Dendrimer encapsulation of [MoVOS4] cores: implications for the DMSO reductase family of enzymes.

Full text of DOI:10.1021/ic0011866

192
Inorg. Chem. 2001, 40, 192-193
Dendrimer Encapsulation of [MoVOS4] Cores: Implications for the DMSO Reductase Family of Enzymes
Sujit Mondal and Partha Basu*,†
Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, Pennsylvania 15282
ReceiVed October 27, 2000
Pyranopterin-containing mononuclear molybdenum enzymes
such as nitrate reductases and dimethylsulfoxide reductases
(DMSOR) play important roles in global nitrogen and sulfur
cycles.1 Substantial evidence from crystallography2-5 and spec-
troscopy6,7 now exists for description of the molybdenum active
center, e.g., in the fully oxidized state the oxo-molybdenum-
(VI) centers are coordinated by four sulfur donors. The oxidized
state undergoes two one-electron reductive steps to regenerate
the catalytically competent Mo(IV) state and, thus, passes through
a molybdenum(V) state. Fundamental understanding of the
electron transfer process is a key step to comprehend the function
of these enzymes.
In addition to describing the details of the active center,
crystallography also reveals that in all cases the Mo centers are
deeply buried (∼15 Å) inside the protein matrix, and the
coordinating ligands are not exposed to the surface. Taken
together, the consistent minimal picture of molybdenum(V)
centers emerges as a [MoVOS4] core buried inside the protein.
Biologically important metal centers such as hemes and iron-
sulfur clusters exhibit a significant modulation of the reduction
potentials upon encapsulation.8 However, the effect of encapsula-
tion on the reduction potential of any oxo-molybdenum center
is unknown, which prompted us to initiate this investigation with
[MoVOS4] cores. Over the past two decades several [MoVOS4]
cores have been reported in the literature; those provide a starting
point for our investigation.9-13 This report focuses on [MoVOS4]-
cores derived from tetrathiophenolate ligands.14 Here, we disclose
for the first time a new class of thiol-containing ligands and their
use toward encapsulating [MoVOS4]- cores.
The thiol group in 4-mercaptobenzoic acid is protected by
oxidizing the thiols to disulfide by iodine, and the synthesis of
polyether amine dendritic units following the literature.15 The
amine groups of these units have been linked with the carboxylato
groups of 4,4′-dithiobenzoic acid, and the resulting materials have
been purified in excellent yields (82-98%) by chromatography
on silica gel. The G0 disulfide (4a) has been isolated as a white
solid, whereas first generation nitrile terminated (4b) and ester
terminated (4c) disulfides have been isolated as yellow liquids.
The disulfides (4b and 4c) have been reduced to dendritic thiols
(5a and 5b) with NaBH4, and the corresponding thiols have been
isolated as light yellow liquids (yield: 90-95%). All ligands and
their precursors have been characterized by NMR and IR
spectroscopy and mass spectrometry (Supporting Information).
Tetraphenylphosphonium salts of dendritic oxomolybdenum-
(V) tetrathiolate complexes, [PPh4][MoO(p-SC6H4CONHCH3)4]
(7a), [PPh4][MoO(p-SC6H4CONHC(CH2O(CH2)2CN)3)4] (7b),
and 7c have been synthesized from 6 (Scheme 1) via ligand
exchange reactions11a either with dendritic thiols (5a, 5b) or the
disulfide (4a). Compound 7a has been prepared by exchanging
the thiophenolato groups of 6 using the disulfide 4a via redox-
coupled ligand exchange reaction in THF11a,16 and has been
isolated as a blue solid. Compounds 7b and 7c have been
synthesized directly from the corresponding thiols by exchanging
dendritic thiophenols with 6 in THF. Both 7b and 7c compounds
have been purified by size exclusion chromatography in good
yields (60-70%).
† E-mail: basu@duq.edu.
(1) (a) Enemark, J. H.; Young, C. G. AdV. Inorg. Chem. 1993, 40, 1-88.
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The molecular mass for compounds 6 and 7a-c has been
determined by negative ion electrospray ionization mass spec-
trometry (ESIMS) from their acetonitrile solutions (Table 1). The
oxo-molybdenum(V) complexes 6 and 7a-c exhibit an intense
low-energy absorption at ∼600 nm (ꢀ ∼ 6000 M-1 cm-1) due to
S f Mo charge transfer (CT) transitions which obscures any d-d
transition. Importantly, the position of the low-energy CT
transition is not affected by the architecture of ligands. In contrast
peripherally substituted thiophenolato complexes show significant
variation in the CT transition as a function of substituents.17 Even
for the more structurally rigid trispyrazolylborate system, change
in the reduction potential is accompanied by concomitant change
in the low-energy CT transition.18 Thus, the insensitivity of the
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10.1021/ic0011866 CCC: $20.00 © 2001 American Chemical Society
Published on Web 12/15/2000

