Chemistry Top Ten: The Secret Site Where Water Gives Up Its Oxygen

October 2012
Reuters/Darren Hauck

The current Hot Ten has most papers about graphene (# 1, #7, and #8), or polymer solar cells (#2, #4, #5, #6, and #10). The remaining two are #3, which is about metal-organic frameworks, and #9, which reveals the molecular structure at the heart of the photosynthesis that sustains life on Earth.

What’s Hot in Chemistry
Rank Paper (Mar-Apr 12) Rank Last Period
(Jan-Feb 12)
1 S. Bae, et al., "Roll-to-roll production of 30-inch graphene films for transparent electrodes," Nature Nanotech., 5(8): 574-8, August 2010.  [8 South Korean, Singaporean, and Japanese institutions]  96 2
2 Y.Y. Liang, et al., "For the bright future—Bulk heterojunction polymer solar cells with power conversion efficiency of 7.4%," Adv. Materials, 22(20): E135-8, 25 May 2010.  [U. Chicago, IL; Solarmer Energy Inc., El Monte, CA]   74 1
3 H. Furukawa, et al., “Ultrahigh porosity in metal-organic frameworks,” Science, 329(5990): 424-8, 23 July 2010. [Soongsil U., Seoul, Korea; U. Calif., Los Angeles; Northwestern U., Evanston, IL] 58 4
4 Z. He, et al., Simultaneous enhancement of open-circuit voltage, short-circuit current density, and fill factor in polymer solar cells,” Adv. Materials, 23(40): 4636, 25 October 2011. [South China U. Sci., Quangzhou; Chinese Acad. Sci., Suzhou; Hong Kong Baptist U.] 43 +
5 T.Y. Chu, et al., “Bulk heterojunction solar cells using thieno[3,4-c]pyrrole-4,6-dione and diethieno[3,2-b:2’,3’-d]silole copolymer with a power conversion efficiency of 7.3%,” J. Am. Chem. Soc., 133(12): 4250-3, 30 March 2011.  [Natl. Res. Council of Canada, Ottawa, Ontario; U. Laval, Quebec, Canada] 39 +
6 S.C. Price, et al., “Fluorine substituted conjugated polymer of medium band gap yields 7% efficiency in polymer-fullerene solar cells,” J. Am. Chem. Soc., 113(12): 4625-31, 30 March 2011. [U. North Carolina, Chapel Hill]    39 5
7 S. Chen, et al., “Graphene oxide—MnO2 nanocomposites for supercapacitors,” ACS Nano, 4(5): 2822-30, May 2010. [Nanjing U. Sci. Tech., China]  37 +
8 J.M. Cai, et al., “Atomically precise bottom-up fabrication of graphene nanoribbons,” Nature, 466(7305): 470-3, 22 July 2010. [6 European institutions]  36 +
9 Y. Umena, et al., “Crystal structure of oxygen-evolving photosystem II at a resolution of 1.9 Å,” Nature, 473(7345): 55-60, 5 May 2011. [Osaka City U., Japan; Okayama U., Japan] 36 3
10 F.C. Krebs, T. Tromholt, M. Jorgensen, "Upscaling of polymer solar cell fabrication using full roll-to-roll processing," Nanoscale, 2(6): 873-86, June 2010. [Tech. U. Denmark, Roskilde]  34 +
SOURCE: Thomson Reuters Web of Science
NB. Only papers indexed by Thomson Reuters since May 2010 are tracked. The “+” sign indicates that the paper was not ranked in the Top Ten during the last period. In the event that two or more papers collected the same number of citations in the most recent bimonthly period, total citations to date determine the rankings

Lightening Our Darkness

Those who speculate on the way Nature transforms light, water, and carbon dioxide into carbohydrate and oxygen gas can now do so with more confidence, thanks to paper #9.

The protein complex known as Photosystem II is where this process happens and this consists of 20 subunits with a molecular mass of 350,000. Previous analysis by X-ray diffraction has provided useful information but it was not done at wavelengths capable of determining exactly how the atoms at the catalytic center were arranged. Without that knowledge, theories as to how these enzymes work lacked detail.

Those who speculate on the way Nature transforms light, water, and carbon dioxide into carbohydrate and oxygen gas can now do so with more confidence, thanks to paper #9.

Now, a group of Japanese chemists, headed by Yasufumi Umena of Osaka City University and Keisuke Kawakami of Okayama University, have examined the target at a resolution of 1.9 Å and located the atoms involved. The key part consists of a cluster of four manganese, one calcium, and five oxygen atoms, together with four water molecules. The oxygen atoms serve as bridges between the metal atoms.

In addition, Umena and Kawakami were able to locate 1,300 other water molecules in photosystem II, and these form a network linked by hydrogen bonds which might well act as a way of channeling protons and oxygen atoms within the enzyme.

The crystals studied came from the thermophilic cyanobacteria Thermosynechococcus vulcanus and, in addition to mapping the active site, they also located 35 chlorophylls, 20 lipids, 11 β-carotenes, and three iron atoms, two of which were part of haemes. There were also three other calcium ions and three chlorine ions.

The atoms at the active center form a distorted cube whose corners consist of three manganese, one calcium, and four oxygen atoms. The bond lengths of the sides of this cubane arrangement are of the order of 2.4–2.5 Å when they involved calcium and 1.8–2.1 Å when they include manganese. Looked at from a different angle, it can be seen that the calcium is linked to all four manganese atoms by oxo bridges in a kind of distorted chair arrangement which the authors of the paper suggest may be the key to the water-splitting reaction.

And what are the chlorine atoms doing? Again the identification of their exact location is new information in #9. These appear to be gatekeepers to the hydrogen-bonding network which extends from the manganese/calcium cluster outwards.

Explaining how photosynthesis strips water molecules of their hydrogens to form O2 is a step nearer, thanks to #9. Clearly there is more to this process than simply the action of the manganese atoms and no doubt the multitude of amino-acid side-chains play a key role.

A Hole Lot of Nothing

Paper #3 is a collaborative effort from groups headed by Jaheon Kim of Soongsil University at Seoul Korea and Omar Yaghi at UCLA. It reports new crystalline metal-organic frameworks (aka MOFs), which have the biggest known holes in their lattices. These are capable of holding large numbers of gaseous molecules like CO2 and possible be a depository for storing them. Alternatively, they might act as catalysts.

The MOF framework is based on zinc carbonate clusters [Zn4(CO3)6] linked by extended organic ligands. Ideally these linker molecules should be as thin as possible although this risks the crystals forming an intertwined lattice, thereby defeating the object of creating materials with large cavities.

What Kim and Yaghi have done is extend two MOFs previously reported, which had linkers of BTE (4,4’,4”-benzene-1,3,5-triyl-tri(ethyne-2,1-diyl)tribenzoate) or BBC (4,4’,4”-benzene-1,3,5-triyl-tris(benzene-4,1-diyl)tribenzoate) both of which gave structures that were very stable.

In #3, these ligands have been extended with either NDC (2.6-naphthalenedicarboxylate) to give MOF-205, or with BPDC (biphenyl-4,4’-dicarboxylate) to give MOF-210, and it is the latter which appears to have reached the upper limit of empty space within its structure, this being 89% of the crystal volume. (Thankfully the new MOFs did not have interpenetrated structures.)

MOF-210’s crystal density was a mere 0.25 g cm-3, and its voids can store almost three times its weight of CO2. Moreover, it has an internal surface area of 2,060 square meters per cubic centimeter, which puts it in the same category as the outer surface of a nanoparticle, suggesting that like those materials it might also have a future as a catalyst.

Arise, Sir Graphene, and Go!

Graphene came to the attention of the wider world when Konstantin Novoselov and Andre K. Geim of the University of Manchester, UK, won the 2011 Nobel Prize for Physics. Following that honor, they were knighted by the Queen, and a grateful UK Government announced a £50 million award to fund a National Graphene Institute.

While graphene does some remarkable things, graphene oxide (GO) is equally remarkable. Seal a container with a membrane of GO and not even helium atoms, or any other gaseous atoms, can escape. Yet water molecules slip through its gaps easily.

Researchers at Manchester realized that this conundrum could be put to use in making alcoholic drinks much stronger. They sealed a bottle of vodka with a graphene oxide membrane and, lo and behold, the vodka got stronger and stronger. Of course, such frivolous research could only be reported in Manchester University’s newsletter.

More academic themes occupy the graphene papers in the current Hot Ten, notably #7, which is on GO-manganese dioxide nanocomposites that have the potential to act as a supercapacitor, and #8, which is on graphene ribbons.

The former is new to the Hot Ten list and its lead authors are Junwu Zhu and Xin Wang of Nanjing University of Science and Technology in China. They report on the combination of graphene oxide with manganese dioxide (GO-MnO2 ) which they synthesized from a water-isopropyl alcohol system in which the graphene came from powdered flake graphite and the MnO2 came from manganese(II) chloride. The mixture was exposed to ultrasonification at 83 oC for 30 minutes, and this resulted in the desired material which, after centrifuging, washing, and drying, was imaged by transmission electron microscopy and characterized by X-ray diffraction. Zhu and Wang speculate that the GO-MnO2 has potential applications in catalysts, electrodes, and other electronic devices.

Dr. John Emsley is based at the Department of Chemistry, Cambridge University, U.K.

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