Superheavy carbonyls Walter Loveland Science 345, 1451 (2014); DOI: 10.1126/science.1259349
This copy is for your personal, non-commercial use only.
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here.
The following resources related to this article are available online at www.sciencemag.org (this information is current as of September 26, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/345/6203/1451.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/345/6203/1451.full.html#related This article cites 7 articles, 1 of which can be accessed free: http://www.sciencemag.org/content/345/6203/1451.full.html#ref-list-1 This article appears in the following subject collections: Chemistry http://www.sciencemag.org/cgi/collection/chemistry
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on September 26, 2014
Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here.
ILLUSTRATION: ADAPTED BY P. HUEY/SCIENCE
frequency at the electron spins via the same interaction. Performing the experiment with a deuterated instead of hydrogenated polymer changes the modulation frequency to the deuteron precession frequency (a factor of 6.6 times smaller), which is a nice confirmation of this picture. In a second experiment, Malissa et al. manipulate both the electron and nuclear spins (in an electron-nuclear double resonance setup) by using, in addition to the microwave pulses, a radiofrequency pulse to rotate the proton spins. Because the hyperfine couplings with surrounding proton spins create a small random magnetic field, the field B0 at which resonance with the microwave field occurs is slightly different for each electron spin. A specific field B0 can therefore select a subensemble of electron spins coupling predominantly to proton spins that are directed, say, upward. After a radiofrequency pulse that swaps the proton spins by 180° downward, the subensemble of these electron spins is no longer in resonance with the microwave field, and therefore these spins become insensitive to subsequent microwave pulses. The swapping of the proton spins can then be observed via the current. This provides an intriguing new approach to detect nuclear spin manipulation electrically. The study of Malissa et al. is an important step toward the use of organic materials in quantum-coherent spin manipulation. A great advantage as compared to other spin manipulation approaches is the operation at room temperature. Nature may have paved the way here for us in the development of a quantum-biological compass, obviously operating at ambient temperature. However, the disadvantages at present are the lack of control over the involved electronic processes, which are mostly due to the morphological disorder of the used conjugated polymer. Coherence times are presently limited by the uncontrolled lifetime of the spin pairs. Increasing and controlling morphological order by embedding a semiconducting organic molecule in the pores of a zeolite crystal have led to a spectacular increase in organic magnetoresistance and the option to address only a few molecules (6). This may also be the way to bring spin manipulation in organic materials to a next level of control. ■ REFERENCES
1. 2. 3. 4.
H. Malissa et al., Science 345, 1487 (2014). T. Ritz et al., Biophys. J. 96, 3451 (2009). S. Engels et al., Nature 509, 353 (2014). T. L. Francis, Ö. Mermer, G. Veeraraghavan, M. Wohlgenannt, New J. Phys. 6, 185 (2004). 5. M. Wohlgenannt, P. A. Bobbert, B. Koopmans, MRS Bull. 39, 590 (2014). 6. R. N. Mahato et al., Science 341, 257 (2013). 10.1126/science.1259655
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
1
2
H 3
He 4
Li
Be
11
12
5
B 13
Na Mg 19
20
21
K
Ca
Sc
37
38
39
Rb Sr 55
Cs 87
Fr
18
Y
56
22
Ti 40
Zr 72
Ba
Hf
88
104
Ra
Rf
57
La 89
Ac
58
Ce 90
Th
23
24
25
26
V
Cr Mn Fe
41
42
Nb Mo 73
74
Ta
W
105
106
Db Sg 59
Pr
60
43
Tc 75
44
27
Co 45
Ru
Rh
Re
Os
76
77
107
108
109
61
62
63
93
94
95
Ir
Bh Hs Mt
Nd Pm Sm Eu
91
Pa
92
U
28
Ni 46
Pd 78
Pt 110
Ds 64
29
Cu 47
30
6
48
Cd
O
14
15
16
Al
Si
31
32
79
80
49
In 81
50
Sn 82
Au Hg
Tl
Pb
111
112
113
114
66
67
Rg 65
Gd
Tb
96
97
Np Pu Am Cm Bk
8
N
Zn Ga Ge
Ag
7
C
Cn Uut Fl
33
As 51
Sb 83
S
34
17
Cl 35
Se
Br
52
53
Te 84
I
85
18
Ar 36
Kr 54
Xe 86
Po
At
Rn
115
116
117
118
Uup Lv Uus Uuo
Er
Tm Yb
69
70
Lu
98
100
101
102
103
99
10
Ne
Bi
Dy Ho Cf
68
P
9
F
Es Fm Md No
71
Lr
The modern Periodic Table. Similar chemical behavior might be expected for the homologs (Mo, W, Sg), (Hg, Cn) and (Pb, Fl), but relativistic effects can cause deviations from the expected behavior.
