato. Both metal ions are in high-spin configurations and
have distorted-octahedral coordination geometries. The
complex exhibits phosphatase-like hydrolytic activity, as parent oxygen atom with Uiso = 1.5 Ueq (parent atom). Other hy-
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drogen atoms were positioned geometrically and constrained to
ride on their parent atoms, with d(C–H) = 0.95–1.00 Å, and Uiso
1.2–1.5 Ueq (parent atom). The crystallographic details are summa-
and (pH-dependent) UV/Vis spectra for complex 1. Calculated ab-
solute energies and energy corrections for reactants, intermediates,
transition states, and products. Computed alternative reaction
mechanisms. Cartesian coordinates of optimized structures.
=
rized in Tables 1 and 2.
Electrochemical Measurements: Cyclic voltammetry scans were re-
corded with a Pine Research WaveNow potentiostat, a platinum
button working electrode, and a platinum-wire counter electrode.
Anhydrous acetonitrile (Aldrich) stored over 4 Å molecular sieves
was used as the solvent with 0.1 m tetraethylammonium perchlorate
(Fluka) as the supporting electrolyte. A silver wire rinsed in HCl,
immersed in supporting electrolyte solution, and isolated by a
Vycor frit was used as a pseudoreference electrode and calibrated
versus ferrocene. All potentials are referenced to the ferrocene/
ferrocenium couple (0.400 V versus NHE). The solutions for the
electrochemical experiments were purged with nitrogen and kept
under an inert atmosphere throughout the measurements.
Acknowledgments
This research has been carried out within the framework of the
International Research Training Group, Metal Sites in Biomole-
cules: Structures, Regulation, and Mechanisms (www.biometals.eu),
sponsored by the Deutsche Forschungsgemeinschaft (DFG) and
the Swedish Research Council, and has also been supported by
COST Action CM1003. B. D. thanks the European Union (EU) for
an Erasmus Mundus fellowship, and F. H. acknowledges financial
support from the Knut and Alice Wallenberg Foundation and the
Göran Gustafsson Foundation.
Reactivity: The phosphodiesterase-like activity of complex 1 was
evaluated using the hydrolysis reaction of BDNPP, which was mon-
itored spectrophotometrically at 400 nm (ε = 12100 Lmol–1 cm–1)
to follow the release of 4-nitrophenolate ion, by taking into account
its acid dissociation constant (pKa = 7.15) using the Henderson–
Hasselbalch equation.[5,7]
The following buffers[8] were used in the experiments: 2-(N-
morpholino)ethanesulfonic acid (MES; pH 5.5–7), 3-(N-morphol-
ino)propanesulfonic acid (MOPS; pH 7.5–8.0), CHES (pH 8.5–
10.0), and 3-(cyclohexylamino)-1-propanesulfonic acid (CAPS; pH
10.5–11.0). The ionic strength was fixed at 0.1 m with sodium per-
chlorate. Reactions were monitored to less than 5% conversion of
BDNPP to 2,4-DNP, and the data were treated using the initial
rate method.
[1] R. H. Holm, P. Kennepohl, E. I. Solomon, Chem. Rev. 1996,
96, 2239–2314.
[2] G. Schenk, Y. B. Ge, L. E. Carrington, C. J. Wynne, I. R.
Searle, B. J. Carroll, S. Hamilton, J. de Jersey, Arch. Biochem.
Biophys. 1999, 370, 183–189.
[3] G. Schenk, C. L. Boutchard, L. E. Carrington, C. J. Noble, B.
Moubaraki, K. S. Murray, J. de Jersey, G. R. Hanson, S. Ham-
ilton, J. Biol. Chem. 2001, 276, 19084–19088.
[4] G. Schenk, L. R. Gahan, L. E. Carrington, N. Mitic, M. Valiz-
adeh, S. E. Hamilton, J. de Jersey, L. W. Guddat, Proc. Natl.
Acad. Sci. USA 2005, 102, 273–278.
[5] M. Högbom, P. Stenmark, N. Voevodskaya, G. McClarty, A.
Gräslund, P. Nordlund, Science 2004, 305, 245–248.
[6] W. Jiang, J. M. Bollinger, C. Krebs, J. Am. Chem. Soc. 2007,
129, 7504.
[7] W. Jiang, D. Yun, L. Saleh, E. W. Barr, G. Xing, L. M. Hoffart,
M. A. Maslak, C. Krebs, J. M. Bollinger, Science 2007, 316,
1188–1191.
[8] N. Voevodskaya, F. Lendzian, A. Ehrenberg, A. Gräslund,
FEBS Lett. 2007, 581, 3351–3355.
