Deficiencies in the mismatch repair (MMR) pathway are associated with several types of cancers, as well as resistance to commonly used chemotherapeutics. Rhodium metalloinsertors have been found to bind DNA mismatches with high affinity and specificity in vitro, and also exhibit cell-selective cytotoxicity, targeting MMR-deficient cells over MMR-proficient cells. Ten distinct metalloinsertors with varying lipophilicities have been synthesized and their mismatch binding affinities and biological activities determined. Although DNA photocleavage experiments demonstrate that their binding affinities are quite similar, their cell-selective antiproliferative and cytotoxic activities vary significantly. Inductively coupled plasma mass spectrometry (ICP-MS) experiments have uncovered a relationship between the subcellular distribution of these metalloinsertors and their biological activities. Specifically, we find that all of our metalloinsertors localize in the nucleus at sufficient concentrations for binding to DNA mismatches. However, the metalloinsertors with high rhodium localization in the mitochondria show toxicity that is not selective for MMR-deficient cells, whereas metalloinsertors with less mitochondrial rhodium show activity that is highly selective for MMR-deficient versus proficient cells. This work supports the notion that specific targeting of the metalloinsertors to nuclear DNA gives rise to their cell-selective cytotoxic and antiproliferative activities. The selectivity in cellular targeting depends upon binding to mismatches in genomic DNA.

Coordination complexes of vanadium(5+) played a key role in understanding the structure and mechanism of vanadium-dependent haloperoxidases, particularly the effects of protonation on peroxide coordination to dioxovanadium(5+) species, and in the activation of the peroxo-oxovanadium(5+) complex for substrate oxidation. There has been no spectroscopic evidence that could test the presence of a hydroxo intermediate in a catalytically active oxovanadium(5+) complex. Herein we report the use of the pre-edge transition in X-ray absorption spectroscopy as a spectroscopic signature for V=O bonding. Displacement of oxo donors with hydrogen peroxide or chloride donors dramatically decreases the pre-edge intensity, confirming that the source of the intense pre-edge feature is closely related to the -bonding associated with the V=O. Protonation of a catalytically active tripodal amine oxovanadium(5+) complex has no affect on the pre-edge intensity and, therefore, rules out the possibility of a hydroxo intermediate in the catalytic cycle.

Density functional theory has been used to investigate the structural, electronic, and reactivity properties of an established functional model for vanadium‐dependent haloperoxidases, K[VO(O2)Hheida] (Hheida2– = 2,2′‐[(2‐hydroxyethyl)imino]diacetate). Possible solution species were determined on the basis of potential exogenous donors present under the conditions necessary for reactivity. The energetically favored solution‐state species is a 1:1 complex of Hheida and vanadium with a coordinated hydroxyethyl donor trans to the vanadium–oxido bond which is in agreement with the reported solid‐state structure for K[VO(O2)Hheida]. Transition states of the oxidation reaction were located for four substrates: chloride, bromide, iodide, and dimethyl sulfide. The role of protonation and its effects on reactivity were examined for each substrate. Protonation of the peroxido moiety leads to a significant drop in the activation barrier for oxidation. In contrast no transition states could be located for an oxido‐transfer process involving the oxido ligand. Barriers of activation calculated for halide oxidation were similar, providing support to the hypothesis that the pKa of the halide in acetonitrile is responsible for the decrease in reactivity between I–, Br–, and Cl–. The results presented herein provide a mechanistic correlation between a functional model and the enzyme, making K[VO(O2)Hheida] a “complete” functional model for vanadium‐dependent haloperoxidase.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

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