J. Am. Chem. Soc. 1998, 120, 3259-3260
3259
Molybdenum and Tungsten Structural Analogues of
the Active Sites of the MoIV + [O] f MoVIO Oxygen
Atom Transfer Couple of DMSO Reductases
James P. Donahue,† Christian Lorber,†,‡ Ebbe Nordlander,‡ and
R. H. Holm*,†
Department of Chemistry and Chemical Biology
HarVard UniVersity, Cambridge, Massachusetts 02138
Inorganic Chemistry 1, Chemical Center
Lund UniVersity, S-22100 Lund, Sweden
Figure 1. Probable minimal structures of the desoxo MoIV and monooxo
MoVIO centers in Rs DMSO reductase and the pterin dithiolene cofactor
(R absent or a nucleotide). Symmetrical chelate rings are depicted;
unsymmetrical binding has been detected in the enzyme by crystal-
lography.2
ReceiVed NoVember 17, 1997
Under the Hille classification scheme for molybdenum oxo-
transferases,1 oxidized enzymes with two pterin dithiolene co-
factors bound to a MoVIX unit (X ) O, S, Se) are placed in the
dimethyl sulfoxide (DMSO) reductase family, named for its
prototypic member. Recent crystallographic and EXAFS analyses
of oxidized (MoVI) and reduced (MoIV) forms of two enzymes of
this family, Rhodobacter sphaeroides2,3 (Rs) and Rhodobacter
capsulatus4,5 (Rc) DMSO reductases, together provide structural
bases for mechanistic analysis. X-ray and EXAFS results for
the Rs enzyme agree with one MoVIdO group in the oxidized
form, the absence of this group in the reduced form, and serinate
ligation in both forms, but are in apparent nonconformity in other
structural aspects. In the [MoVIO(S2‚pterin)2(O‚Ser)] coordination
unit of the Rs enzyme, X-ray results indicate one symmetrical
(Mo-S 2.4 Å) and one unsymmetrical (Mo-S 2.4, 3.1 Å)
dithiolene chelate ring forged with the two pterin cofactors6
(Figure 1). The symmetrical ligand is retained in the reduced
form, but the other dithiolene very weakly interacts with the metal
(Mo‚‚‚S 2.9, 3.7 Å). The EXAFS results show two tightly bound
dithiolene ligands (Mo-S 2.44 Å) in the oxidized form and
coordination of 3-4 sulfur atoms (Mo-S 2.33 Å) in the reduced
form together with two different Mo-O/N interactions. Finally,
resonance Raman spectra for this enzyme are consistent with tight
binding of both cofactor ligands in the oxidized, reduced, and
inhibited forms.7 The apparent nonuniformity of dithiolene
binding in these enzymes renders desirable the synthesis of well-
defined active site analogues.
pated in all earlier oxo transfer systems developed as models for
enzymatic reactions.9 Indeed, synthetic species of these types
are far less recurrent than MoIVO and MoVIO2 complexes on which
the oxo transfer systems are based, including bis(dithiolene)Mo
complexes prepared as structural and reactivity models.10 While
it catalyzes a different reaction, we note that an apparently typical
member of the principal family of tungsten enzymes,11 Pyrococcus
furiosus aldehyde oxidoreductase (AOR), possesses the minimal
crystallographic coordination unit [W(S2‚pterin)2], in which there
may be an additional one or two oxygen ligands bound.12 The
few known bis(dithiolene)tungsten complexes contain the WVIO2
and WV,IVO units.13 Here, we disclose synthetic routes to and
structures of bis(dithiolene) desoxo MIV and MVIO complexes
related to the metal centers in these enzymes.
Reaction of [MoIVOCl(MeNC)4](PF6)14 with 2 equiv of Li2(bdt)
and excess Et4NCl in acetonitrile followed by standard workup
and recrystallization (MeCN/MeOBut) afforded bright orange
(Et4N)2[MoIVO(bdt)2]15a ((Et4N)2[1], 44%, bdt ) benzene-1,2-
dithiolate), previously reported10a but prepared conveniently by
this procedure. Treatment of a slurry of (Et4N)2[1] in acetonitrile
with 1 equiv of Ph2ButSiCl for 1 h resulted in silylation of the
oxo ligand; recrystallization of the crude product (MeCN/MeOBut)
produced green-black diamagnetic (Et4N)[MoIV(OSiPh2But)(bdt)2]
((Et4N)[2], 83%).15b The corresponding tungsten complex has
The majority of these results, together with the earlier
demonstration using 18O-labeling that an oxygen atom is trans-
ferred from substrate to the reduced molybdenum center,8 lead
to the minimal oxo transfer reaction couple [MoIV(S2‚pterin)2-
(O‚Ser)] + Me2SO f [MoVIO(S2‚pterin)2(O‚Ser)] + Me2S (Figure
1). The desoxo MoIV and monooxo MoVI centers were unantici-
16
also been prepared. Reaction of WOCl3(THF)2 with 2 equiv
(9) (a) Holm, R. H. Coord. Chem. ReV. 1990, 100, 183. (b) Schultz, B. E.;
Holm, R. H. Inorg. Chem. 1993, 32, 4244. (c) Enemark, J. H.; Young, C. G.
