Structural models of the bimetallic subunit at the A-cluster of acetyl coenzyme A synthase/CO dehydrogenase: Binuclear sulfur-bridged Ni-Cu and Ni-Ni complexes and their reactions with CO

Article abstract of DOI:10.1021/ja045284s

The Ni(II)-dicarboxamido-dithiolato complexes (Et₄N)₂[Ni(NpPepS)] (1) and (Et₄N)₂[Ni(PhPepS)] (2) were used as Ni_d metallosynthons in the construction of higher nuclearity dinuclear Ni-Cu and Ni-Ni species to model the bimetallic M_p-N_id site of the A-cluster of acetyl coenzyme A synthase/CO dehydrogenase (ACS/CODH). Reaction of 1 with [Cu(neo)Cl] and [Ni(terpy)Cl₂] in MeCN affords the dinuclear complexes (Et₄N)[Cu(neo)Ni(NpPepS)] (3) and [Ni(terpy)Ni(NpPepS)] (4), respectively. Reaction of 2 with [Ni(dppe)Cl₂] in MeCN yields [Ni(dppe)Ni(PhPepS)] (6). The Ni-Cu complex 3 exhibits no redox chemistry at the Ni_d site and no reaction with CO. In contrast, the Ni_p sites in 4 and 6 are readily reduced (characterized by their Ni(I) EPR spectra) and bind CO, exhibiting ν_co bands at 2044 and 1997 cm^-1, respectively, indicating terminal CO binding. The present Ni-Ni systems replicate the structural and chemical properties of the A-cluster site in ACS/CODH and support the presence of Ni at M_p in the catalytically active enzyme. Copyright

Full text of DOI:10.1021/ja045284s

Published on Web 10/22/2004
Structural Models of the Bimetallic Subunit at the A-Cluster of Acetyl
Coenzyme A Synthase/CO Dehydrogenase: Binuclear Sulfur-Bridged Ni-Cu
and Ni-Ni Complexes and Their Reactions with CO
Todd C. Harrop,† Marilyn M. Olmstead,‡ and Pradip K. Mascharak*,†
Department of Chemistry and Biochemistry, UniVersity of California, Santa Cruz, California 95064, and
Department of Chemistry, UniVersity of California, DaVis, California 95616
Received August 4, 2004; E-mail: pradip@chemistry.ucsc.edu
The bacterial metalloenzyme acetyl coenzyme A synthase/CO
dehydrogenase (ACS/CODH) catalyzes two very important biologi-
cal processes, namely the reduction of atmospheric CO2 to CO and
the synthesis of acetyl coenzyme A from CO, CH3 from a
methylated corrinoid iron-sulfur protein, and the thiol coenzyme
A.1 The structure of the A-cluster, where ACS synthesis occurs,
has recently been determined by crystallography.2,3 Two separate
A-cluster structures of ACS/CODH from the bacterium Moorella
thermoacetica reveal a multimetallic active site containing a
cuboidal Fe4S4 unit bridged to a bimetallic subunit through one
Cys-S moiety. The bimetallic site contains a square-planar Ni(II)d
site (distal to the cubane) coordinated to two deprotonated car-
boxamido nitrogens from the peptide backbone and two cysteine
sulfurs. The other metal center, designated as the proximal metal
(Mp), contains either a Cu,2 Ni,3 or Zn3 ion and is bridged to the
Nid site through two Cys-S residues and to the Fe4S4 unit through
one other Cys-S, affording an Mp(Cys-S)3 coordination sphere. In
addition to the three bridging Cys-S moieties, a fourth, still
unidentified, ligand is also bound to complete the coordination
sphere. An intense debate over the roles of three different metal
ions has recently been subsided following careful biochemical
studies4 and publication of a third A-cluster structure from the
hydrogenogenic bacterium Carboxydothermus hydrogenoformans.5
It is now evident that Ni occupies the Mp site in the catalytically
active enzyme; both Cu and Zn found in the crystal had their origin
in the promiscuity of adventitious metal ion capture by the Nid site.4
The quest for insight into the mechanism of acetyl coenzyme A
synthesis on the basis of the crystal structures has prompted us6
and others7 to synthesize structural analogues of the active site. In
these attempts, the Fe4S4 unit has often been ignored (unlikely
substrate binding site) and the focus has been primarily on the
Mp-Nid bimetallic site. In this Communication, we report the re-
sults of our modeling attempt(s) that utilizes the Ni(II)-dicarbox-
amido-dithiolato complexes (Et4N)2[Ni(NpPepS)] (1)6b and (Et4N)2-
[Ni(PhPepS)] (2) as Nid synthons in the construction of Ni-Cu
and Ni-Ni dinuclear analogues (Figure 1).