Communications
Inorganic Chemistry, Vol. 40, No. 2, 2001 193
Scheme 1
Table 1. Molecular Ion Peaks and Redox Potentials (vs Fc+/Fc)
functionality facilitates the reduction of the metal center by ∼150
mV as compared to 6. Second, molecules 7a-c display an
increasingly larger potential difference between the current
maxima of the reduction and return oxidation waves (∆E),
indicative of increasing kinetic difficulty of reduction and
oxidation processes. The electrochemical data suggests that
increasing the steric bulk of the ligand far from the metal center
induces sluggish redox chemistry.8c,21 Third, the reduction be-
comes increasingly difficult (by ∼100 mV for 7a to 7c; by 70
mV for 7a to 7b) for more sterically demanding ligands.
Interestingly, the shift in the reduction potential in our molecules
is larger than that reported for encapsulated iron-sulfur clusters
(19-25 mV).8c The trend shows that by increasing the ligand
bulk, reduction becomes increasingly difficult. Because accurate
reporters of electronic structures of the molybdenum center such
as the position of the charge transfer transition and the EPR g
values show little variation, we suggest that, for molecules of
the present study, the reduction potential is modulated by solvent
accessibility to the electroactive metal center. Differential solvent
accessibility conceivably could result in changing the effective
dielectric constant at the metal center.
M-
calcd
base peak
obsd
E1/2,b mV
complexes
base peaka
(∆Ep,b mV)
6
550
778
1775
2339
550.7
778.7
1774
-1151 (63)
-993 (71)
-1063 (136)
-1092 (172)
7a
7b
7c
2339
a By electrospray ionization mass spectrometry (ESIMS). b Condi-
tions: ∼10-3 M solutions of the complex in MeCN at 25 °C; scan
rate, 100 mV/s; Pt working and reference electrodes; supporting
1
electrolyte, [NEt4][BF4]; E1/2 ) /2(Epa + Epc) and ∆Ep ) (Epc - Epa).
CT transition in current molecules demonstrates a similar
electronic structure of the metal center. For molybdenum com-
plexes, the ModOt vibrations have also been probed. The ModOt
vibration has been observed at the same position as other
tetrathiophenolato complexes.17 No variation in the ModOt
stretching frequency (941-943 cm-1) has been observed within
the complexes 6, 7a-c, suggesting that the bond strength of
ModOt in these molecules is essentially the same.
The d1 oxo-molybdenum(V) complexes (7a-c) are paramag-
Interestingly, the Mo(V/IV) reduction potential for DMSOR
isolated from different sources varies widely from -90 mV for
Escherichia coli22 (membrane-bound protein) to +140 mV for
Rhodobacter sphaeroides (water-soluble protein).23 The results
of the present investigation led us to suggest that the variation in
the Mo(V/IV) reduction potential in different DMSO reductases
may be modulated by solvent.
1
netic with an S ) /2 ground state and are amenable to electron
paramagnetic resonance (EPR) spectroscopy. EPR spectra of
complexes 7a-c have been recorded in frozen (20 K) acetoni-
trile-toluene (1:1) solutions. The molybdenum complexes exhibit
axial EPR spectra similar to those observed for [MoVO(SPh)4]-
with characteristic molybdenum hyperfine structures.19 No sig-
nificant variation in the g values has been observed in these
complexes; this indicates that the integrity of the square pyramidal
[MoVOS4]- core (C4V local symmetry) is retained. Taken together,
in the present case, the addition of bulky and symmetric dendritic
architecture does not impose any further distortion at the metal
center; therefore the electronic structure remains unaltered.
The redox property of tetraethylammonium salts of [MoVO-
(SPh)4]- has been investigated in detail.20 The anion displays a
well-defined couple due to the reduction of Mo(V) to Mo(IV).
The room-temperature cyclic voltammograms of 6 and 7a-c in
acetonitrile exhibit a well-defined one-electron couple (Table 1);
however, no reversible oxidation couple has been observed within
the solvent window. Several conclusions can be made from cyclic
voltametry. First, introduction of electron-withdrawing amide
Acknowledgment. We are grateful to Prof. M. Hendrich for
recording EPR spectra. We also thank Drs. M. Bier and K.
Somyatullya, Mr. A. Ghosh, and Mr. S. Sengupta for experimental
assistance. Stimulating discussions with Profs. D. Wright and J.
Stolz are acknowledged. Financial support from Research Cor-
poration (RI 249), Noble J. Dick Foundation, and National
Institutes of Health (GM 6155501) is acknowledged.
Supporting Information Available: Characterization data and cyclic
voltammograms. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC0011866
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