NATURE |Vol 441|29 June 2006
NEWS & VIEWS
1055
to glue together other RNA structures 8. Just like guanine or adenine in the purine-sensing riboswitches, TPP and SAM become deeply buried in the RNA structures, embedded between interacting helices. The similar mode of substrate recognition observed in diverse riboswitch classes suggests a common func-tional mechanism, whereby conformational adaptability enables the RNA to encapsulate the substrate.
Highly specific substrate recognition by proteins exploits an arsenal of 21 amino acids that vary significantly in charge, size and polarity. By contrast, RNA has only four com-ponents at its disposal — a
denine, guanine,cytosine and uracil — that are all similar in size and chemistry. The riboswitches described in this issue 1,2demonstrate how RNA overcomes its limited chemical diversity by adaptively folding into remarkably complex architectures to create highly specific binding pockets. The overall architectures created by these two riboswitches are surprisingly similar and resemble small RNA enzymes (ribozymes).The close structural similarity of ribo-switches from bacteria 1and plants 7empha-sizes how difficult it may be for a single RNA molecule to combine substrate recognition with genetic control. This similarity has fuelled interest in riboswitches as attractive targets for antibacterial drugs, particularly because there is an apparent absence of such RNA systems in humans. Indeed, a TPP ana-logue (pyrithiamine pyrophosphate, PTPP)shows antimicrobial activity by blocking the binding site of the TPP-sensing riboswitch,shutting down thiamine metabolism in bacteria.
Unfortunately, these RNAs may be surpris-ingly proficient at developing drug resistance.Microbes develop resistance to PTPP by allow-ing mutations to emerge within the riboswitch,despite the need to preserve the RNA ’s struc-tural architecture. A single mutation is suffi-cient to override the inhibitory effects of the drug, rescuing thiamine synthesis 1,7. It seems that the structural flexibility that allows a
reactive materials studiesriboswitch to alter its conformation upon sub-strate binding may also facilitate the emer-gence of resistance to drugs targeted against it.Since their discovery a few years ago,numerous riboswitches ha
ve been found that specifically recognize substrates as diverse as nucleotides, amino acids, vitamins and co-enzymes 9. These RNAs are evolutionarily ancient, and are widespread in bacteria, plants
and fungi. Studies of aptamers — RNAs selected experimentally to recognize small molecules — demonstrate that RNA can dis-criminate between closely related structures at least as well as antibodies can 10. It is now clear that natural selection can also produce RNAs with extraordinary substrate specificity. Does this mean that small-molecule drugs could be found that bind to these RNAs and inhibit their function? Would bacteria and fungi become resistant to such drugs, or do riboswitches represent a chink in microbial armour?■
Steve Reichow and Gabriele Varani are in the Departments of Biochemistry and Chemistry,University of Washington, Seattle, Washington 98195-1700, USA.
e-mail: varani@chem.washington.edu
1.Serganov, A., Polonskaia, A., Phan, A. T., Breaker, R. R. &Patel, D. J. Nature 441,1167–1171 (2006).
2.Montange, R. K. & Batey, R. T. Nature 441,1172–1175 (2006).
3.Mandal, M. et al.Cell 113,577–586 (2003).
4.Sudarsan, N. et al.RNA 9,644–647 (2003).
5.Serganov, A. et al.Chem. Biol. 11,1729–1741 (2004).
6.Batey, R. T., Gilbert, S. D. & Montange, R. K. Nature 432,411–415 (2004).
7.Thore, S., Leinbundgut, M. & Ban, N. Science 312,1208–1211(2006).
8.Cate, J. H., Hanna, R. L. & Doudna, J. A. Nature Struct. Biol. 4,553–558 (1997).
9.Winkler, W. C. Curr. Opin. Chem. Biol.9, 594–602 (2005).10.Jenison, R. D. et al.Science 263,1425–1429 (1994).
As semiconductor materials are shrunk to the nanoscale, their physical properties begin to alter: colours change, melting points decrease,electron energy bands turn into discrete levels,and reactive surface areas become proportion-ately larger as particle size decreases 1. This applies not only to discrete semiconductor particles, but also for extended ‘mesoporous’framework structures of nanometre scale. In this issue, two papers 2,3detail approaches to the synthesis of periodic porous structures made of germanium. The optical properties of these materials are shown to depend on their dimensions, composition and the presence of other molecules attached to their surface.Porosity can b
e an extremely useful charac-teristic in a material, with the attendant large surface area benefiting applications in which molecules interact with a surface, such as sen-sors and catalysts. Silicon with nanoscale holes has been widely studied since its luminescent properties were discovered 4. But porous forms of a fellow member of group 14 of the periodic table — named ekasilicon by Mendeleev, but
MATERIALS SCIENCE
Germanium takes holey orders
Andreas Stein
Soap-like molecules serve as a scaffold for remarkably well-ordered, porous germanium skeletons. The nanometre-sized features of these
semiconductor frameworks confer unique optical and electronic properties.