Figure 1. Ni(II) synthons (Nid mimics) used in this study.
Figure 2. ORTEP diagram of the anion of (Et4N)[Cu(neo)Ni(NpPepS)]
(3) (50% probability) with the atom-labeling scheme. H atoms are omitted
for the sake of clarity. Selected bond distances (Å) and bond angles (°):
Cu-N(3) 2.037(5), Cu-S(1) 2.2670(16), Ni-N(1) 1.889(5), Ni-S(1)
2.1876(15), Ni-Cu 3.0740(11), N(3)-Cu-N(4) 82.66(18), N(3)-Cu-S(1)
129.79(14), N(4)-Cu-S(2) 115.91, S(1)-Cu-S(2) 87.46(6), N(1)-Ni-
N(2) 88.75(19), N(2)-Ni-S(1) 177.80(15), S(1)-Ni-S(2) 92.49(6),
C(1)-S(1)-Ni 101.17(18).
Interestingly, the cathodic cyclic voltammogram of 3 in DMF does
not exhibit any reduction wave up to -1.8 V (vs SCE). It is
therefore evident that the Ni(II) center of 3 (Nid mimic) is very
resistant to reduction. The metal centers of 3 show no affinity
toward CO,7c and passage of CO through solutions of 3 results in
no breakdown of the complex. This latter behavior of 3 contrasts
that of similar Ni(II)-Cu(I) sulfur-bridged complexes.7b
The reaction of [Ni(terpy)Cl2] (terpy ) 2,2′:6′,2′′-terpyridine)
and 1 (1:1 ratio) in MeCN resulted in the isolation of neutral
[Ni(terpy)Ni(NpPepS)] (4) as a red-brown powder (75%). Since
the A-cluster exhibits an EPR spectrum (NiFeC signal) upon one-
electron reduction and CO binding (Ared-CO), we explored the
possibility of reduction and CO binding with the Ni-Ni dimer 4.
Upon reduction of 4 to 4red with Na2S2O4, an axial EPR spectrum
(100 K, DMF glass) is observed (g ) 2.226, 2.125). This spectrum
is characteristic of five-coordinate Ni(I) in a trigonal bipyramidal
geometry.8 Reaction of CO with 4red in DMF results in 4red-CO
adduct with a terminal Ni(I)-CO band at 2044 cm-1. In DMF glass,
Treatment of an MeCN solution of 1 with 1 equiv of [Cu(neo)-
Cl] (neo ) 2,9-dimethyl-1,10-phenanthroline) afforded the brown
dinuclear Ni-Cu complex (Et4N)[Cu(neo)Ni(NpPepS)] (3) (Figure
2) in 70% yield. Crystallographic analysis of 3 reveals a square
planar Ni(II) ion bridged to a distorted tetrahedral Cu(I) center via
the thiolato-S donors of 1. The average Cu-S and Ni-S distances
of 3 (2.2934 and 2.1950 Å, respectively) compare well with those
observed in the Cu form of ACS/CODH.2 The Cu-Ni distance of
3 (3.0740 Å) is somewhat longer than that of the enzyme (2.792
Å). Steric constraints imposed by the NpPepS4- ligand frame are
presumably responsible for the longer metal-metal distance.
4red-CO displays a rhombic EPR spectrum with g ) 2.223, 2.128,
2.019 (Figure S1, Supporting Information), typical of similar six-
coordinate Ni(I) compounds with terminally bound CO molecule.8
Since 4 does not exhibit any affinity toward CO, it is evident that
the Nip mimic in this model only binds CO in the reduced (+1)
† University of California, Santa Cruz.
‡ University of California, Davis.
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C O M M U N I C A T I O N S
can be reduced to the Ni(I) state and binds CO in terminal fashion
(Nip mimic).
Several groups have shown that the ACS activity and the NiFeC
signal of the enzyme diminish upon treatment with 1,10-phenan-
throline (phen), whereupon part of the Ni is removed (labile Ni).1,12
Treatment of 1 or 2 with excess phen (up to 100 equiv) does not
lead to the formation of [Ni(phen)3]2+. However, when 6 is treated
with excess phen (∼25 equiv) in CH2Cl2, the electronic absorption
spectrum of 6 changes to that of the monomer 2 and [Ni(phen)3]2+
over a time period of 35 min, with kobs ) 1.502 × 10-4 s-1 (Figure
S4, Supporting Information). This reaction confirms that the
thiolato-S bridges between the Nid and the Nip centers in models
such as 6 are vulnerable to phen treatment. Since phen does not
remove Ni from the Nid synthons 1 and 2, it is apparent that the
labile Ni in the enzyme arises from the Nip site. Interestingly,
addition of excess (up to 100 equiv) neocuproine, a Cu(I) chelator,
to 6 does not result in any Ni loss. These observations are in line
with those observed with the enzyme.4a,c
In summary, we report two Ni-Ni models, namely 4 and 6, that
exhibit structural features and chemical properties very similar to
those of the binuclear active site of ACS/CODH. These models
are the first examples of sulfur-bridged dinuclear Ni complexes
that bind CO at the bridged Ni(I) center, as proposed in the
mechanism of acetyl coenzyme A synthesis.3 Also, in one case,
the corresponding Ni-Cu model has been characterized.
