Bis(dithiolene)molybdenum analogues relevant to the DMSO reductase enzyme family: Synthesis, structures, and oxygen atom transfer reactions and kinetics
Bs. Lim et Rh. Holm, Bis(dithiolene)molybdenum analogues relevant to the DMSO reductase enzyme family: Synthesis, structures, and oxygen atom transfer reactions and kinetics, J AM CHEM S, 123(9), 2001, pp. 1920-1930
A series of dithiolene complexes of the general type [Mo-IV(QR')(S2C2Me2)(2
)](1-) has been prepared and structurally characterized as possible structu
ral and reactivity analogues of reduced sites of the enzymes DMSOR and TMAO
R (QR' = PhO-, 2-AdO(-),-(PrO-)-O-i), dissimilatory nitrate reductase (QR'
= 2-AdS(-)), and formate dehydrogenase (QR' = 2-AdSe(-)). The complexes are
square pyramidal with the molybdenum atom positioned 0.74-0.80 Angstrom ab
ove the S-4 mean plane toward axial ligand QR'. In part on the basis of a r
ecent clarification of the active site of oxidized Rhodobacter sphaeroides
DMSOR (Li, H.-K.; Temple, C.; Rajagopalan, K. V.; Schindelin, H. J. Am. Che
m. Sac. 2000, 122, 7673), we have adopted the minimal reaction paradigm Mo-
IV + XO reversible arrow (MoO)-O-VI + X involving desoxo Mo(IV), monooxo Mo
(VI), and substrate/product XO/X for direct oxygen atom transfer of DMSOR a
nd TMAOR enzymes. The [Mo(OR')(S2C2Me2)(2)](1-) species carry dithiolene an
d anionic oxygen ligands intended to simulate cofactor ligand and serinate
binding-in DMSOR and TMAOR catalytic sites. In systems with N-oxide and S-o
xide substrates, the observed overall reaction sequence is [Mol(IV)(OR')(S2
C2Me2)](1-) + XO --> [(MoO)-O-VI(OR')(S2C2Me2)(1-) --> [(MoO)-O-V(S2C2Me2)(
2)](1-). Direct oxo transfer in the first step has been proven by isotope l
abeling. The reactivity of [Mo(OPh)(S2C2Me2)(2)](1-) has been the most exte
nsively studied. In second-order reactions, 1 reduces DMSO and (CH2)(4)SP (
k(2) approximate to 10(-6), 10(-4) M-1 s(-1); DeltaS(double dagger) = -36,
-39 eu) and Me3NO (k(2) = 200 M-1 s(-1); DeltaS(double dagger) = -21 eu) in
acetonitrile at 298 K. Activation entropies indicate an associative transi
tion state, which from relative rates and substrate properties is inferred
to be concerted with X-O bond weakening and Mo-O bond making. The (MoO)-O-V
I product in the first step; Such as [(MoO)-O-VI(OR')(S2C2Me2)(2)](1-), is
an intermediate in the overall reaction sequence, inasmuch as it is too uns
table to isolate and decays by an internal redox process to a MoVO product,
liberating an equimolar quantity of phenol. This research affords the firs
t analogue reaction systems of biological N-oxide and S-oxide substrates th
at are based on desoxo Mo(IV) complexes with biologically relevant coordina
tion. Ore-transfer reactions in analogue systems are substantially slower t
han enzyme systems based on a k(cat/)K(M) criterion. An interpretation of t
his behavior requires more information on the rate-limiting step(s) in enzy
me catalytic cycles. (2-Ad = 2-adamantyl, DMSOR = dimethyl sulfoxide reduct
ase, TMAOR = trimethylamine N-oxide reductase).