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Solar-powered ‘nanoalloys’
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can convert polluting nitrates into ammonia fertilizer without releasing
carbon dioxide
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Nourishing crops with synthetic ammonia (NH3) fertilizers has increasingly pushed agricultural
yields higher, but such productivity comes at a price. Over-application of this chemical can build
up nitrate ion (NO3–) concentrations in the soil—a potential groundwater poison and food source
for harmful algal blooms. Furthermore, industrial manufacturing of ammonia is an energy-intensive
process that contributes significantly to atmospheric greenhouse gases.
A research team led by Miho Yamauchi and Masaki Takata from the RIKEN SPring-8 Center in
Harima has now discovered an almost ideal way to detoxify the effects of ammonia fertilizers1.
By synthesizing photoactive bimetallic nanocatalysts that generate hydrogen gas from water
using solar energy, the team can catalytically convert NO3– back into NH3 through an efficient
route free from carbon dioxide emissions.
Replacing the oxygen atoms of NO3– with hydrogen is a difficult chemical trick, but chemists
can achieve this feat by using nanoparticles of copper–palladium (CuPd) alloys to immobilize
nitrates at their surfaces and catalyzing a reduction reaction with dissolved hydrogen atoms.
However, the atomic distribution at the ‘nanoalloy’ surface affects the outcome of this procedure:
regions with large domains of Pd atoms tend to create nitrogen gas, while well-mixed alloys
preferentially produce ammonia.
According to Yamauchi, the challenge in synthesizing homogenously mixed CuPd alloys is getting
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the timing right—the two metal ions transform into atomic states at different rates, causing phase
separation. Yamauchi and her team used the powerful x-rays of the SPring-8 Center’s synchrotron
to characterize the atomic structure of CuPd synthesized with harsh or mild reagents. Their
experiments revealed that a relatively strong reducing reagent called sodium borohydride gave
alloys with near-perfect mixing down to nanoscale dimensions.
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Most ammonia syntheses use hydrogen gas produced from fossil fuels, but the use of solar energy
by the researchers avoids this. They found that depositing the nanoalloy onto photosensitive
titanium dioxide (TiO2) yielded a material able to convert ultraviolet radiation into energetic
electrons; in turn, these electrons stimulated hydrogen gas generation from a simple
water/methanol solution . When they added nitrate ions to this mixture, the CuPd/TiO2
catalyst converted nearly 80% into ammonia—a remarkable chemical selectivity that the
researchers attribute to high concentrations of reactive hydrogen photocatalytically produced
near the CuPd surface.
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Yamauchi is confident that this approach can help reduce the ecological impact of many
classical chemical hydrogenation reactions. “Considering the environmental problems we face,
we have to switch from chemical synthesis using fossil-based hydrogen to other clean processes,”
she says.
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Encoding unchartered territory
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Ensembles of neurons in the brain’s hippocampus inform about future as well
as past experiences
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When a mammal explores an unfamiliar environment, ensembles of ‘place’ cells in
the hippocampus fire individually, recording specific locations in a cognitive map that
aid future spatial navigation of the area. Once the relationship between place cell
activity and location has been established, the activity of the cells can be used to
predict the animal’s location within its environment. Activity patterns in the ensembles
are later ‘replayed’ during rest and sleep, and neuroscientists believe this is important
for consolidating the spatial memories of the new environment.
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Neuroscientists also contend that the sequence of place cell firing corresponding
to the new environment is established during the first exploration of that environment.
Now George Dragoi and Susumu Tonegawa from the RIKEN-MIT Center for Neural
Circuit Genetics at the Massachusetts Institute of Technology in Cambridge,
Massachusetts, report that the activity of place cell circuits is also preconfigured to
encode novel environments1.
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Dragoi and Tonegawa recorded the activity patterns of place cells in the CA1
region of the hippocampus while mice navigated a familiar environment. They
also recorded from the same cells afterwards, while the mice rested or slept. As
expected, some of the place cell activity patterns they observed corresponded to
the familiar environment that the animals had explored, but they also recorded
new patterns from place cells that were previously silent.
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The researchers found that the novel activity patterns corresponded strongly to
the sequences of place cell firing that were recorded when the mice subsequently
explored an unfamiliar part of the environment. This suggests that the
activity patterns represent ‘preplay’ of the unexplored locations rather than replay
of the familiar part of the environment. Thus, the activity of hippocampal place cells
appears not only to consolidate spatial memories of newly experienced environments,
but also to predict how novel, unexplored environments can be encoded when they
are navigated in the future. The researchers also suggest that hippocampal preplay
may accelerate spatial memory formation once the novel environment is eventually
explored.
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Encoding of new information makes use of the pre-existing organization of the
hippocampal network, and will stabilize faster compared to a case when the neuronal
network has to re-organize to a new state that does not resemble the pre-existing one,”
says Dragoi. “In an immediate follow-up to this study, we will address the role of the
intact hippocampal circuitry in the mechanisms and dynamics of the preplay phenomenon,”
he adds.
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Keeping to time counter-intuitively
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.Experimental work proves the theory that a circadian body clock requires a delay
to function properly
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For more than 20 years, theoretical mathematical models have predicted that a delay built into
a negative feedback system is at the heart of the molecular mechanism that governs circadian
clocks in mammalian cells. Now, the first experimental proof of this theory has been provided
by an international research team led by molecular biologists and information scientists from
the RIKEN Center for Developmental Biology in Kobe1. The demonstration of the feedback
delay should lead to a better understanding of how cellular clocks function, and therefore how
mammals adjust to the regular daily and seasonal changes in their environment. The work
could also open the way to the development of treatments for circadian disorders, such as
seasonal affective disorder, jet lag and even bipolar disorder.
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Mammals not only show daily rhythms of waking and sleeping, but also body temperature,
hormone secretion, and many other biological activities. The master cellular clocks that act
as timers for these patterns are found in the suprachiasmatic nucleus of the brain. The molecular
which code for proteins that repress their own activation by binding with the products of two other
genes Bmal1 and Clock. The whole clock system is orchestrated by the interaction of these
proteins with a complex array of promoters and enhancers, genetic sequences that regulate
gene activity.
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Within these clock-gene regulators are short sequences often known as clock-controlled
elements. Different clock-controlled elements bind with the different proteins likely to be
prevalent at different times of the day or night. The researchers carefully modified these
sequences, and observed the impact on circadian rhythms of cells. They focused their
studies in particular on the gene Cry1, and observed how the rhythm of its activity was
affected by the modifications of clock-controlled elements within promoters and enhancers.
In addition to revealing a previously unknown clock-controlled element in the Cry1 promoter,
the researchers also found that different combinations of clock-controlled elements led to
different lengths of delay in the activation of Cry1. They demonstrated that this delay of Cry1
was required for the circadian clock to function.
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Based on these findings, they proposed a simple model of the mammalian circadian clock
and now want to construct it using artificial components. “We think further experimental and
theoretical analyses of this minimal circuit will lead to a deeper understanding of the mammalian
circadian clock,” say team members Rikuhiro Yamada and Maki Ukai-Tadenuma.
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