rechristened germanium by its discoverer,
Clemens Winkler, in honour of his home country — have been investigated much less.Germanium’s int
erest lies in its semiconduct-ing nature and use in transistors, and also in its applications as a component of fibre-optic cables, as the focusing element in infrared-sensitive night-vision systems and as a poly-merization catalyst in the manufacture of soft-drinks bottles. Size-dependent properties can also be achieved with germanium at rela-tively large feature sizes, and, in a mesoporous form, unusual electronic and optical proper-ties are to be expected.
But few procedures exist to synthesize porous germanium. Etching and vapour-deposition techniques have been used to form germanium films with somewhat random pore structures and feature sizes 5,6. Now,Armatas and Kanatzidis (page 1122)2and Tolbert and colleagues (Sun et al., page 1126)3describe the use of a versatile technique known as surfactant templating to synthesize mesoporous germanium structures with
Figure 1|Gene regulation by riboswitches. The RNA of a riboswitch contains two functional domains: a metabolite-sensing domain (blue) and a gene-expression signal (green). These domains adopt interdependent conformations in response to the presence or absence of a particular metabolite
(yellow). In this example, the gene-expression signal is required for the initiation of protein synthesis.When the metabolite is absent, the metabolite-sensing domain adopts a conformation that re
veals the gene-expression signal and allows protein synthesis to occur (indicated by the red star). When the metabolite binds, the ensuing structural reorganization leads to the sequestration of the gene-expression signal, shutting off protein production (indicated by the grey star).
Nature Publishing Group
©2006
NEWS & VIEWS
NATURE |Vol 441|29 June 2006
1056
cubic and hexagonal geometry, respectively. Surfactant templating involves the self-assembly of inorganic building-blocks through their electrostatic interactions with organic ‘amphiphilic’ surfactant molecules that, like those in soap, have both hydrophilic and hydrophobic properties 7. According to how these interactions vary, different geometries are obtained as the inorganic components link to form a framework. Forms of silica with well-ordered cubic or hexagonal channel systems, or with disordered worm-like tunnels or layered structures, have already been syn-thesized using this techniqu
e 7,8. I n these mesostructured materials, inorganic regions alternate with organic template regions at typ-ical distances of several nanometres. To obtain a mesoporous solid with accessible channels and large surface areas, the organic template must be removed, often by combustion, with-out the inorganic framework collapsing. If this procedure works, it results in a structure that,although not necessarily crystalline on the atomic scale (mesoporous silica, for example,has amorphous walls), often shows regular order on the scale of the templated pore size.The successful synthesis of mesostructured germanium must address several challenges.Chief among these is finding a soluble germa-nium precursor that self-assembles in the presence of an amphiphilic surfactant and that can be linked into a continuous framework.Self-assembly is itself promoted by the choice of a solvent that forms a hydrogen-bonded network but does not react with the precur-sor. The current papers 2,3use precursors that, although different, are both related to the compounds formed when highly electro-positive metals combine with elements from the middle of the periodic table 9. These com-pounds were pioneered by Eduard Zintl in the 1920s and 1930s. That of Armatas and Kanatzidis 2has the chemical formula Mg 2Ge and contains Ge 4ǁunits in a crystalline lattice,whereas that of Tolbert and colleagues 3, K 2Ge 9,consists of more complex (Ge 92ǁ)n polymer chains derived from Ge 94ǁcluster ions.
To form an ordered, mesostructured germa-nium, Armatas and Kanatzidis add GeCl 4to their reaction mixture to provide Ge 4+bridges that connect the Ge 4ǁunits of the precursor.Tolbert and colleagues, on the other hand, link up their germanium chains by mild oxidation,taking care to avoid germanium oxide phases that would result from over-oxidation. The end result is, in the first case, cubic channel structures 2and, in the second, two-dimen-sional, hexagonal channel structures 3.
As in the case of mesoporous silica, the walls of both materials are amorphous on an atomic scale, but periodic on the mesoscale of the frameworks themselves. As anticipated, the materials exhibit size-dependent effects that result from so-called quantum confinement, a phenomenon that affects the allowed energy values of the material’s electrons at smaller scales. In particular, compared with spectra in
bulk germanium, substantial shifts towards shorter, ‘bluer’ wavelengths are observed in the optical absorption or luminescence spectra of 1-nm-thick germanium walls.