Figure 3. ORTEP diagram of [Ni(dppe)Ni(PhPepS)] (6) (50% probability)
with the atom-labeling scheme. H atoms are omitted for the sake of clarity.
Selected bond lengths (Å) and bond angles (°): Ni1-Ni2 2.8255(4),
Ni1-N1 1.8909(17), Ni1-N2 1.8895(17), Ni1-S1 2.1558(5), Ni1-S2
2.1438(5), Ni2-S1 2.2354(6), Ni2-S2 2.2413(5), Ni2-P1 2.1781(6),
Ni2-P2 2.1816(6), S2-Ni1-S1 79.29(2), P1-Ni2-P2 86.18(2),
Ni1-S1-Ni2 80.077(19), C1-S1-Ni 111.15(17).
state. The Nid mimic 1 shows no affinity toward CO under similar
reducing conditions, and no EPR signal is observed.
Since the Nip site in ACS is four-coordinate in the resting state,
we attempted synthesis of appropriate Ni-Ni models with a second
Nip synthon, [Ni(dppe)Cl2] (dppe ) 1,2-bis(diphenylphosphino)-
ethane). This Nip synthon is expected to favor an easier reduction
to the Ni(I) oxidation state and further facilitate binding of CO.
However, due to the geometric constraints imposed by the ligand
frame in 1, all attempts to synthesize the Ni-Ni complex with
[Ni(dppe)Cl2] eventually led to the formation of trimeric (Et4N)2-
[Ni(DMF)2{Ni(NpPepS)}2] (5, Scheme S1, Supporting Informa-
tion). We therefore synthesized the second Nid synthon
[Ni(PhPepS)]2- (anion of 2, structure shown in Figure S2, Sup-
porting Information), employing a ligand with less steric demands.
Reaction of 2 with 1 equiv of [Ni(dppe)Cl2] in MeCN afforded
[Ni(dppe)Ni(PhPepS)] (6, Figure 3) as a blue-green crystalline solid
in 85% yield. In this Ni-Ni model, the two Ni(II) centers are
bridged through both thiolato-S donors of the PhPepS4- ligand
frame, and both exist in square planar geometry. The dihedral angle
between the two square planes is 111.4°, and the Ni- - -Ni separation
is 2.8255(4) Å. The metric parameters of the bridged Nid synthon
in 6 are very similar to that noted with 2. This suggests that sulfur
metalation does not change any structural feature of this Nid mimic.
Complex 6 can be easily reduced with Na2S2O4 or NaBH4, and
6red exhibits a strong Ni(I) EPR spectrum similar to other Ni(I)-
P2S2 complexes.9 Since 2 does not exhibit any reduction wave up
to -1.8 V vs SCE in solvents such as DMF, it is clear that the
reduction occurs at the Nip site of 6. Although 6 displays no reac-
tivity toward CO, the one-electron-reduced species 6red is different.
Passage of CO through a DMF solution of 6red generates the CO
adduct 6red-CO (EPR spectrum shown in Figure S3, Supporting
Acknowledgment. T.C.H. received support from the NIH IMSD
grant GM58903.
Supporting Information Available: Spectroscopic and analytical
data for the complexes; Scheme S1 summarizing the synthetic reactions;
X-band EPR spectra of 4red and 4red-CO (Figure S1) and 6red-CO
(Figure S3); ORTEP diagram of (Et4N)2[Ni(PhPepS)] (2) (Figure S2);
changes in the electronic absorption spectrum of 6 upon phen addition
(Figure S4); and X-ray crystallographic files in CIF format. This
material is available free of charge via the Internet at http://pubs.acs.org.
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ion.7c This species displays two νCO bands at 1948 and 1866 cm-1
.
Since these νCO values are lower than that of the enzyme, it is quite
possible that the Nip site in ACS does not attain the 0 oxidation
state during catalysis. Recently, two groups have reported sulfur-
bridged dinuclear Ni complexes with P2S2 coordination at the
bridged Ni center that exhibit low reduction potentials.7a,e However,
no spectroscopic data are available on the CO adducts of these
complexes in the reduced state. Complex 6 is therefore the first
structurally characterized Ni-Ni model that includes dicarboxam-
ide-dithiolate ligation (Nid mimic) with a bridged Ni(II) center that
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