Chemical & Chemical Engineering News (80th Anniversary Issue), Vol. 81, No. 36, 2003, Sept. Edited by X. Lu Introduction

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.

VANADIUM AT A GLANCE

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.

NIOBIUM

PAMELA S. ZURER, C&EN WASHINGTON

I never handled any niobium until my 50th birthday. As a science journalist, I'd heard of superconducting alloys of niobium, of sophisticated porous materials made from niobium oxide, and of the interesting chemistry of some niobium complexes. But it was my husband's birthday gift of a niobium necklace that put the metal into my hands and made me want to learn more about it.

My necklace is a simple choker made of small metal rectangles linked together. Yet it never fails to draw compliments when I wear it. A design on the surface of each link shimmers in soft luminous blues, greens, and purples--reflecting light more like the wings of a butterfly than the cold luster of gold or silver.

The captivating colors, I figured, must be due to a variety of enamels applied to the metal's surface. Wrong. The rainbow of colors is generated by thin films of niobium oxide, with the thickness of the oxide layer governing the color perceived. Jewelry artists produce brilliant colors on niobium by anodizing the metal.

"Thin-film interference is responsible for the color," says Bill Seeley, founder of Reactive Metals Studio in Clarksdale, Ariz., which sells niobium, titanium, and anodizing equipment to jewelry makers. "The oxide is transparent and has a high refractive index. Light waves bounce off the oxide, but some go through and reflect off the metal below, reappearing at the surface after a time delay that depends on the thickness of the oxide layer. Those two sets of waves either interfere with or reinforce each other, creating the color you see."

Surface oxide layers can be produced by heating niobium in air. Jewelry artists, however, prefer the control over the oxide thickness that they can achieve with electrochemistry.

"The thickness of the oxide is controlled by the voltage in the anodizing bath," Seeley explains. "An extremely thin layer--600 to 1,000 Å--grows at the interface. The oxide itself is resistant to the passage of current. If you set the voltage at 30 V, for example, the oxide film grows to a certain thickness and stops. Artists make multicolored pieces by using masks to temporarily protect parts of their pieces from the electrolyte."

In addition to niobium, interference colors can be created by anodizing titanium, zirconium, molybdenum, and tantalum, says Seeley, whose master of fine arts thesis was on studio preparation and coloring of titanium. Niobium, however, is particularly attractive to work.

"It's a beautiful, ductile metal," Seeley notes. "You can form it, re-form it, chase it, repousse it, spin it, shape it any number of ways. And it needs no special cleaning to create the beautiful anodized colors."

Dianne deBeixedon, professor of metalworking at Old Dominion University, Norfolk, Va., agrees. "Niobium is very malleable, very cooperative, a wonderful metal to shape. Plus there are the vibrant colors.

"For example, at 60 V, niobium produces a beautiful deep yellow," deBeixedon says. "But at about 65 V, it starts turning a pinky peach. At various voltages there are purples, fuchsia, a gorgeous turquoise, greens, a beautiful cobalt blue. The only color you can't get is red."

VIBRANT The colors in Seeley's niobium and silver broach (top) and deBeixedon's "Night Color Squid" (below) spring from thin-film interference.