A final challenge was the removal of the sur-factant without destroying the mesostructure or converting germanium to its oxide. In the system derived from Mg 2Ge, this proved to be very difficult: Armatas and Kanatzidis state that “attempts to remove the surfactant from mesostructured germanium by therm
al decomposition led to contraction of the inor-ganic framework with consequent loss of pore ordering”2. Interesting properties are, how-ever, observed in this system even without sur-factant removal. I ts optical bandgap — a crucial parameter in determining the conduc-tion properties of a material — could, for instance, be smoothly varied through con-trolled partial oxidation and the generation of sub-oxide GeO x species in the framework. In the (Ge 92ǁ)-based system, mild oxidation followed by an ion-exchange step generates cross-linkages in the (Ge 92ǁ)n that allow sur-factant removal to take place. This results in a large surface area of 500 m 2per gram of mate-rial, and the opened structure permits incor-poration of guest molecules that shift the energies of electrons at the semiconductor’s surface and so can modify its conductivity.These papers 2,3both represent significant advances in the structuring of germanium frameworks. In a sense, the papers are com-plementary: they show the versatility of Zintl-compound chemistry in combination with surfactant templating and provide different avenues towards designed, ordered semicon-
ductor geometries. Tolbert and colleagues 3illustrate a way to produce true mesoporosity;Armatas and Kanatzidis 2show that a cubic structure with continuous walls and continu-ous channels can be achieved, which is often a more desirable architecture to facilitate the access of guests to the whole pore system. This work will no doubt spawn new meso-porous semiconductor systems of relevance in
sensing, detection and communications.Opportunities exist for tuning the properties of these systems with mixed compositions,including the mixed Ge/Si systems 3and Zintl clusters involving other group-14 elements, or by using other linking molecules to con-nect Zintl compounds using Armatas and Kanatzidis’s method. With the precedents of structural and compositional variety set by mesoporous oxides, the stage is set for meso-porous semiconductors with properties that can be engineered.■
Andreas Stein is in the Department of Chemistry,University of Minnesota, Minneapolis,Minnesota 55455, USA.
e-mail: stein@chem.umn.edu
1.Brus, L. J. Phys. Chem.90,2555–2560 (1986).
2.Armatas, G. S. & Kanatzidis, M. G. Nature 441,1122–1125(2006).
3.Sun, D. et al. Nature 441,1126–1130 (2006).
4.Canham, L. T. Appl. Phys. Lett. 57, 1046–1048 (1990).
5.Choi, H. C. & Buriak, J. M. Chem. Commun. 1669–1670(2000).
6Shieh, J. et al. Adv. Mater.16,1121¬1124 (2004).7.Kresge, C. T. et al. Nature 359, 710–712 (1992).
8.Huo, Q., Margolese, D. I. & Stucky, G. D. Chem. Mater. 8,1147–1160 (1996).
9.Sevov, S. C. in Intermetallic Compounds Vol. 3 (eds Westbrook, J. H. & Fleischer, R. L.) 113–132 (Wiley,Hoboken, NJ, 2002).
ASTRONOMY
Supernovae brought to light
Carl Heiles
The existing catalogue of Galactic supernova remnants contains only a small fraction of the true number of these stellar explosions. A different observational technique is being employed to find the missing ones.
Just as nuclear bombs on Earth leave their legacy of devastation, stellar explosions —supernovae — leave their mark on the cosmos in the form of supernova remnants (SNRs) in the interstellar gas. The rate of supernova explosions in our Galaxy can be inferred from historical records of visual sightings a
nd through comparison with other galaxies.These data, together with theories of SNR expansion and evolution, allow a prediction of the number of SNRs that should currently reside in our Galaxy. The result is tens of times larger than the number of known remnants,which are catalogued almost exclusively from the radiation they emit at radio frequencies 1.Writing in The Astrophy sical Journal , Koo,
Kang and Salter 2use a new technique to iden-tify the more numerous older SNRs, which
were missing from the catalogue because they are radio-dim.
The legacies left by supernovae, extending beyond cosmology, are often unappreciated.First, there is the anthropocentric point that we wouldn’t be here without the elements, such as iron and carbon, that supernovae have caused to disperse into the interstellar gas. Over Earth’s lifetime, several dozen nearby supernovae (within a distance of around 30 light years)would also have seriously modified Earth’s environment, each for several hundred years 3.The most serious consequences would have been the destruction of the ozone layer and a
Nature Publishing Group
©2006