Mechanism of Oxo-Transfer Catalysis of Monoperoxo
Vanadium Complexes with Tripodal Amine Ligands:
A DFT Investigation.
by
Curtis J. Schneider
A proposal of original research
Submitted in partial fulfillment
of the requirements for the
CBI training program
December 30th, 2004
Specific Aims
Vanadium dependent haloperoxidases (VHPO) are enzymes that catalyze the two
electron oxidation of a halide ion in the presence of hydrogen peroxide and subsequently
halogenate an organic substrate. VHPO have been shown to stereoselectively oxidize
achiral sulfides to sulfoxides. The resting state of the enzyme contains a vanadium cofactor with four oxygens and one histidine in a trigonal bipyramidal geometry3. The
structurally characterized peroxide bound form of vanadium dependent chloroperoxidase
shows a significantly distorted tetragonal pyramidal geometry3. The peroxo moiety is
bound in a 2 fashion in the pseudo axial and pseudo equatorial position. There is no
evidence for redox cycling of the vanadium co-factor. It is proposed that the vanadium
atoms acts as a strong lewis acid to activate the hydrogen peroxide for oxygen atom
transfer in both halide and sulfide oxidation. The Pecoraro group has focused on the
development of tripodal amine complexes that mimic the oxidation ability of
haloperoxidases1,4. The stereoselectivity of sulfide oxidation has not been examined for
this set of complexes. My current efforts are devoted to the development of a ligand to
impose a stereochemical preference for sulfide oxidation. The efforts are hindered by the
solution state structure of [VO(O2)heida]- (heida = N-(2-hydroxyethyl)iminodiacetate).
O
O
O
O
O O
O O
O O
O
V
N
O
O
V
O
V
N
O
O
O
HO O
O
O
O
OH
OH
1
N
2
3
Figure 1: Possible solution structures of VO(O2)heida. 1 is an illustration of the solid state structure
of the complex. 2 and 3 represent two possible orientations of the peroxo moiety after dissociation of
the weakly coordinated hydroxyl group. All complexes are vanadium(V), overall complex charge is
ambiguous as the the appropriate protonation state will be assessed
In fulfillment of the ten week student sabbatical requirement of the CBI training program,
I plan, in collaboration with Prof. Luca De Gioia at the University of Milano-Bicocca, to:



Determine the energetic difference in various protonation states of VO(O2)heida-1
complexes
Calculate the energetic differences between 1-3 in chemically feasible
protonation states.
Determine the orientation of the peroxo moiety at the transition state associated
with oxygen transfer
Using computational methods I will identify the mechanistic and coordination similarities
between models and the enzyme. This information will then be applied to design ligands
capable of stereospecific oxidation of sulfides.
Introduction
Vanadium dependent haloperoxidases are a class of enzymes found in a variety of marine
algae, seaweed, and some lichens5. This class of enzymes catalyzes the two electron
oxidation of halides by hydrogen peroxide. VHPOs are named for the most
electronegative halide they are capable of
oxidizing (i.e. chloroperoxidase). The first
bromoperoxidase (VBPO) requiring vanadium
was isolated from Ascophyllum nodosum by
Vilter in 19846. The crystal structures of a
number of vanadium dependent haloperoxidases
have been solved, including the VBPO isolated
from As. nodosum7 and one from Curvularia
inaequalis3. There is a high degree of sequence
homology within the active site residues of the
bromo- and chloroperoxidases8.
VHPOs require 1 equivalent of vanadium per
Figure 2: Atomic Structure of active
protein for activity. The resting state of the
site of the vanadium co-factor from C.
enzyme is characterized by a trigonal bipyramidal
inaequalis as obtained from X-ray
geometry with an axial hydroxo and histidine
crystallography2.
ligands (Figure 2). One hydroxo and two oxo
groups define the equatorial plane and are
stabilized by a hydrogen bonding network within the active site2. EXAFS9 and X-ray
crystallography3 have been used to examine the structure of the peroxide bound form of
vanadium dependent chloroperoxidase. The peroxo unit appears to bind in a 2 fashion
with one peroxo in the pseudo axial position of a distorted tetragonal pyramidal
geometry7.
VHPOs catalyze the oxidation of halide and its subsequent addition to an organic
substrate. In the absence of an organic substrate a halide disproportionation reaction
leads to the generation of molecular oxygen from hydrogen peroxide. In vitro VHPOs
are capable of stereoselectively oxidizing a variety of sulfides with no double oxidation
products10. The slight pH dependence for oxygen evolution and halide oxidation alludes
to the importance of protonation states within the active site11. The pH dependence of
reactivity in small model complexes has led to the development of a mechanism in which
protonation of the peroxo moiety significantly enhances the rate of reaction. There is no
experimental evidence for redox cycling of the co-factor during oxidation, consequently
an oxo-transfer type mechanism has been proposed, in which the vanadium acts as a
Lewis acid activating the terminal oxidant hydrogen peroxide. The proposed mechanism
for both halide and sulfide oxidation (Figure 3) involves nucleophilic attack on
protonated peroxo complex followed by product release and the regeneration of 1 equiv.
of acid1. This mechanism is consistent with isotopic labeling of hydrogen peroxide, in
which the generated sulfoxide contains almost exclusively 18O. Recently density
functional theory was used to evaluate protonation state and transition state geometry of
oxygen transfer to both halides and thioethers12; their results indicate that protonation of
the peroxo moiety followed by
nucleophilic attack on the
unprotonated oxygen is
energetically favorable
A large number of model
complexes have been developed to
mimic the activity of VHPOs.
Colpas et. al. explored a number of
tetradentate tripodal amine ligands
as an approximation for the active
site coordination environment. Of
these complexes, VO(O2)heida (1)
was found to have the fastest rate
for halide oxidation. Smith et. al.
showed 1 to be catalytically
Figure 3: Complete scheme for the catalytic cycle of
competent for sulfide oxidation
sulfide oxidation for peroxovanadium complexes
with a rate comparable to VHPOs1.
with tripodal amine ligands1
The pH dependence of both halide
and sulfide oxidation was
consistent with the proposed mechanism for the enzyme system1,4. It should be noted
that these models do not effectively mimic coordination geometry in the peroxo bound
form of VHPOs due to an extra oxygen atom in the first coordination sphere. These
geometry discrepancies prevent a direct correlation of mechanistic details between the
model and enzyme systems. The solid state structure of K[VO(O2)heida] has been
determined by X-ray crystallography7. 51V NMR4 of K[VO(O2)heida] shows one species
in solution at room temperature1 with a chemical shift consistent with a monoperoxo
vanadium complex, but this information does not address fast exchange with a labile
hydroxyl group or the orientation of the peroxo unit.
In an attempt to develop further the above models for VHPOs by introducing steric bulk
using chiral centers, an asymmetric environment will be created forcing a stereoselective
oxidation of achiral sulfides. These efforts are hindered by the lack of knowledge about
the catalytically competent species. A number of questions can be raised about the
specific coordination environment, protonation state, preferred protonation sites, and
transition state geometry, none of which are readily answerable using current
experimental techniques.
The development of modern quantum chemical calculations has shed light on the
electronic and reactivity properties of both transition metal complexes13 and
metalloenzymes14. Recently density functional calculations were used to probe the
sulfide oxidation ability of VHPOs2 and Schiff base vanadium complexes15. These
studies, though related, do not address coordination environment or geometry imposed by
an O3N donor set. Using computation methodology previously employed in the study of
the enzyme system2, the electronic structure of 1 and the coordination geometry of the
catalytically competent species along the proposed reaction coordinate will be explored.
By understanding the geometry and electronic structure of the catalytically competent
species, specific ligand design can be employed to create the most effective steric or
electronic interactions.
Proposed Methodology
I propose to examine the VO(O2)heida system for 10 weeks in the laboratory of Prof.
Luca De Gioia at the University of Milano-Bicocca. The expertise of De Gioia
laboratory in DFT calculations on transition metal systems2,12,16-21, specifically with their
previous study on the peroxide bound form VHPOs2,12, makes this laboratory the ideal
place to learn the techniques involved with quantum chemical calculations on peroxo
vanadium complexes. Below is an itinerary detailing a ten week sabbatical period to
begin June 2005:
Preparations: Background reading on the uses and abilities of quantum chemical
calculations including but not limited to: choice of basis sets, correlation
functions, density functionals, available software, and complications on transition
metal systems. Discussions with Chemistry Faculty members specializing in
computational chemistry will supplement these readings.
Week 1-3: Using crystal coordinates for [VO(O2)heida]- develop a reasonable
quantum chemical system that includes sulfide completely separated from the
complex. Progress in this area will depend primarily on the size of the system and
the complexity of surrounding environment (gas phase or solvent
approximations). This optimization will include various protonation states
ranging from dianionc to a cationic complex.
Week 4-6: Examine the free energies of various coordination environments of the
optimized peroxo vanadium species with bound and unbound hydroxyl arm in an
attempt to find the most feasible geometry for the peroxo moiety during
nucleophilic attack.
Week 7-10: The preferential site for nucleophilic attack by sulfide will be
examined and the optimal geometry for the transition state will be determined. It
will be important to make sure that the transition states identified lead to the
desired products.