AdV. Inorg. Chem. 1993, 40, 1. (d) Young, C. G.; Wedd, A. G. J. Chem.
Soc., Chem. Commun. 1997, 1251.
(10) (a) Boyde, S.; Ellis, S. R.; Garner, C. D.; Clegg, W. J. Chem. Soc.,
Chem. Commun. 1986, 1541. (b) Oku, H.; Ueyama, N.; Kondo, M.; Nakamura,
A. Inorg. Chem. 1994, 33, 209. (c) Ueyama, N.; Oku, H.; Kondo, M.;
Okamura, T.; Yoshinaga, N.; Nakamura, A. Inorg. Chem. 1996, 35, 643. (d)
Das, S. K.; Chaudhury, P. K.; Biswas, D.; Sarker, S. J. Am. Chem. Soc. 1994,
116, 9061.
† Harvard University.
‡ Lund University.
(1) Hille, R. Chem. ReV. 1996, 96, 2757.
(2) Schindelin, H.; Kisker, C.; Hilton, J.; Rajagopalan, K. V.; Rees, D. C.
Science 1996, 272, 1615.
(3) George, G. N.; Hilton, J.; Rajagopalan, K. V. J. Am. Chem. Soc. 1996,
118, 1113.
(11) Johnson, M. K.; Rees, D. C.; Adams, M. W. W. Chem. ReV. 1996,
96, 2817.
(4) The X-ray structure of Rc DMSO reductase reveals the [MoVIO2-
(S2‚pterin)(O‚Ser)] coordination unit in the oxidized enzyme (Schneider, F.;
Lo¨we, J.; Huber, R.; Schindelin, H.; Kisker, C.; Kna¨blein, J. J. Mol. Biol.
1996, 263, 53). The enzyme has also been crystallized by others (McAlpine,
A. S.; McEwan, A. G.; Shaw, A. L.; Bailey, S. JBIC 1997, 2, 690). The reasons
for the different structures of an enzyme from the same organism and from
the highly homologous Rs enzyme2 are unclear, but may relate to the conditions
of crystallization.
(12) (a) Chan, M. K.; Mukund, S.; Kletzin, A.; Adams, M. W. W.; Rees,
D. C. Science 1995, 267, 1463. (b) Schindelin, H.; Kisker, C.; Rees, D. C.
JBIC 1997, 2, 773.
(13) (a) Ueyama, N.; Oku, H.; Nakamura, A. J. Am. Chem. Soc. 1992,
114, 7310. (b) Das, S. K.; Biswas, D.; Maiti, R.; Sarkar, S. J. Am. Chem.
Soc. 1996, 118, 1387.
(14) Novotny, M.; Lippard, S. J. Inorg. Chem. 1974, 13, 828.
(15) All reactions were performed under anaerobic conditions at ambient
temperature using dry solvents. All compounds gave satisfactory elemental
analyses. IR spectra were determined in KBr, and UV/vis and 1H NMR spectra
were in acetonitrile. (a) (Et4N)2[1]: νMoO 903 cm-1. (b) (Et4N)[2]: νSiO 947
cm-1; λmax (ꢀM) 301 (10 500), 352 (22 200), 388 (sh), 461 (sh), 570 (265)
nm. (c) (Et4N)[3]: νWO 949 cm-1. (d) (Et4N)2[4]: νWO 905 cm-1. (e) (Et4N)[5]:
νSiO 963 cm-1; λmax (ꢀM) 307 (21 100), 345 (6740), 420 (420), 553 (85) nm.
(f) (Et4N)[6]: 1H NMR (253 K) δ 7.25 (dd), 6.96 (dd) (bdt); 1.02 (s, But);
(5) Baugh, P. E.; Garner, C. D.; Charnock, J. M.; Collison, D.; Davies, E.
S.; McAlpine, A. S.; Bailey, S.; Lane, I.; Hanson, G. R.; McEwan, A. G.
JBIC 1997, 2, 634.
(6) (a) Hilton, J. C.; Rajagapolan, K. V. Arch. Biochem. Biophys. 1996,
325, 139. (b) Solomon, P. S.; Lane, I.; Hanson, G. R.; McEwan, A. G. Eur.
J. Biochem. 1997, 246, 200.
(7) (a) Garton, S. D.; Hilton, J.; Oku, H.; Crouse, B. R.; Rajagopalan, K.
V.; Johnson, M. K. J. Am. Chem. Soc. 1997, 119, 12906. (b) Johnson, M. K.;
Garton, S. D.; Oku, H. JBIC 1997, 2, 797.
(8) Schultz, B. E.; Hille, R.; Holm, R. H. J. Am. Chem. Soc. 1995, 117,
827.
FAB-MS m/z 649 (M-). (g) (Et4N)[7]: νWO 888 cm-1, νSiO 933 cm-1 1H
;
NMR δ 7.23 (dd), 6.87 (dd) (bdt); 1.06 (s, But); FAB-MS m/z 735 (M-); λmax
(ꢀM) 296 (14 300), 325 (sh), 340 (sh), 375 (sh), 456 (3720), 600 (1610) nm.
(16) Persson, C.; Andersson, C. Inorg. Chim. Acta 1993, 203, 235.
S0002-7863(97)03917-6 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/20/1998