[9] J. M. Younker, C. M. Krest, W. Jiang, C. Krebs, J. M. Bollinger,
M. T. Green, J. Am. Chem. Soc. 2008, 130, 15022–15027.
[10] M. Carboni, J.-M. Latour, Coord. Chem. Rev. 2011, 255, 186–
202.
[11] a) M. Olczak, W. Watorek, B. Morawiecka, Biochim. Biophys.
Acta 1997, 1341, 14–25; b) M. Olczak, W. Watorek, Arch. Bio-
chem. Biophys. 1998, 360, 85–92.
Computational Details: The DFT calculations were performed with
the B3LYP functional[58] as implemented in the Gaussian 09 pack-
age.[59] The functional was modified to include 15% Hartree–Fock
exchange, as this has been shown to yield more accurate results for
transition-metal systems.[60] For geometry optimizations, the
LANL2DZ pseudopotential[61] was used for Fe and Mn, the 6-
311+G(d) basis set was used for the phosphorus and the oxygen
atoms coordinated to it, and the 6-31G(d,p) basis set was used
for all other atoms. To obtain more accurate energies, single-point
calculations were then performed with the 6-311+G(2d,2p) basis
set for all atoms except Fe and Mn, for which LANL2DZ was
used. Solvation effects were evaluated by performing single-point
calculations on the optimized geometries at the same level of theory
as the geometry optimizations using the conductor-like polarized
continuum model (C-PCM)[62] with water (ε = 78.36) as the sur-
rounding solvent. These calculations used the UAKS atomic radii
and parameters taken from the Gaussian 03 version.[63] Frequency
calculations were performed at the same level of theory as the ge-
ometry optimizations to confirm the nature of the stationary points
and to obtain the free-energy corrections at 298.15 K. Dispersion
effects were taken into account by applying the DFT-D3 correc-
tion.[64] The energies reported are thus Gibbs free energies cor-
rected for solvation and dispersion effects.
[12] C. S. Andersson, M. Högbom, Proc. Natl. Acad. Sci. USA
2009, 106, 5633–5638.
[13] N. Mitic, S. J. Smith, A. Neves, L. W. Guddat, L. R. Gahan,
G. Schenk, Chem. Rev. 2006, 106, 3338–3363.
[14] D. E. Wilcox, Chem. Rev. 1996, 96, 2435–2458.
[15] M. B. Twitchett, A. G. Sykes, Eur. J. Inorg. Chem. 1999, 2105–
2115.
[16] P. Nordlund, P. Reichard, Annu. Rev. Biochem. 2006, 75, 681–
706.
[17] M. Högbom, Metallomics 2011, 3, 110–120.
[18] J. A. Cotrouvo Jr., J. Stubbe, Metallomics 2012, 4, 1020–1036.
[19] A. K. Boal, J. A. Cotruovo Jr., J. Stubbe, A. C. Rosenzweig,
Science 2010, 329, 1526–1530.
[20] M. Jarenmark, S. Kappen, M. Haukka, E. Nordlander, Dalton
Trans. 2008, 993–996.
[21] M. Jarenmark, E. Csapo, J. Singh, S. Wöckel, E. Farkas, F.
Meyer, M. Haukka, E. Nordlander, Dalton Trans. 2010, 39,
8183–8194.
Different spin states were considered, and the energetically lowest
one was found to correspond to an antiferromagnetic state between
high-spin FeIII and MnII metal centers. The energies were corrected
according to the broken-symmetry method.[65] The J coupling con-
stant was calculated using the same procedure. As discussed above,
the coupling in this case is very weak and the broken symmetry
correction is therefore very small.
[22] H. Carlsson, M. Haukka, E. Nordlander, Inorg. Chem. 2002,
41, 4981–4983.
Supporting Information (see footnote on the first page of this arti-
cle): Mass spectra, cyclic voltammograms, Mössbauer spectrum,
[23] H. Carlsson, M. Haukka, A. Bousseksou, J. M. Latour, E.
Nordlander, Inorg. Chem. 2004, 43, 8252–8262.
Eur. J. Inorg. Chem. 0000, 0–0
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FULL PAPER
[24] M. Jarenmark, M. Haukka, S. Demeshko, F. Tuczek, L. Zuppi-
roli, F. Meyer, E. Nordlander, Inorg. Chem. 2011, 50, 3866–
3887.
[50]
[51]
The possibility of further hydrolysis of P has not been consid-
ered in the current study.
B. Das, H. Daver, M. Pyrkosz-Bulska, E. Persch, S. K. Bar-
man, R. Mukherjee, E. Gumienna-Kontecka, M. Jarenmark,
F. Himo, E. Nordlander, J. Inorg. Biochem. 2014, 132, 6–17.
Z. Otwinowski, W. Minor, in: C. W. Carter, J. Sweet, Methods
Enzymol. 1997, 276, 307–326.
