A LOO K BAC K AT 2002
C&EN REVISITS 2002 A decade ago, in its annual Chemistry Highlights feature, C&EN looked at some of that year’s KEY RESEARCH ADVANCES in chemistry. C&EN reporters have revisited several of those highlighted discoveries to see what became of them.
Carbohydrate microarrays have become essential screening tools for revealing the functions of biomolecules that interact with sugars, providing essential information on the mechanisms of diseases. The devices, which were developed a decade ago, are each made up of solid substrates displaying hundreds of different oligosaccharides (also called glycans) or carbohydrate-containing macromolecules. In 2002, four initial types were designed and constructed: polysaccharide and glycoconjugate microarrays by Denong Wang’s group at Columbia University (Nat. Biotech., DOI: 10.1038/nbt0302-275); monosaccharide chips by Milan Mrksich and coworkers at the University of Chicago (Chem. Biol. 2002, 9, 443); natural and synthetic oligosaccharide arrays by Ten Feizi’s group at Imperial College London (Nat. Biotech., DOI: 10.1038/nbt735); and synthetic oligosaccharide arrays on microtiter plates by a group from Scripps Research Institute in California led by Chi-Huey Wong, now president of Academia Sinica, in Taiwan (Chem. Biol. 2002, 9, 713; J. Am. Chem. Soc., DOI: 10.1021/ ja020887u). The carbohydrate microarray field has matured since then, thanks in part to the establishment of research centers that specialize in the technology—for example, the Carbohydrate Microarray Facility at Imperial College and the Consortium for Functional Glycomics based at Scripps and at Emory University. Carbohydrate arrays can be constructed with either underivatized or chemically modified oligosaccharides. Columbia’s Wang, for example, has specialized in
COURTESY OF RYAN MCBRIDE/SCR IPPS
CARBOHYDRATE CHEMISTRY: HAPPY BIRTHDAY TO MICROARRAYS
This carbohydrate microarray from the Consortium for Functional Glycomics, only a small part of which is shown, was screened with 17 lectin proteins. Hits are visualized with fluorescent markers, from black representing no binding, to various colors representing increasing binding intensity.
making arrays by attaching underivatized saccharides covalently or noncovalently to substrates. “The use of underivatized saccharides for microarray construction has the advantage of preserving the native structures of the carbohydrate molecules,” Wang says, “but requires a ready-to-use microarray surface with appropriate surface chemistry.” Because desired surface chemistries for unmodified oligosaccharides are often difficult to produce, microarrays made from modified carbohydrates have become far more popular. For instance, Feizi and her coworkers at Imperial College’s Carbohydrate Microarray Facility construct microarrays by releasing oligosaccharides from glycosylated proteins or polysaccharides and tagging them with lipids, forming “neoglycolipids.”
