In This Issue
APPLIED PHYSICAL SCIENCES
Nanoporous hydrogen storage
Metal–organic frameworks (MOFs) are nanoporous materials researchers are investigating as storage media for hydrogen fuel. MOF-5 comprises 1,4-benzenedicarboxylate linkers joining Zn4O clusters in a cubic system. Previous work has shown that MOF-5 can store 1.3 wt% of hydrogen at 78 K and ≈1 atm. Andreas Blomqvist et al. report calculations showing that the adsorption of lithium atoms on the organic linkers would greatly increase the amount of hydrogen that MOF-5 can store. The authors carriedout ab initio computations to optimize the MOF structure before adding lithium atoms to the center of the linker carbon rings and reoptimizing. The carbon rings stretched, indicating that charge transfer had taken place. Each lithium atom had donated 0.9 electron and was therefore positively charged. The lithium-decorated MOF-5 was capable of holding one, two, or three hydrogen molecules per lithium atom, with binding energies in the range of 12–18 kJ(mol H2)−1. Molecular dynamics simulations show that each lithium atom holds two hydrogen atoms up to 200 K, for a storage of 2.9 wt%; this decreases to 2.0 wt% at 300 K. This is the highest capacity yet reported for MOF-5, although the authors believe it would increase under applied pressure. — K.M.
Structure of the hydrogen-storing metal-organic framework MOF-5.
“Li-decorated metal–organic framework 5: A route to achieving a suitable hydrogen storage medium” by A. Blomqvist, C. Moysés Araújo, P. Srepusharawoot, and R. Ahuja (see pages 20173–20176)
BIOCHEMISTRY
Environmental pollutant concentrates in breast milk
Perchlorate (ClO4 −) inhibits the Na+/I− symporter (NIS), which uses the sodium gradient to transport iodide into the thyroid and other tissues. Although environmental levels of perchlorate are generally low, recent results have shown that they may affect thyroid function. Perchlorate has been thought to merely block iodide transport. However, Orsolya Dohán et al. now find that NIS actively transports perchlorate, which accumulates in the breast milk of nursing rats. The authors say it is possible that nursing human mothers exposed to perchlorate not only may be supplying less iodide to their infants, but also may be passing on perchlorate via milk. This could pose a risk of hypothyroidism at a crucial developmental stage. The authors injected rats with perchlorate and extracted breast milk, which inhibited iodide transport in NIS-expressing cells. Showing that perchlorate was actively transported by NIS was difficult because 36Cl is not sufficiently radioactive for direct observation of ClO4 − accumulation. The authors studied the kinetics of 186ReO4 −, similar in size to ClO4 −, and found that only one 186ReO4 − (or one ClO4 −) is transported per Na+, the first finding that a symporter may transport different substrates with different stoichiometries. — K.M.
Rats treated with perchlorate (Left) absorb less iodide in the thyroid (T) than controls (Right).
“The Na+/I− symporter (NIS) mediates electroneutral active transport of the environmental pollutant perchlorate” by Orsolya Dohán, Carla Portulano, Cécile Basquin, Andrea Reyna-Neyra, L. Mario Amzel, and Nancy Carrasco (see pages 20250–20255)
CELL BIOLOGY
Watching the clock
Circadian rhythms cycle over a 24-hour period to help keep an organism in sync with its environment. Light cues affect the rhythms by stimulating gene transcription, but the response can vary greatly depending on the timing of the light exposure. Sandhya Pulivarthy et al. used real-time monitoring of oscillation in mammalian cells to study the relationship of gene transcription levels, or amplitude, to the timing of their appearance, or phase. After attaching a bioluminescent reporter to the Per2 gene and introducing the melanopsin receptor to confer light responsiveness, the researchers measured the activity of this clock gene in response to light stimulation. They found that Per2's activity differed depending on the timing of the light pulse. During the day portion of the cycle, when Per2 is most active, light increased the amplitude of the rhythm, but had little effect on phase. During the night portion, however, the phase shifted but amplitude decreased. The authors say that their results demonstrate a reciprocal relationship between phase change and amplitude that may serve to sensitize organisms to environmental cues. — T.H.D.
Phase plots of circadian time (CT) and the time of the light pulse.
“Reciprocity between phase shifts and amplitude changes in the mammalian circadian clock” by Sandhya R. Pulivarthy, Nobushige Tanaka, David K. Welsh, Luciano De Haro, Inder M. Verma, and Satchidananda Panda (see pages 20356–20361)
ENVIRONMENTAL SCIENCES
Bio-nanotubes from arsenic
Certain species of bacteria are able to respire heavy metals, including Shewanella, which has attracted attention as a possible biological agent for nuclear waste cleanup. Ji-Hoon Lee et al. find that Shewanella sp. strain HN-41 alters the oxidation state of arsenic and also precipitates it, with sulfur, into nanotubes. The authors found that although HN-41 does not directly respire arsenic(V), it reduces arsenate(V) to arsenite(III). HN-41 acquires energy by oxidizing lactate in the presence of thiosulfate ions. One byproduct of this process is hydrogen sulfide, which the bacteria detoxifies via an unknown mechanism by combining it with arsenite (III) to produce insoluble arsenic–sulfide compounds. These are deposited as a yellow precipitate, which microscopic examination reveals to be a tangle of nanotubes, each with a diameter of 20–100 nm. The authors measured the electrical properties of the nanotubes and found that although initially nonconductive, after 8 days the nanotubes behaved as if metallic, which the authors attributed to the increasing external density of arsenic. The nanotubes also exhibit a semiconductor-like optical bandgap and are photoconductive, possibly making the nanotubes suitable for use in optoelectronic devices, according to the authors. — K.M.
Arsenic–sulfur nanotubes produced by Shewanella strain HN-41.
“Biogenic formation of photoactive arsenic-sulfide nanotubes by Shewanella sp. strain HN-41” by Ji-Hoon Lee, Min-Gyu Kim, Bongyoung Yoo, Nosang V. Myung, Jongsun Maeng, Takhee Lee, Alice C. Dohnalkova, James K. Fredrickson, Michael J. Sadowsky, and Hor-Gil Hur (see pages 20410–20415)
IMMUNOLOGY
Myofibroblasts evade immune surveillance in tissue fibrosis
Healing after an injury is primarily regulated by fibroblasts, via the synthesis or organization of matrix components. After the damage is repaired, a controlled healing response involves killing of the fibroblasts, which is essential for minimal scarring. T cells facilitate removal of myofibroblasts by triggering Fas ligand-induced apoptosis. In patients with pulmonary fibrosis, not only are the alveoli structurally abnormal, but myofibroblasts and collagen accumulate in the extracellular matrix, causing lung scarring that then inhibits gas exchange. Shulamit Wallach-Dayan et al. examined the relationship between T cells and lung myofibroblasts in vivo and in vitro and showed that, in conditions like lung fibrosis, myofibroblasts enjoy an immune-privileged status that enabled them to avoid immune surveillance. The authors showed that myofibroblasts from mice with bleomycin-induced lung fibrosis resisted Fas-induced apoptosis, killed a variety of T cells, and escaped allogeneic immune surveillance. Wallach-Dayan et al. suggest that this immune-privileged status may lead to fibrosis in other organs. — B.T.
Cellular profile of fluid recovered in allogeneic mice.
“Evasion of myofibroblasts from immune surveillance: A mechanism for tissue fibrosis” by Shulamit B. Wallach-Dayan, Regina Golan-Gerstl, and Raphael Breuer (see pages 20460–20465)