M. C. Burla, R. Caliandro, M. Camalli, B. Carrozzini, G. L.
Cascarano, L. De Caro, C. Giacovazzo, G. Polidori, R.
Spagna, J. Appl. Crystallogr. 2005, 38, 381–388.
L. Farrugia, J. Appl. Crystallogr. 1999, 32, 837–838.
G. M. Sheldrick, Bruker AXS, Inc., Madison, Wisconsin, USA,
2003.
G. Sheldrick, Acta Crystallogr., Sect. A 2008, 64, 112–122.
M. Nardelli, J. Appl. Crystallogr. 1999, 32, 563–571.
a) C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785–
789; b) A. D. Becke, Phys. Rev. A 1988, 38, 3098–3100; c) A. D.
Becke, J. Chem. Phys. 1992, 96, 2155–2160; d) A. D. Becke, J.
Chem. Phys. 1992, 97, 9173–9177; e) A. D. Becke, J. Chem.
Phys. 1993, 98, 5648–5652.
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B.
Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Son-
nenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hase-
gawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai,
T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M.
Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Starov-
erov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell,
J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M.
Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Ad-
amo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,
A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Mar-
tin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,
J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B.
Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09, Re-
vision A.02, Gaussian, Inc., Wallingford, CT, 2009.
[25] M. Jarenmark, H. Carlsson, V. M. Trukhan, M. Haukka, S. E.
Canton, M. Walczak, W. Fullagar, V. Sundstrom, E. Nord-
lander, Inorg. Chem. Commun. 2010, 13, 334–337.
[26] E. Lambert, B. Chabut, S. Chardon Noblat, A. Deronzier, G.
Chottard, A. Bousseksou, J. P. Tuchagues, J. Laugier, M.
Bardet, J. M. Latour, J. Am. Chem. Soc. 1997, 119, 9424–9437.
[27] J. H. Satcher, M. W. Droege, T. J. R. Weakley, R. T. Taylor, In-
org. Chem. 1995, 34, 3317–3328.
[28] V. Zelenák, Z. Vargová, K. Györyová, Spectrochim. Acta Part
A 2007, 66, 262–272.
[29] M. Carboni, M. Clemancey, F. Molton, J. Pecaut, C. Lebrun,
L. Dubois, G. Blondin, J.-M. Latour, Inorg. Chem. 2012, 51,
10447–10460.
[30] a) R. M. Buchanan, M. S. Mashuta, J. F. Richardson, R. J.
Webb, K. J. Oberhausen, M. A. Nanny, D. N. Hendrickson, In-
org. Chem. 1990, 29, 1299–1301; b) P. Karsten, A. Neves, A. J.
Bortoluzzi, M. Lanznaster, V. Drago, Inorg. Chem. 2002, 41,
4624–4626; c) T. R. Holman, Z. Wang, M. P. Hendrich, L.
Que Jr, Inorg. Chem. 1995, 34, 134–139.
[31] A. S. Borovik, V. Papaefthymiou, L. F. Taylor, O. P. Anderson,
L. Que, J. Am. Chem. Soc. 1989, 111, 6183–6195.
[32] N. Sträter, T. Klabunde, P. Tucker, H. Witzel, B. Krebs, Science
1995, 268, 1489–1492.
[33] T. Klabunde, N. Sträter, R. Frohlich, H. Witzel, B. Krebs, J.
Mol. Biol. 1996, 259, 737–748.
[34] L. W. Guddat, A. S. McAlpine, D. Hume, J. de Jersey, S. Ham-
ilton, J. L. Martin, Acta Crystallogr., Sect. D 1999, 55, 1462–
1464.
[52]
[53]
[54]
[55]
[56]
[57]
[58]
[59]
[35] J. Uppenberg, F. Lindqvist, C. Svensson, B. Ek-Rylander, G.
Andersson, J. Mol. Biol. 1999, 290, 201–211.
[36] N. Sträter, B. Jasper, M. Scholte, B. Krebs, A. P. Duff, D. B.
Langley, R. L. Han, B. A. Averill, H. C. Freeman, J. M. Guss,
J. Mol. Biol. 2005, 351, 233–246.
[37] Y. Lindqvist, E. Johansson, H. Kaija, P. Vihko, G. Schneider,
J. Mol. Biol. 1999, 291, 135–147.
[38] H. Flack, Acta Crystallogr., Sect. A 1983, 39, 876–881.
[39] B. Burger, S. Demeshko, E. Bill, S. Dechert, F. Meyer, Angew.
Chem. Int. Ed. 2012, 51, 10045–10049; Angew. Chem. 2012,
124, 10190–10195.