WWW.CEN-ONLIN E .ORG
38
D EC E M BE R 24, 20 1 2
These constructs are immobilized noncovalently on solid matrices for binding experiments. The researchers currently use arrays that display about 800 unique oligosaccharides. There’s no charge to users for screening limited numbers of samples at the facility, which is supported by the Wellcome Trust. Researchers at the Consortium for Functional Glycomics also use derivatized oligosaccharides to construct arrays. But in their case, the oligosaccharides are attached covalently to glass slides via amine linkers. The current arrays each have 610 synthetic oligosaccharides, including oligosaccharides with up to 37 sugar units. The arrays are available at no cost to users through the support of the National Institute of General Medical Sciences, says James C. Paulson, who led development of the arrays. The devices are produced at Scripps, and screening data are analyzed at Emory. Carbohydrate microarrays have not been commercialized, owing in part to difficulties encountered in optimizing and standardizing the way oligosaccharides are immobilized and their density and distribution on array surfaces. Nevertheless, they are proving valuable in a range of applications. In one such application, microarray analysis of the pandemic H1N1 flu virus uncovered a potential mechanism linking viral receptor binding to disease severity. Another oligosaccharide array study identified immunogenic carbohydrate moieties of anthrax spores. And carbohydrate array studies of neutralizing antibodies that target oligosaccharides from HIV-infected patients are aiding the design of HIV vaccines. “Carbohydrate microarray technology has come of age as a tool in the biological sciences,” Feizi says.—STU BORMAN
In 2002, chemists at Scripps Research Institute in California tricked the Alzhei mer’s-linked enzyme acetylcholinesterase into making a powerful inhibitor of its own activity. It was a test case for streamlining drug design by involving the target as early as possible. They used a selective form of the cycloaddition reaction between azides and alkynes that’s better known as click chemistry (Angew. Chem. Int. Ed. 2002, 41, 1053). And they used the enzyme target’s own shape constraints and noncovalent interactions as a molecular-scale reaction vessel. The research team, led by K. Barry Sharpless and M. G. Finn, dubbed the technique in situ click chemistry. Researchers began to explore the method’s potential as a screen for drugs, lightharvesting compounds, and more. Ten years on, the technique has had some successes, but it hasn’t been widely adopted. The method was a newcomer to the area of target-guided synthesis, which aims to streamline discovery of functional molecules by combining screening and synthesis into one process. The click approach has a few differences from other target-guided techniques, including minimal side reactions. A handful of teams have carried the technique forward. In 2009, a UCLA lab developed a microfluidic platform to perform 1,024 in situ click reactions in parallel. Then in 2010, scientists at France’s National Institute of Health & Medical Research applied the technique to examine the protein EthR, a possible tuberculosis drug target. Also in 2009, Caltech’s James R. Heath collaborated with the Scripps team to make agents that act like antibodies. Integrated Diagnostics, a company Heath cofounded, plans to develop the so-called protein-catalyzed capture agents into clinical diagnostic tools for diseases such as cancer. “There’s no reason why our approach should be restricted to enzyme inhibitors,” Finn says. “That’s part of the reason why I rave about Heath’s work.”
A few pharmaceutical companies have tried the click technique. A team at Amgen explored it while hunting for tight-binding inhibitors of β-secretase, a potential Alzheimer’s drug target. In a 2007 meeting presentation, Amgen researchers praised the technique’s ability to account for the dynamic nature of the target proteins but noted that the click reaction is slow. This click chemistry variant “certainly hasn’t taken the world by storm,” Finn says. “It is still a hit-or-miss proposition. No one’s taken the time to figure out what conditions make it work consistently.”
COURTESY OF M. G. FINN
CHEMICAL BIOLOGY: IN SITU CLICK CHEMISTRY
Compared with this work, Sharpless has garnered more citations for a copper(I) catalyzed version of click chemistry he also published in 2002 (Angew. Chem. Int. Ed. 2002, 41, 2596). That paper is the fourth-most-cited work in chemistry from 2002, according to statistics from the American Chemical Society’s Chemical Abstracts Service. Whether or not the click technique proves to be a panacea for molecular discovery, Sharpless says, chemists should keep searching for better screens. “Every year that goes by it becomes clearer that the intentional designing of inhibitors is very difficult, and luck plays a bigger role than we’d like,” he says. Even if established techniques tip the odds in chemists’ favor, Sharpless adds, they are far from costefficient.—CARMEN DRAHL A 2002 model of an inhibitor (top center) in the active site of acetylcholinesterase is the first example of in situ click chemistry in action.
WWW.CEN-ONLIN E .ORG
39
D EC E M BE R 24, 20 1 2
CHEMICAL STRUCTURE: DATABASES GROW IN POPULARITY If journal citations measure a piece of work’s impact on the scientific community, then it’s clear what’s been most important to chemists during the past decade: molecular structural data. “The Cambridge Structural Database: A Quarter of a Million Crystal Structures and Rising,” is a treatise published in 2002 on the exponentially expanding collection of small-molecule crystal structures curated by the Cambridge Crystallographic Data Centre (CCDC), in England (Acta Cryst. 2002, B58, 380). The paper had garnered 4,689 citations as of Dec. 12, the most of any paper in the chemical sciences published in 2002, according to an analysis conducted for C&EN by the American Chemical Society’s Chemical Abstracts Service (CAS). The Cambridge Structural Database (CSD) was designed to house molecular structures and property data of small molecules of interest to chemists and life scientists, including organic and organometallic compounds of up to 1,000 atoms. Since its inception in 1965, the database has grown from a fledgling project managed by a few staff members who collected data by hand, to a state-of-the-art facility with dozens of team members who design and manage software for molecular searches, analysis, and visualization. CSD complements, and sometimes overlaps with, three other major structural databases, also started in the 1960s and 1970s. These are the Protein Data Bank, managed by Rutgers University and the University of California, San Diego; the Inorganic Crystal Structure Database, managed by FIZ Karls ruhe–Leibniz Institute for Information Infrastructure, in Germany; and CRYSTMET, a database for metals and alloys, managed by Toth Information Systems, in Canada. In the 2002 paper, CCDC’s thendirector Frank H. Allen predicted that CSD would collect half-a-million crystal structures by 2010—it came in just under the wire, reaching that number on Dec. 1, 2010. The database now contains more than 630,000 structures and is growing by more than 40,000 structures each year. “We are clearly heading for a million rather rapidly,” says Allen, who retired from the CCDC directorship in 2008. By comparison, the CAS Registry, the world’s largest collection of publicly disclosed substance information, ended
A LOO K BAC K AT 2002
2002 with information on just more than 20.7 million small molecules, Schenck notes. “Reflecting the phenomenal growth in chemistry research worldwide, the CAS Registry added its 70 millionth small molecule in early December,” he adds. “I’m not surprised CSD is being so heavily used and cited, as it does contain so much information about molecular structure and
intermolecular interactions,” says Sarah L. (Sally) Price, a professor of physical chemistry at University College London. Price, who has cited CSD dozens of times in her publications, says her ability to retrieve crystal structures of families of related molecules and then analyze them using CCDC’s software has been“an essential starting point for looking at supramolecular behavior.”
SOLID-STATE NMR: BIOMOLECULES VIEWED AT ATOMIC RESOLUTION
J. AM . CHEM. SOC.
determining structures of amyloid fibrils, which are insoluble protein aggregates associated with several neurodegenerative disorders, including Alzheimer’s disease. A decade ago, researchers determined the One key to advancing the field of first atomic-resolution structure of a biosolid-state NMR has been the availability molecule using solid-state NMR spectrosof high-field magnets. Another was the copy. Although the structure determination development of dynamic nuclear polarizawas of a mere tripeptide, says Ann McDertion, a technique that increases the NMR mott, a protein NMR specialist at Columbia signal of a sample by adding a stable radical University, “it was a pivotal piece of work.” compound. Irradiating the sample-radical Scientists in the combination with highfield anticipated that frequency microwaves solid-state NMR would transfers polarization be able to produce of the radical’s electron structures as detailed as spin to the nuclear those obtained by X-ray spins of the atoms in a crystallography, McDerprotein or other species mott adds. “But to see it of interest. executed that elegantly European scientists raised the bar.” are investing more heavThe tripeptide work ily in advanced NMR was led by Robert G. instruments than are Griffin, a chemistry scientists in the U.S., At 159 amino acids, matrix metalloproteinase-12 is the largest professor at MIT, and MIT’s Griffin says, largegraduate student Chad protein structurally characterized by ly because of funding solid-state NMR so far, shown here M. Rienstra, now a availability. Commercial with its two-dimensional 13C spectrum. chemistry professor at dynamic nuclear polarthe University of Illiization and high-field nois, Urbana-Champaign (Proc. Natl. Acad. instruments cost upward of $2 million, an Sci. USA, DOI: 10.1073/pnas.152346599). amount that necessitates U.S. researchers The solid-state NMR approach incorpogather money from multiple sources. rates magic-angle spinning, in which NMR Griffin says he expects it will be another signals are narrowed by spinning samples at decade before solid-state NMR of biomola specific angle relative to the direction of ecules becomes routine. And after biomolthe magnetic field. Since their 2002 paper, ecules, the technique’s next big hit will likely Griffin, Rienstra, and others have continbe surface science. ued to work on the technique, with an eye Two years ago, Emsley and his colleagues toward solving the structures of proteins demonstrated the use of dynamic nuclear that are not amenable to crystallization. polarization solid-state NMR to study “Today the most exciting papers are tarfunctionalized silica surfaces prepared as geting membrane proteins, amyloid fibrils, a “translucent slush” of porous silica in a or large protein assemblies, such as bacteradical solution. Materials scientists are rial secretion needles or elements of the interested in the approach, Emsley says, and ribosome,” says Lyndon Emsley, scientific his group is now collecting and analyzing director of the European Center for High NMR spectra of the surfaces of materials as Field NMR, in Lyon, France. Solid-state varied as catalysts, cement, and solar-cell NMR has been particularly important for components.—JYLLIAN KEMSLEY WWW.CEN-ONLIN E .ORG
40
D EC E M BE R 24, 20 1 2
CSD data have innumerable other uses, Allen notes, for scientists studying conformer generation, protein-ligand docking, and solid-state phenomena such as drug polymorphism and cocrystallization. “The types of papers that use and cite CSD continue to surprise and delight us,” says Colin Groom, current director of CCDC.—ELIZABETH WILSON
ORGANIC SYNTHESIS: ONE-STEP ARYLBORONATES When it comes to assembling molecules, synthetic chemists find that the Suzuki coupling reaction is as indispensable to them as a screwdriver is to a carpenter. A 2011 review article estimates that the method, which traditionally cross-couples an organoboron compound with an organohalide to form a new carbon-carbon bond, accounts for 40% of all C–C bond-forming reactions used in the pursuit of drug candidates (J. Med. Chem., DOI: 10.1021/ jm200187y). Its utilitarian nature earned its discoverer, Akira Suzuki of Hokkaido University in Japan, a share of the 2010 Nobel Prize in Chemistry. But even a great reaction can suffer from tough-to-obtain reagents. So in 2002 when two groups independently announced they had discovered iridium catalysts that turn arenes into arylboronic esters in one step, as opposed to traditional multistep approaches, it was as if chemists had been handed a cordless electric screwdriver. The reaction gives chemists a tool for taking a C–H bond and turning it into something that can readily undergo a subsequent chemical transformation, says Milton R. Smith III of Michigan State University, who led one of the research efforts (Science, DOI: 10.1126/science.1067074). “The key to this chemistry is that it allows us to get to these compounds much more efficiently,” adds Robert E. Maleczka Jr., Smith’s Michigan State collaborator. “It allows access to many sorts of substitution patterns that were traditionally unavailable. It really opens up the opportunity to make new building blocks.” The Michigan State chemists took advantage of the chemistry to start BoroPharm, a company that sells boronic acids and related chemicals. Smith and Maleczka also received a 2008 Presidential Green Chemistry Challenge Award for their work.
John F. Hartwig, then of Yale University, whose team independently developed an iridium catalyst for the transformation, notes that the reaction has been widely used in academic and industrial laboratories. “One medicinal chemist stated to me that this reaction has become part of their regular repertoire,” says Hartwig, now at the University of California, Berkeley. He carried out the original work with Hokkaido chemists Tatsuo Ishiyama and Norio Miyaura (J. Am. Chem. Soc., DOI: 10.1021/ja0173019). “It is remarkable that only about a decade separates the initial observation of stoichiometric functionalization of arenes and alkanes with metal-boryl complexes in high yield and the development of catalysts for the borylation of arenes that are
The first step in the synthesis of the alkaloid natural product rhazinicine was made possible by an iridium-catalyzed borylation reaction.
now widely used,” Hartwig and colleagues noted in a 2010 review (Chem. Rev., DOI: 10.1021/cr900206p). Tweaking the catalyst ligands over time has transformed the chemistry to make it even more powerful, Hartwig adds. “If the two catalysts the two groups published in 2002 were all that were available, the synthetic community would never be using this chemistry as commonly as they use it today,” he says.
A123 SYSTEMS
MATERIALS SCIENCE: LITHIUM-ION BATTERIES 2.0 Ten years ago, the first generation of lithium-ion batteries enjoyed prominence for markedly advancing portable power technology for small electronics. Cell phones, laptop computers, and other mobile devices commercialized in the early 1990s typically used nickel-based batteries. But by the new millennium, those devices’ popularity had soared, thanks to lithium-ion batteries that weighed less and lasted longer. Then came an advance that changed the lithium-ion battery scene substantially: In 2002, MIT’s Yet-Ming Chiang and coworkers reported that doping the inexpensive and stable cathode nanomaterial LiFePO4 with Mg2+, Nb5+, and other ions boosted the electrical conductivity by a whopping eight orders of magnitude relative to the undoped material (Nat. Mater., DOI: 10.1038/ nmat732). The discovery, which led to batteries that could be charged and discharged repeatedly at extraordinary rates, ushered in a new era—what Chiang refers to as “lithium-ion batteries 2.0.” Within a few years, the company that Chiang cofounded to commercialize this battery technology, A123 Systems, was able to move into high-powered applications previously untouched by lithium-ion batteries. For example, the batteries were used in a new line of DeWalt power tools. And they ran electric motorcycles that repeat-
The iridium-catalyzed chemistry is now being used to synthesize natural products and fine chemicals, as well as in materials science. The reaction takes place under mild conditions and tolerates a broad range of functional groups. “These reactions are inherently clean, with hydrogen gas being the only stoichiometric by-product,” Maleczka points out. “The functional group tolerance really set the stage for us to be able to combine these reactions with subsequent chemical events, so you can do multiple steps in one pot.” “I think there’s still a lot that’s going to be developed in the future on this,” Smith adds. “In 10 years there are likely to be more advances that will put this into even more people’s hands.”—BETHANY HALFORD
Lab analysis of lithium-ion battery electrodes, seen here on spools, ensures their suitability for applications demanding high power.
edly set speed records. In short, Chiang’s materials advance helped give rise to lithium-ion batteries with some muscle. The batteries continue to move into areas that once were unimaginable. In the transportation sector, for example, lithium-ion batteries now power some
WWW.CEN-ONLIN E .ORG
41
D EC E M BE R 24, 20 1 2
3,000 hybrid-electric city buses across North America and have collectively racked up more than 300 million miles of road service, Chiang says. Commercial versions of the batteries are now used in several lines of electric and hybrid-electric passenger cars. In addition, specialty batteries have been used to power Formula 1 race cars, replacing liquid fuels, and the world’s fastest electric car, the Buckeye Bullet, which topped 307 mph in 2010. LiFePO4 batteries have also made inroads in the electric power industry. The batteries are used in commercial facilities for grid-scale storage, short-term peakpower stabilization, backup power, and integrating power generated at wind farms into the electric grid. Despite these successes, however, A123 Systems has fallen on hard times. It filed for bankruptcy in October, and as of C&EN press time it was in the process of being split up and sold. As Chiang explains, A123 undertook major manufacturing scaleups to keep pace with the projected rapid growth in lithium-ion battery demand by the electric-vehicle industry. “But that market simply has not grown as quickly as was forecast,” he says. “We’re looking now beyond lithium-ion 2.0,” Chiang adds. His group and others are searching for materials that may lead to a third generation of lithium-ion batteries, ones that provide even lower cost and longer peak-power solutions in the area of electric-grid storage.—MITCH JACOBY
