Structural analogues of the bimetallic reaction center in acetyl CoA synthase: A Ni - Ni model with bound CO

Article abstract of DOI:10.1021/ja035606c

Models for the active site of the acetyl CoA synthase (ACS) were synthesized by attachment of Cu⁺ and Ni(0) to nickel diaminodithiolate (S₂N₂) and diamidodithiolate (S₂N₂-) complexes. The Ni-Ni species form stable CO adducts, i.e., [{(CO)₂Ni}{NiS₂N₂-}]^2-, whereas the Cu-NiS₂N₂ and Cu-NiS₂N₂- models do not. These results provide supporting evidence for a biological role for reduced nickel in ACS. Copyright

Full text of DOI:10.1021/ja035606c

Published on Web 06/25/2003
Structural Analogues of the Bimetallic Reaction Center in Acetyl CoA
Synthase: A Ni-Ni Model with Bound CO
Rachel C. Linck, Cameron W. Spahn, Thomas B. Rauchfuss,* and Scott R. Wilson
Department of Chemistry, UniVersity of Illinois at UrbanasChampaign, Urbana, Illinois 61801
Received April 13, 2003; E-mail: rauchfuz@uiuc.edu
The enzymatic processing of industrially important small mol-
ecules, e.g., dihydrogen, carbon monoxide, and dinitrogen, presents
inspiration for organometallic catalysis based on evolutionarily
optimized reaction centers. In the case of CO, carbon monoxide
dehydrogenase/acetyl CoA synthase bifunctional enzymes (CODH/
ACS), as found in Moorella thermoacetica (f. Clostridium ther-
moaceticum), are important in the global carbon cycle.1,2
The crystallographic characterization of the ACS enzyme, as
described separately by Doukov et al. and Darnault et al.,3,4 revealed
an unprecedented hexametallic three-component active site. The
three components are (i) an Fe4S4 cluster attached via a µ-SR group
to an extra-cluster metal center, a motif also seen in the Fe-only
hydrogenases;5 (ii) a four-coordinate, proximal metal site, featuring
Cu, Ni, or Zn, which in turn is bound to the third component; (iii)
a square planar distal Ni ligated to the protein backbone via two
amides (deprotonated) and two thiolates (Figure 1).2 Initial crystal-
lographic characterization found copper in the proximal position,3
but more recent work proposes that the active form of the enzyme
in fact contains nickel in this site, which oscillates between square
planar (oxidized) and tetrahedral (reduced) geometries.4 Both
Cu(I) and Ni(0) would have similar coordination preferences (as
would Ni(I)6), and both metals are plausible binding sites for CO.
As a first step in understanding the catalytic mechanism implied
by the ACS structure, we have synthesized structural analogues of
the active site. Our approach, adopted also by Riordan et al.,7 is to
set aside the [Fe4S4(SR)4]2-, which is unlikely to bind substrate,
and to focus on the two monometallic subunits, i.e., the metal center
that is bridged to square planar Ni via a pair of thiolato bridges.
Dithiolato-bridged bimetallic structures have often been achieved
using nickel diamino dithiolates as building blocks,8-11 and we
adopted this approach.
The 1:1 reaction of [Cu(NCMe)4]BF4 and the nickel hexam-
ethylated diamino dithiolate (abbreviated NiS2N2)12 yielded the red-
brown derivative [Cu2{NiS2N2}2]2+, which was crystallographically
shown to adopt the dimeric “stair-case” structure. This dication
proved to be a versatile precursor to heterobimetallic derivatives
via ligand-induced scission of the Cu2Ni2 core. Thus, treatment of
[Cu2{NiS2N2}2]2+ with PR3 (R ) iPr, NMe2) afforded the bimetallic
derivative [{(R3P)(MeCN)Cu}{NiS2N2}]+ (Scheme 1), as shown
by NMR and ESI-MS measurements. Excess phosphine effected
no further reaction. Survey studies assayed using ESI-MS shows
that excess methylimidazole (MeIm) cleaves [Cu2{NiS2N2}2]2+ to
form [{(MeIm)(MeCN)Cu}{NiS2N2}]+, which does not react with
CO. Addition of 1 equiv of MeNC to [Cu2{NiS2N2}]2+ produced
[{(MeNC)(MeCN)Cu}{NiS2N2}]+, but excess MeNC resulted in
the extraction of Cu+, yielding [Cu(CNMe)4]+. Crystallographic
characterization of [{(iPr3P)(MeCN)Cu}{NiS2N2}]+ revealed, as in
the ACS active site, a tetrahedral metal center bound to both
thiolates of the planar NiS2N2 metallo ligand. The fourth coor-