[40] M. A. Camargo, A. Neves, B. Szpoganicz, A. J. Bortoluzzi,
F. L. Fischer, H. Terenzi, E. E. Castellano, Inorg. Chem. 2010,
49, 3057–3063.
[41] W. Jiang, J. J. Xie, H. Norgaard, J. M. Bollinger, C. Krebs, Bio-
chemistry 2008, 47, 4477–4483.
[42] M. Scarpellini, A. Neves, R. Hoerner, A. J. Bortoluzzi, B. Szpo-
ganics, C. Zucco, R. A. N. Silva, V. Drago, A. S. Mangrich,
W. A. Ortiz, W. A. C. Passos, M. C. B. De Oliveira, H. Terenzi,
Inorg. Chem. 2003, 42, 8353–8365.
[43] A. Neves, H. Terenzi, R. Horner, A. Horn Jr., B. Szpoganicz,
J. Sugai, Inorg. Chem. Commun. 2001, 4, 388–391.
[44] S. C. Batista, A. Neves, A. J. Bortoluzzi, I. Vencato, R. A. Per-
alta, B. Szpoganicz, V. V. E. Aires, H. Terenzi, P. C. Severino,
Inorg. Chem. Commun. 2003, 6, 1161–1165.
[45] D. Volkmer, B. Hommerich, K. Griesar, W. Haase, B. Krebs,
Inorg. Chem. 1996, 35, 3792–3803.
[46] N. Mitic, K. S. Hadler, L. R. Gahan, A. C. Hengge, G. Schenk,
J. Am. Chem. Soc. 2010, 132, 7049–7054.
[47] M. Lanznaster, A. Neves, A. J. Bortoluzzi, V. V. E. Aires, B.
Szpoganicz, H. Terenzi, P. C. Severino, J. M. Fuller, S. C. Drew,
L. R. Gahan, G. R. Hanson, M. J. Riley, G. Schenk, J. Biol.
Inorg. Chem. 2005, 10, 319–332.
[60]
M. Reiher, O. Salomon, B. A. Hess, Theor. Chem. Acc. 2001,
107, 48–55.
P. J. Hay, W. R. Wadt, J. Chem. Phys. 1985, 82, 270–283.
M. Cossi, N. Rega, G. Scalmani, V. Barone, J. Comput. Chem.
2003, 24, 669–681.
[61]
[62]
[63]
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T.
Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar,
J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N.
Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y.
Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P.
Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R.
Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R.
Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morok-
uma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzew-
ski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K.
Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V.
Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B.
Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L.
Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A.
Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W.
Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian 03, Re-
vision D.01, Gaussian, Inc., Wallingford, CT, 2004.
[64]
[65]
a) S. Grimme, J. Antony, S. Ehrlich, H. Krieg, J. Chem. Phys.
2010, 132, 154104–154122; b) S. Grimme, S. Ehrlich, L. Go-
erigk, J. Comput. Chem. 2011, 32, 1456–1465.
L. Noodleman, J. Chem. Phys. 1981, 74, 5737–5743.
Received: October 25, 2013
[48] M. Lanznaster, A. Neves, A. J. Bortoluzzi, B. Szpoganicz, E.
Schwingel, Inorg. Chem. 2002, 41, 5641–5643.
[49] A. Neves, M. Lanznaster, A. J. Bortoluzzi, R. A. Peralta, A.
Casellato, E. E. Castellano, P. Herrald, M. J. Riley, G. Schenk,
J. Am. Chem. Soc. 2007, 129, 7486–7487.
Published Online: ᭿
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9
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I31375
/KAP1
Date: 25-03-14 16:52:41
Pages: 10
www.eurjic.org
FULL PAPER
Catalytic Hydrolysis
B. Das, H. Daver, A. Singh, R. Singh,
M. Haukka, S. Demeshko, F. Meyer,
G. Lisensky, M. Jarenmark,* F. Himo,*
E. Nordlander* ................................ 1–10
A Heterobimetallic FeIIIMnII Complex of
an Unsymmetrical Dinucleating Ligand: A
Structural and Functional Model Complex
for the Active Site of Purple Acid Phos-
phatase of Sweet Potato
Catalytic hydrolysis of a phosphodiester
bond was investigated by using a mixed-
valence heterodinuclear FeIIIMnII complex
of the dinucleating ligand as catalyst. Com-
putational modeling and kinetic results
indicate that a μ-hydroxo FeIIIMnII com-
plex is the active catalyst that binds the
substrate in a monodentate fashion at the
Mn ion.
Keywords: Metalloenzymes / Mixed-valent
compounds / Moessbauer spectroscopy /
Density functional calculations /
Transition states / Hydrolysis
Eur. J. Inorg. Chem. 0000, 0–0
10
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim