Download Vanadium bromoperoxidase, an example of a peroxo

A DENSITY FUNCTIONAL THEORY STUDY OF THE OXIDATION OF BROMIDE BY
PEROXOMETAL COMPLEXES
Maruf Khan and Martha S. Reynolds
Colgate University
A peroxometal complex is involved in the oxidation of bromide by vanadium bromoperoxidase,
an enzyme found in seaweed. The resulting hypobromite ion combines with an organic molecule
to form an irritant that protects against bacteria, fungi and predators. This project focuses on the
mechanism for the oxidation of bromide by several synthetic analogues of the enzyme.
A simplified system with MoO(O2)(NH3)(H2O) and MoO2(NH3) as the metal-containing reactant
and product, respectively, was examined. Molybdenum was used because experimental data have
shown that peroxomolybdenum complexes are more reactive than their vanadium counterparts.1
Gaussian98 was employed to optimize geometries using density functional theory (B3PW91)
with the MIDI! basis set, and double zeta valence polarized basis (DZVP) functions for the
metal. After optimization of reactant and product geometries, a transition state search was done.
The transition state was sought using the QST2 function of Gaussian, which searches for a saddle
point on the energy surface by requiring that one eigenvalue of the matrix of second-order
energy derivatives be negative at every optimization step. Gaussian returned a transition state
with the bromide attached to one of the oxygen atoms in the peroxo group. The following
diagram shows the positions of the different species on the reaction profile:
O
H3N
O
Mo
H2O
transtion
state
O
Br-
O
H3N
O
H3N
O
Mo
Mo
H2O
O
H2O
O
Br-
+ OBr product
reactant
A transition state is now being sought for bromide oxidation by an experimentally more realistic
molecule— MoO(O2)(dipic)(H2O) (dipic is 2,6-pyridinedicarboxylate). Optimization of reactant
and product geometries is complete, but the transition state structure is still being sought. Similar
calculations will then be done for MoO(O2)(mpic)(H2O) (mpic is 6-hydroxymethyl-2pyridinecarboxylate), MoO(O2)(pdm)(H2O) (pdm is 2,6-pyridinedimethanol), and the
peroxovanadium complexes of the same ligands.
1
Reynolds, M. S.; Butler, A. Inorg. Chem. 1996, 35, 2378—2383