Still, our quest to elucidate the role of V-BrPO in the biogenesis of halogenated marine natural products dazzles the interests of my students and me every day. We have just discovered how V-BrPO can catalyze the bromination and cyclization of terpenes, forming the bromocyclic polyenes and bromocyclic ethers in many halogenated marine natural products [J. Am. Chem. Soc., 125, 3688 (2003)]. But the route to this discovery was circuitous. We started off with enzyme kinetic investigations and explored the general substrate selectivity of this enzyme. We continued on to functional biomimetic studies using small-molecule vanadium(V) complexes (as well as other metal ions), which established the Lewis acid role of the vanadium(V) center and revealed the importance of the protein scaffold and the importance of hydrogen bonding to activate the V(V)-bound peroxide toward halide oxidation. Along the way, our investigations have taken us around the world in search of algae that contain vanadium haloperoxidase enzymes that might be involved in the biogenesis of the interesting halogenated marine natural products. Our algal collections come from as far away as Antarctica to as nearby as our backyard in the Santa Barbara Channel and to points in between, such as the North Sea, Australia, and the Bahamas. V-BrPO is a clear example of the adaptation of a living organism (algae) to its chemical environment: V is the second most abundant transition-metal ion in surface seawater after molybdenum; halide ion concentrations are also high (about 0.5 M Cl, mM in Br, and µM in I), and sufficient levels of hydrogen peroxide are available as a by-product of other enzymatic processes in algae or in surface seawater during daylight hours as a result of photochemical reactions. In many cases, the algae use the halogenated natural products as a chemical defense, such as against microbial colonization or to prevent fish from feeding on them. Yet many of the halogenated marine natural products have attractive biological activities of interest to the pharmaceutical industry. Given the abundance of vanadium in seawater; the beautiful array of colors displayed by vanadium complexes; and the importance of vanadium in nature, steel refinement (which accounts for the vast majority of V production), and catalysis and new materials (interesting stories unto themselves), it is fitting that the element vanadium was named after Vanadis, the Scandinavian goddess of love, beauty, and abundance.
Alison Butler is a professor of chemistry at the University of California, Santa Barbara. Her research takes her around the world in search of new bioinorganic chemistry in diverse environments.
Chemical & Engineering News
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