Project Scope
The purposed quantum chemical calculations, although physical in nature, connect the
VHPOs with functional model complexes. The direct connection made between the
proposed and previous studies will provide a better understanding of the mechanism for
VHPOs that may be expanded to similar transition metal oxo-transfer enzymes (e.g.
Molybdenum containing oxo transfer).
Additionally this research directly effects the development of effective asymmetric
sulfoxidation catalysts. The increasing synthetic utility of chiral sulfoxides and the
expanding field of chiral sulfoxides containing pharmaceuticals22 (i.e. gastric PPIs), has
created a demand for a selective asymmetric oxidants for these processes. The model
complexes developed from this study could provide a direct synthetic route to some of
these sulfoxide containing molecules.
The Chemical and Biological Interface training grant fosters the collaborative efforts of
researchers at the forefront and chemistry and biology. The proposed research uniquely
connects the chemical understanding of a model system with that of the native enzyme.
By providing an effective ligand design methodology, this work can further impact the
biological community by providing a potential asymmetric oxidation pathway to
pharmaceuticals and other biologically relevant optically active sulfur derivatives.
References
(1)
Smith, T. S.; Pecoraro, V. L. Inorg. Chem. 2002, 41, 6754-6760.
(2)
Zampella, G.; Kravitz, J. Y.; Webster, C. E.; Fantucci, P.; Hall, M. B.;
Carlson, H. A.; Pecoraro, V. L.; De Gioia, L. Inorg. Chem. 2004, 43, 4127-4136.
(3)
Messerschmidt, A.; Wever, R. Proc. Natl. Acad. Sci., USA 1996, 93, 392396.
(4)
Colpas, G. J.; Hamstra, B. J.; Kampf, J. W.; Pecoraro, V. L. J. Am. Chem.
Soc. 1996, 118, 3469-3478.
(5)
Bolm, C.; Bienewald, F. Angew. Chem., Int. Ed. Engl. 1996, 34, 26402642.
(6)
Vilter, H. Metal Ions in Biological Systems 1995, 31, 325.
(7)
Weyand, M.; Hecht, H.-J.; Kiess, M.; Liaud, M.-F.; Vilter, H.;
Schomburg, D. Journal of Molecular Biology 1999, 293, 595-611.
(8)
Crans, D. C.; Smee, J. J.; Gaidamauskas, E.; Yang, L. Chemical Reviews
2004, 104, 849-902.
(9)
Arber, J. M.; De Boer, E.; Garner, C. D.; Hasnain, S. S.; Wever, R.
Biochemistry 1989, 28, 7968-7973.
(10) Andersson, M.; Willetts, A.; Allenmark, S. J. Org. Chem. 1997, 62, 84558458.
(11) Everett, R. R.; Kanofskyen, J. R.; Butler, A. Journal of Biological
Chemistry 1990, 265, 4908-4914.
(12) Zampella, G.; Fantucci, P.; Pecoraro, V. L.; De Gioia, L. J. Am. Chem.
Soc. 2004, ASAP.
(13) Niu, S.; Hall, M. B. Chemical Reviews (Washington, D. C.) 2000, 100,
353-405.
(14) Siegbahn, P. E. M.; Blomberg, M. R. A. Chemical Reviews (Washington,
D. C.) 2000, 100, 421-437.
(15) Balcells, D.; Maseras, F.; Lledo´s, A. J. Org. Chem. 2003, 68, 4267-4274.
(16) Bruschi, M.; Fantucci, P.; De Gioia, L. Inorg. Chem. 2002, 41, 1421-1429.
(17) De Riso, A.; Gullotti, M.; Casella, L.; Monzani, E.; Profumo, A.; Gianelli,
L.; De Gioia, L.; Gaiji, N.; Colonna, S. Journal of Molecular Catalysis A: Chemical
2003, 204-205, 391-400.
(18) Franzini, E.; Fantucci, P.; De Gioia, L. Journal of Molecular Catalysis A:
Chemical 2003, 204-205, 409-417.
(19) Bruschi, M.; Fantucci, P.; De Gioia, L. Inorg. Chem. 2003, 42, 4773-4781.
(20) Franzini, E.; De Gioia, L.; Fantucci, P.; Zampella, G.; Bonacic-Koutecky,
V. Inorganic Chemistry Communications 2003, 6, 650-653.
(21) Bruschi, M.; Fantucci, P.; De Gioia, L. Inorg. Chem. 2004, 43, 3733-3741.
(22) Ferna´ndez, I.; Khiar, N. Chemical Reviews 2003, 103, 3651-3705.