dination site on the copper center is occupied by a labile MeCN
ligand.
Figure 1. Drawing of the active site of ACS (M ) Cu, Ni, Zn).
Scheme 1. Preparation of Cu-Ni Models Using Nickel Diamino
Dithiolate
Although [{(iPr3P)(MeCN)Cu}{NiS2N2}]+ shows no affinity for
CO, it does form a stable adduct with MeNC (νCN ) 2194 cm-1),
an electron-rich analogue of CO. The binding of MeNC demon-
strates the lability of the MeCN ligand and the Lewis acidity of
the Cu(I) site.
Following these studies on NiS2N2, we pursued models in which
the ligand superstructure was more clearly biomimetic. The planar
Ni center in ACS is bound by a diamido-dithiolato donor set
provided by a cys-gly-cys sequence.3,4 This coordination environ-
ment, while otherwise unprecedented in nature, has been previously
investigated in synthetic systems7,13,14 and is reminiscent of the
nitrile hydratase binding site.15-17 The diamido dithiolate derived
from 3-mercaptopropionic acid and ethylenediamine18 had not been
previously used as a ligand, although it can be readily prepared on
a large scale and it features an unsaturated backbone as in ACS.
The salt (Et4N)2[NiS2N′2] adopts the expected square planar
structure. Capitalizing on the aforementioned methodology, reaction
of (Et4N)2[NiS2N′2] and [Cu(NCMe)4]PF6 in the presence of PiPr3
afforded (Et4N)[{(iPr3P)Cu}{NiS2N′2}] (Scheme 2). Crystallo-
graphic studies confirm that copper is trigonal planar (∑ angles )
359.3°). The Cu-S and Ni-S distances are 2.287 and 2.202 Å,
respectively, vs 2.204 and 2.186 Å seen in the Cu form of ACS.3
The Cu‚‚‚Ni distance is 2.672 Å in the model and 2.792 Å in the
enzyme.
3 The absence of the MeCN ligand (the solvent for crystal
growth) may reflect the enhanced donor power of [NiS2N′2]2- vs
[NiS2N2], which stabilizes three-coordinate 16e Cu(I). Again, this
bimetallic complex shows no affinity for CO, despite the coordi-
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C O M M U N I C A T I O N S
al. for the reduced bimetallic subunit of the active ACS enzyme.
Detailed comparison between our model and the Darnault structure
is complicated by the uncertainty in the oxidation state of the
enzyme crystals as well as variable site occupancy.
The synthesis of [{(CO)2Ni}{NiS2N′2}]2- can plausibly be
proposed to proceed via the intermediacy of [{(cod)Ni}{NiS2N′2}]2-
,
but we have not yet isolated this species. Whereas [{(CO)2Ni}-
{NiS2N′2}]2- is quite stable in solution and in the solid state under
inert atmosphere, it is labile with respect to donor ligands and to
air, like the biological system. Addition of PPh3 removed the
Ni(0) center, affording Ni(CO)2(PPh3)2,19 and exposure to air rapidly
degraded the complex to give [Ni{NiS2N′2}2]2- (ESI-MS).
In summary, we report a series of d8-d10 bimetallic systems
bearing a stoichiometric and structural resemblance to the bimetallic
site of ACS as described by Doukov et al. and by Darnault et al.3,4
The preparation of such species supports the mechanism for ACS
activity proposed by Darnault et al.4 The nonreactivity of our
Ni-Cu derivatives toward CO contrasts with the stability of the
NiCO derivative. Although further work is indicated, especially
on the CO-localized reactivity, our findings provide first-generation
models for a biological role for nickel carbonyls.4
Figure 2. Molecular structure of the anion in (Et4N)2[{(CO)2Ni}{NiS2N′2}]
(50% thermal ellipsoids). Distances (Å) and angles (°): Ni(1)-Ni(2) 2.805,
S(1)-Ni(1)-S(2) 83.2, Ni(1)-S(av) 2.195, Ni(2)-S(av) 2.347, Ni(2)-
C(av) 1.754.
Scheme 2. Preparation of Cu-Ni and Ni-Ni Models Using Nickel
Diamido Dithiolate
Acknowledgment. This research was supported by NIH.
Supporting Information Available: Synthesis and characteriza-
tion of [Cu2{NiS2N2}2](BF4)2, [{(iPr3P)(NCMe)Cu}{NiS2N2}]PF6, (Et4N)2-
[NiS2N′2], (Et4N)[{(iPr3P)Cu}{NiS2N′2}], and (Et4N)2[{(CO)2Ni}-
{NiS2N′2}] (PDF, CIF, JPEG). This material is available free of charge
via the Internet at http://pubs.acs.org.
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