My intrigue with vanadium began in college when I discovered the brilliant colors characteristic of vanadium complexes. The multiple oxidation states stable in aqueous solution captured my interest, and I kept a careful eye out for reports on the biological role of vanadium. Since the turn of the past century, vanadium has been known to be accumulated to very high levels by ascidians (also known as tunicates or sea squirts), yet even today, the biological function of the sequestered vanadium remains a mystery. In the 1970s, a vanadium nitrogenase had been reported, then retracted, and finally rediscovered through genetic manipulations in the late 1980s as the alternative nitrogenase.

When I started out as an assistant professor in 1986 and wanted to work on a new metalloprotein in biology, I was delighted by the initial report of a vanadium haloperoxidase enzyme that is found in marine algae. Vanadium bromoperoxidase (V-BrPO) is abundant in marine algae and catalyzes the oxidation of halides (Cl2, Br2, or I2) by hydrogen peroxide, which results in the halogenation of certain organic substrates or the formation of singlet oxygen in the absence of appropriate organic substrates.

Name: Named for Vanadis, a Scandinavian goddess, because of its many colorful compounds.
Atomic mass: 50.94.
History: Discovered by Mexican chemist Andrés Manuel del Rio in 1801, but he withdrew his claim when the discovery was disputed. Rediscovered in 1830 by Swedish chemist Nils G. Sefström.
Occurrence: Makes up about 0.02% of Earth's crust and is found in trace quantities in more than 60 different minerals. The most important source of the metal is vanadinite.
Appearance: Bright, shiny, gray metal.
Behavior: Soft, ductile, and very resistant to corrosion.
Uses: Essential to some organisms; acts to stimulate metabolism. Used as an additive to steel for tools, construction materials, springs, and jet engines. Vanadium pentoxide is used commercially as a catalyst in the contact process for preparing sulfuric acid, and as a mordant, a material that permanently fixes dyes to fabrics.
RAINBOW Vanadium forms various bright colors in solution depending on oxidation state. From left to right, V(II), V(III), V(IV), and V(V). Solutions were prepared by Jens Uwe Kuhn.
The active site of V-BrPO characterized by vanadate coordination to the protein by one histidine ligand is deceptively simple because the protein scaffolding and the vast hydrogen bonding network are essential for efficient catalytic activity. Soon after I began working on vanadium bromoperoxidase, I found out that this vanadium(V) enzyme unfortunately displayed neither the brilliant colors of other vanadium complexes nor the redox properties that had captured my interests as a graduate student and postdoctoral fellow. Instead, V-BrPO is colorless and appears to remain in the V(V) oxidation state during catalysis.

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.


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