CHEMISTRY
Superheavy carbonyls The radioactive superheavy element seaborgium can form a carbonyl compound during its short lifetime By Walter Loveland
E
lements with atomic numbers Z > 94 are radioactive and cannot be found naturally in Earth. Efforts to create these elements, particularly the superheavy elements (Z ≥ 104), in the laboratory typically use “hot fusion” reactions involving extremely high temperatures. On page 1491 of this issue, Even et al. (1) show how these atoms can be cooled down and used to synthesize a new class of chemical compounds, the superheavy metal carbonyls. The results confirm a 15-yearold prediction of relativistic quantum chemistry. The known superheavy elements, referred to as the transactinides, range in atomic number from 104 (Rf ) to the as yet unnamed element 118 (see the first figure). The lifetimes of many of these elements range from milliseconds to minutes. In the case of Z = 112 (Cn) to Z = 118, current methods only allow the production of a few atoms per week. Chemistry must be done on a “one atom at a time” basis, making these studies very difficult. Relativistic effects shape the chemistry of these elements. The speeds of the innermost electrons in these atoms approach the speed of light, requiring the use of relativistic quantum mechanics to describe the atoms and molecules. The relativistic effects consist of a contraction and stabilization of the innermost s and p orbitals, the splitting
SCIENCE sciencemag.org
of electron energy levels due to spin-orbit coupling, and an expansion (and destabilization) of the outer d orbital and all f orbitals. In the transactinides, stabilization of the 7s orbital of Cn doubles its binding energy. In Sg, the 7s and 6d orbital ordering is inverted, possibly changing its oxidation properties. The spin-orbit splitting, like the other relativistic properties, increases roughly as Z 2, approaching the energies of chemical bonds in magnitude. The periodic table of the elements is not a list of the elements ordered by increasing atomic number, but rather a grouping of the elements by chemical properties. It is thus a living document. The placement of new elements such as the transactinides in the table depends on measurements of their chemical properties. Experimental studies of the chemistry of the transactinides (2) support the placement of the elements Rf (104), Db (105), Sg (106), Bh (107), Hs (108), and Cn (112) into groups 4, 5, 6, 7, 8, and 12 of the periodic table, respectively, forming the fourth row of transition metals; however, the real possibility of deviations exists. For example, element 112 (Cn) is nominally a homolog of Hg; element 114 (Fl) is a homolog of Pb; thermochromatography experiments have shown that Cn behaves like Hg (3); but it has been difficult to determine whether Fl is a noble gas or a noble Oregon State University, Corvallis, OR 97331, USA. E-mail: [email protected] 19 SEP TEMBER 2014 • VOL 345 ISSUE 6203
Published by AAAS
1451
INSIGHTS | P E R S P E C T I V E S
Rotating target wheel
Recoil separator Q Mo/W/Sg trajectories
Q
Beam Beam/background trajectories
D
N2 (I) cryostat
Gas chromatography column RTC
Structures of a multisubunit protein-RNA complex reveal how the CRISPR system recognizes DNA targets By Yan Zhang and Erik J. Sontheimer
Tefon capillary 2⫻ 32 SiO2-covered detectors for α and β particles and fssion fragments Making Sg carbonyl. In Even et al.’s experiment, hot Sg atoms are produced in a nuclear reaction, transported through the GARIS magnetic separator, and slowed down in the recoil transfer chamber (RTC), where they react with CO. The resulting Sg(CO)6 products are transported to the thermochromatography apparatus for analysis.
metal (4, 5). Gas-phase chemical studies have been important tools for studying the chemical behavior of short-lived elements. The use of physical recoil separators that isolate the nuclear reaction products has allowed the synthesis of compounds, such as the metal carbonyls. The metal carbonyls of the group 6 elements—Cr(CO)6, Mo(CO)6, and W(CO)6—are well known. U(CO)6 is unstable above 30 K, in accord with predictions of relativistic quantum chemistry (6). However, Sg(CO)6 is predicted (6) to be stable because of relativistic effects that lead to stronger molecular bonding. Actinide carbonyls have been difficult both to synthesize and characterize. Laser ablation experiments produce U(CO)6 and Th(CO)6 only as transient species. The synthesis of transactinide carbonyls looked to be forbidding because of the harsh synthesis conditions required to create these elements. Even et al. use a novel separation of the recoiling reaction products to overcome these problems. Having synthesized a Sg carbonyl complex, they used thermochromatography to characterize it. The authors carried out the experiment at the GARIS recoil separator at the RIKEN accelerator facility in Japan. They first generated Mo, W, and Sg nuclei in nuclear reactions at the target portion of the separator (see the second figure). The recoiling nuclei were then separated from the incident projectile beam by the magnetic elements of the GARIS separator and entered a recoil transfer chamber. In that chamber, the Mo, W, and Sg nuclei were thermalized in a He/ CO mixture and reacted with the CO molecules to form carbonyls. These compounds were transported to the gas chromatography 1452
Cascading into focus
column, where a temperature gradient from 25°C to –120°C was maintained. The column consisted of 32 pairs of Si semiconductor detectors covered with SiO2 that recorded the decay of the radioactive nuclei. Mo and W formed volatile carbonyls that adsorbed on the column in temperature regions characteristic of their known enthalpies of adsorption. A volatile Sg species adsorbed on the column at a similar position. Monte Carlo simulations of the adsorption process allowed the scientists to deduce an adsorption enthalpy of 50 ± 4 kJ/mol for the Sg species, in good agreement with the theoretical predictions (7) and similar to the measured value for W(CO)6. Even et al. conclude that they have formed a volatile Sg carbonyl complex, probably Sg(CO)6. This represents the first synthesis of a new class of superheavy compounds— a finding that could lead to the synthesis of carbonyl complexes of elements 104 to 109. The implications of this development are widespread. For example, the chemistry of element 109, Mt, has not been studied. Using carbonyl complexes, it should now be possible to study Mt chemistry. Thermal dissociation experiments with the carbonyls should enable studies of the strengths of metal-carbon bonds in all the superheavy elements. ■ REFERENCES
1. 2. 3. 4. 5. 6.
J. Even et al., Science 345, 1491 (2014). M. Schädel, Radiochim. Acta 100, 579 (2012). R. Eichler et al., Nature 447, 72 (2007). R. Eichler et al., Radiochim. Acta 98, 133 (2010). A. Yakushev et al., Inorg. Chem. 53, 1624 (2014). C. S. Nash, B. E. Bursten, J. Am. Chem. Soc. 121, 10830 (1999). 7. V. Pershina, J. Anton, J. Chem. Phys. 138, 174301 (2013). 10.1126/science.1259349
A
n adaptive immune pathway in bacteria and archaea is specified by clustered, regularly interspaced, short palindromic repeat (CRISPR) sequences (1). Research into CRISPR RNAs (crRNAs) and the CRISPRassociated (Cas) proteins have revealed the ability of most of these systems to target foreign DNA molecules for destruction. On pages 1473 and 1479 of this issue, Jackson et al. (2) and Mulepati et al. (3), respectively, and Zhao et al. (4) describe high-resolution structures of the multiprotein assembly called CRISPR-associated complex for antiviral defense (“Cascade”), which drives CRISPR interference in many strains of Escherichia coli. The structures show how Cascade presents crRNA to its DNA target, and demonstrate that DNA recognition occurs through a configuration that, surprisingly, is not double-helical. Diverse flavors of CRISPR systems have been organized into three families (types I, II, and III) with additional subdivisions (5). All use crRNA precursors (pre-crRNAs) with multiple repeat sequences that are separated by “spacers.” These spacers match “protospacer” sequences that are usually present within phage genomes or plasmids. Pre-crRNAs are processed into individual crRNAs, each with a single spacer that guides the effector machinery to its DNA target. Type II systems require only a single protein, Cas9, for crRNA-guided DNA targeting. By contrast, crRNAs in type I and type III systems assemble into large multiprotein complexes. Cascade was first defined for the type I-E system from E. coli strain K12 (6) and includes one to six copies each of five distinct Cas proteins. Many of E. coli Cascade’s important interactions, mechanistic features (6–12), and structural properties (7–9, 12) have been described, including its seahorse-like shape. Double-stranded DNA (dsDNA) recognition by Cascade requires not only crRNA-DNA complementarity, but also a 3–base pair sciencemag.org SCIENCE
19 SEP TEMBER 2014 • VOL 345 ISSUE 6203
Published by AAAS
ILLUSTRATION: ADAPTED BY P. HUEY/SCIENCE
Mylar window
D
STRUCTURAL BIOLOGY
This copy is for your personal, non-commercial use only.
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here.
The following resources related to this article are available online at www.sciencemag.org (this information is current as of September 26, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/345/6203/1451.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/345/6203/1451.full.html#related This article cites 7 articles, 1 of which can be accessed free: http://www.sciencemag.org/content/345/6203/1451.full.html#ref-list-1 This article appears in the following subject collections: Chemistry http://www.sciencemag.org/cgi/collection/chemistry
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on September 26, 2014
Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here.
ILLUSTRATION: ADAPTED BY P. HUEY/SCIENCE
frequency at the electron spins via the same interaction. Performing the experiment with a deuterated instead of hydrogenated polymer changes the modulation frequency to the deuteron precession frequency (a factor of 6.6 times smaller), which is a nice confirmation of this picture. In a second experiment, Malissa et al. manipulate both the electron and nuclear spins (in an electron-nuclear double resonance setup) by using, in addition to the microwave pulses, a radiofrequency pulse to rotate the proton spins. Because the hyperfine couplings with surrounding proton spins create a small random magnetic field, the field B0 at which resonance with the microwave field occurs is slightly different for each electron spin. A specific field B0 can therefore select a subensemble of electron spins coupling predominantly to proton spins that are directed, say, upward. After a radiofrequency pulse that swaps the proton spins by 180° downward, the subensemble of these electron spins is no longer in resonance with the microwave field, and therefore these spins become insensitive to subsequent microwave pulses. The swapping of the proton spins can then be observed via the current. This provides an intriguing new approach to detect nuclear spin manipulation electrically. The study of Malissa et al. is an important step toward the use of organic materials in quantum-coherent spin manipulation. A great advantage as compared to other spin manipulation approaches is the operation at room temperature. Nature may have paved the way here for us in the development of a quantum-biological compass, obviously operating at ambient temperature. However, the disadvantages at present are the lack of control over the involved electronic processes, which are mostly due to the morphological disorder of the used conjugated polymer. Coherence times are presently limited by the uncontrolled lifetime of the spin pairs. Increasing and controlling morphological order by embedding a semiconducting organic molecule in the pores of a zeolite crystal have led to a spectacular increase in organic magnetoresistance and the option to address only a few molecules (6). This may also be the way to bring spin manipulation in organic materials to a next level of control. ■ REFERENCES
1. 2. 3. 4.
H. Malissa et al., Science 345, 1487 (2014). T. Ritz et al., Biophys. J. 96, 3451 (2009). S. Engels et al., Nature 509, 353 (2014). T. L. Francis, Ö. Mermer, G. Veeraraghavan, M. Wohlgenannt, New J. Phys. 6, 185 (2004). 5. M. Wohlgenannt, P. A. Bobbert, B. Koopmans, MRS Bull. 39, 590 (2014). 6. R. N. Mahato et al., Science 341, 257 (2013). 10.1126/science.1259655
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
1
2
H 3
He 4
Li
Be
11
12
5
B 13
Na Mg 19
20
21
K
Ca
Sc
37
38
39
Rb Sr 55
Cs 87
Fr
18
Y
56
22
Ti 40
Zr 72
Ba
Hf
88
104
Ra
Rf
57
La 89
Ac
58
Ce 90
Th
23
24
25
26
V
Cr Mn Fe
41
42
Nb Mo 73
74
Ta
W
105
106
Db Sg 59
Pr
60
43
Tc 75
44
27
Co 45
Ru
Rh
Re
Os
76
77
107
108
109
61
62
63
93
94
95
Ir
Bh Hs Mt
Nd Pm Sm Eu
91
Pa
92
U
28
Ni 46
Pd 78
Pt 110
Ds 64
29
Cu 47
30
6
48
Cd
O
14
15
16
Al
Si
31
32
79
80
49
In 81
50
Sn 82
Au Hg
Tl
Pb
111
112
113
114
66
67
Rg 65
Gd
Tb
96
97
Np Pu Am Cm Bk
8
N
Zn Ga Ge
Ag
7
C
Cn Uut Fl
33
As 51
Sb 83
S
34
17
Cl 35
Se
Br
52
53
Te 84
I
85
18
Ar 36
Kr 54
Xe 86
Po
At
Rn
115
116
117
118
Uup Lv Uus Uuo
Er
Tm Yb
69
70
Lu
98
100
101
102
103
99
10
Ne
Bi
Dy Ho Cf
68
P
9
F
Es Fm Md No
71
Lr
The modern Periodic Table. Similar chemical behavior might be expected for the homologs (Mo, W, Sg), (Hg, Cn) and (Pb, Fl), but relativistic effects can cause deviations from the expected behavior.
CHEMISTRY
Superheavy carbonyls The radioactive superheavy element seaborgium can form a carbonyl compound during its short lifetime By Walter Loveland
E
lements with atomic numbers Z > 94 are radioactive and cannot be found naturally in Earth. Efforts to create these elements, particularly the superheavy elements (Z ≥ 104), in the laboratory typically use “hot fusion” reactions involving extremely high temperatures. On page 1491 of this issue, Even et al. (1) show how these atoms can be cooled down and used to synthesize a new class of chemical compounds, the superheavy metal carbonyls. The results confirm a 15-yearold prediction of relativistic quantum chemistry. The known superheavy elements, referred to as the transactinides, range in atomic number from 104 (Rf ) to the as yet unnamed element 118 (see the first figure). The lifetimes of many of these elements range from milliseconds to minutes. In the case of Z = 112 (Cn) to Z = 118, current methods only allow the production of a few atoms per week. Chemistry must be done on a “one atom at a time” basis, making these studies very difficult. Relativistic effects shape the chemistry of these elements. The speeds of the innermost electrons in these atoms approach the speed of light, requiring the use of relativistic quantum mechanics to describe the atoms and molecules. The relativistic effects consist of a contraction and stabilization of the innermost s and p orbitals, the splitting
SCIENCE sciencemag.org
of electron energy levels due to spin-orbit coupling, and an expansion (and destabilization) of the outer d orbital and all f orbitals. In the transactinides, stabilization of the 7s orbital of Cn doubles its binding energy. In Sg, the 7s and 6d orbital ordering is inverted, possibly changing its oxidation properties. The spin-orbit splitting, like the other relativistic properties, increases roughly as Z 2, approaching the energies of chemical bonds in magnitude. The periodic table of the elements is not a list of the elements ordered by increasing atomic number, but rather a grouping of the elements by chemical properties. It is thus a living document. The placement of new elements such as the transactinides in the table depends on measurements of their chemical properties. Experimental studies of the chemistry of the transactinides (2) support the placement of the elements Rf (104), Db (105), Sg (106), Bh (107), Hs (108), and Cn (112) into groups 4, 5, 6, 7, 8, and 12 of the periodic table, respectively, forming the fourth row of transition metals; however, the real possibility of deviations exists. For example, element 112 (Cn) is nominally a homolog of Hg; element 114 (Fl) is a homolog of Pb; thermochromatography experiments have shown that Cn behaves like Hg (3); but it has been difficult to determine whether Fl is a noble gas or a noble Oregon State University, Corvallis, OR 97331, USA. E-mail: [email protected] 19 SEP TEMBER 2014 • VOL 345 ISSUE 6203
Published by AAAS
1451
INSIGHTS | P E R S P E C T I V E S
Rotating target wheel
Recoil separator Q Mo/W/Sg trajectories
Q
Beam Beam/background trajectories
D
N2 (I) cryostat
Gas chromatography column RTC
Structures of a multisubunit protein-RNA complex reveal how the CRISPR system recognizes DNA targets By Yan Zhang and Erik J. Sontheimer
Tefon capillary 2⫻ 32 SiO2-covered detectors for α and β particles and fssion fragments Making Sg carbonyl. In Even et al.’s experiment, hot Sg atoms are produced in a nuclear reaction, transported through the GARIS magnetic separator, and slowed down in the recoil transfer chamber (RTC), where they react with CO. The resulting Sg(CO)6 products are transported to the thermochromatography apparatus for analysis.
metal (4, 5). Gas-phase chemical studies have been important tools for studying the chemical behavior of short-lived elements. The use of physical recoil separators that isolate the nuclear reaction products has allowed the synthesis of compounds, such as the metal carbonyls. The metal carbonyls of the group 6 elements—Cr(CO)6, Mo(CO)6, and W(CO)6—are well known. U(CO)6 is unstable above 30 K, in accord with predictions of relativistic quantum chemistry (6). However, Sg(CO)6 is predicted (6) to be stable because of relativistic effects that lead to stronger molecular bonding. Actinide carbonyls have been difficult both to synthesize and characterize. Laser ablation experiments produce U(CO)6 and Th(CO)6 only as transient species. The synthesis of transactinide carbonyls looked to be forbidding because of the harsh synthesis conditions required to create these elements. Even et al. use a novel separation of the recoiling reaction products to overcome these problems. Having synthesized a Sg carbonyl complex, they used thermochromatography to characterize it. The authors carried out the experiment at the GARIS recoil separator at the RIKEN accelerator facility in Japan. They first generated Mo, W, and Sg nuclei in nuclear reactions at the target portion of the separator (see the second figure). The recoiling nuclei were then separated from the incident projectile beam by the magnetic elements of the GARIS separator and entered a recoil transfer chamber. In that chamber, the Mo, W, and Sg nuclei were thermalized in a He/ CO mixture and reacted with the CO molecules to form carbonyls. These compounds were transported to the gas chromatography 1452
Cascading into focus
column, where a temperature gradient from 25°C to –120°C was maintained. The column consisted of 32 pairs of Si semiconductor detectors covered with SiO2 that recorded the decay of the radioactive nuclei. Mo and W formed volatile carbonyls that adsorbed on the column in temperature regions characteristic of their known enthalpies of adsorption. A volatile Sg species adsorbed on the column at a similar position. Monte Carlo simulations of the adsorption process allowed the scientists to deduce an adsorption enthalpy of 50 ± 4 kJ/mol for the Sg species, in good agreement with the theoretical predictions (7) and similar to the measured value for W(CO)6. Even et al. conclude that they have formed a volatile Sg carbonyl complex, probably Sg(CO)6. This represents the first synthesis of a new class of superheavy compounds— a finding that could lead to the synthesis of carbonyl complexes of elements 104 to 109. The implications of this development are widespread. For example, the chemistry of element 109, Mt, has not been studied. Using carbonyl complexes, it should now be possible to study Mt chemistry. Thermal dissociation experiments with the carbonyls should enable studies of the strengths of metal-carbon bonds in all the superheavy elements. ■ REFERENCES
1. 2. 3. 4. 5. 6.
J. Even et al., Science 345, 1491 (2014). M. Schädel, Radiochim. Acta 100, 579 (2012). R. Eichler et al., Nature 447, 72 (2007). R. Eichler et al., Radiochim. Acta 98, 133 (2010). A. Yakushev et al., Inorg. Chem. 53, 1624 (2014). C. S. Nash, B. E. Bursten, J. Am. Chem. Soc. 121, 10830 (1999). 7. V. Pershina, J. Anton, J. Chem. Phys. 138, 174301 (2013). 10.1126/science.1259349
A
n adaptive immune pathway in bacteria and archaea is specified by clustered, regularly interspaced, short palindromic repeat (CRISPR) sequences (1). Research into CRISPR RNAs (crRNAs) and the CRISPRassociated (Cas) proteins have revealed the ability of most of these systems to target foreign DNA molecules for destruction. On pages 1473 and 1479 of this issue, Jackson et al. (2) and Mulepati et al. (3), respectively, and Zhao et al. (4) describe high-resolution structures of the multiprotein assembly called CRISPR-associated complex for antiviral defense (“Cascade”), which drives CRISPR interference in many strains of Escherichia coli. The structures show how Cascade presents crRNA to its DNA target, and demonstrate that DNA recognition occurs through a configuration that, surprisingly, is not double-helical. Diverse flavors of CRISPR systems have been organized into three families (types I, II, and III) with additional subdivisions (5). All use crRNA precursors (pre-crRNAs) with multiple repeat sequences that are separated by “spacers.” These spacers match “protospacer” sequences that are usually present within phage genomes or plasmids. Pre-crRNAs are processed into individual crRNAs, each with a single spacer that guides the effector machinery to its DNA target. Type II systems require only a single protein, Cas9, for crRNA-guided DNA targeting. By contrast, crRNAs in type I and type III systems assemble into large multiprotein complexes. Cascade was first defined for the type I-E system from E. coli strain K12 (6) and includes one to six copies each of five distinct Cas proteins. Many of E. coli Cascade’s important interactions, mechanistic features (6–12), and structural properties (7–9, 12) have been described, including its seahorse-like shape. Double-stranded DNA (dsDNA) recognition by Cascade requires not only crRNA-DNA complementarity, but also a 3–base pair sciencemag.org SCIENCE
19 SEP TEMBER 2014 • VOL 345 ISSUE 6203
Published by AAAS
ILLUSTRATION: ADAPTED BY P. HUEY/SCIENCE
Mylar window
D
STRUCTURAL BIOLOGY
