Why have Murillo's skies turned grey?

 

Smalt was one of the blue pigments the most commonly used by the artists between the 16th and 18th centuries. Unfortunately, this pigment is unstable and tends to fade with time. Researchers from the new European platform for research on ancient materials, the SOLEIL synchrotron, the National Gallery, London and the C2RMF found the key of this fading, described for four centuries. These results, obtained through the synchrotron analysis of microsamples of paint from works by Baroque painter Murillo and other artists, have been published in the journal Analytical Chemistry.

Smalt is a pigment that was widely used by artists between the sixteenth and eighteenth centuries, among which were the painters Veronese and Murillo. To produce this pigment, a mixture of cobalt ore, silica (e.g. sand) and potash was fired to form a deep blue glass, which was then ground to a powder. The intensity of the blue colour depended on the fineness of the pigment particles and the cobalt content.

This pigment tends to lose its colour with time, resulting in drastic changes in the appearance of art works – a blue sky turned grey can completely distort the interpretation of a painting. By the end of the eighteenth century smalt was less commonly used, perhaps because other more stable artificial blue pigments had become available. To explain this discoloration phenomenon, described since the seventeenth century, several hypotheses have been advanced, but the exact physicochemical origin of this colour change has until now remained uncertain.

An original analytical approach to this question was developed by scientists at the CNRS, the SOLEIL synchrotron, the National Gallery and the C2RMF under the auspices of IPANEMA, the European research platform for ancient materials. This pigment discoloration is due to a change in the environment of the cobalt ions, which are responsible for the colour. These new results show that there is a direct link between the migration of potassium ions out of the pigment particles, a common process in glass alteration, and this change in coordination of the cobalt ion resulting in loss of the blue colour. 

These results were obtained by analysis of microsamples from works in the National Gallery and the Louvre by X-ray absorption spectroscopy on the LUCIA beamline at SOLEIL synchrotron. The unique combination of the micron-sized X-ray beam delivered by LUCIA and its broad energy range has been crucial in allowing individual smalt particles to be probed in the paint samples and as a result putting an end to an old mystery.

More information: “Investigation of the discoloration of smalt pigment in historic paintings by micro X-ray absorption spectroscopy at the Co K-edge”, Laurianne Robinet, et al., Analytical Chemistry. (2011)

Provided by CNRS (news : web)

Nanoparticles help scientists harvest light with solar fuels

 The humble alga, hated by boaters and pool owners, may someday help provide us with the raw machinery to power our appliances.

Utschig and Tiede are part of Argonne's Photosynthesis Group, which has worked for fifty years to understand photosynthesis—one of the most mysterious and wonderful chemical processes in the world. Photosynthesis built a green Earth out of the bare, meteor-blistered planet which had sat empty for a billion years; it tipped the composition of the atmosphere towards oxygen, allowing all kinds of life to blossom, including us.

The chemistry group is part of a larger effort to develop efficient ways to produce what are termed solar fuels. Most people think of solar panels when they think of solar energy, but the energy that solar panels generate has to be used right away—they directly create electricity, which can't be stored easily.

The alternative is solar fuels, which pull energy from the sun to create fuel that can be stored for later, such as hydrogen. Hydrogen, a promising fuel in the effort to reduce carbon dioxide emissions, is appealingly clean: when it's burned as fuel, water is the byproduct. But we have yet to discover a low-cost way to manufacture large amounts of hydrogen.

"Basically, we've been reverse-engineering photosynthesis," said Argonne chemist David Tiede, who co-authored the paper. "If we understand how Nature does it, we can tweak the process to produce hydrogen."

Most solar fuel efforts focus on a type of protein complex called Photosystem I, or PSI, which is the first half of the photosynthetic duo found in all green plants.

When light strikes the PSI complex, it momentarily knocks an electron into an "excited" state. The goal is to separate this electron from its home atom—leaving behind a "hole" of positive charge—and channel it to an artificial catalyst to make hydrogen. But the electron only remains excited for the tiniest fraction of a second; the catalyst needs to grab it during this tiny window.

With co-author Nada Dimitrijevic, the team designed platinum nanoparticle catalysts. These catalysts have a size and surface chemistry that allows them to stick to PSI molecules at the point where the light-generated electrons accumulate. When the modified platinum nanoparticles and PSI are mixed in water, the two link together.

"The platinum nanoparticles have the same size and surface charge as the molecule that PSI would bind to naturally," Tiede said.

Because the study design used platinum as a catalyst, which is too expensive to be cost-effective, the research serves as proof-of-concept. Further studies hope to improve the method's efficiency, reliability and economics.

"The next step we'll take is experimenting with non-platinum catalysts," Utschig said. "Hopefully we can find a catalyst that can be made with a cheaper metal, which would make the process much more attractive on a large scale."

The paper, "Photocatalytic Hydrogen Production from Noncovalent Biohybrid Photosystem I/Pt Nanoparticle Complexes," was published in the Journal of Physical Chemistry Letters and is available online.

Provided by Argonne National Laboratory (news : web)

Resolving water's electrical properties

An old confusion about the electrical properties of water's surface has ended, thanks to scientists at Pacific Northwest and Lawrence Livermore National Laboratories. The conflict arose because two types of measurements gave two radically different interpretations of what was happening at the surface of water. The team showed, through careful analysis, that the measurements weren't wrong, but rather the behavior of water's electrons influenced one measurement more than the other. The team's results provided a consistent interpretation of the different measurements and grace The Journal of Physical Chemistry B cover.

"This could change how we think about water," said Dr. Gregory Schenter, a chemical physicist at PNNL who worked on the study.

While most people may not think of water as having electrical properties, when the behavior and movement of the electrons in this ubiquitous liquid comes into play in designing alternatives to today's fossil fuels, water is often part of the conversation. The electrical forces that exist in water, a simple V-shaped molecule made from two hydrogen atoms and an oxygen atom, are vital to understanding and controlling how molecules, ions, and other chemical components move and behave. For example, understanding water is necessary to convert agricultural waste into bio-fuels. Further, water's behavior impacts work on storing energy from solar cells, wind turbines, and other renewable sources, allowing more flexibility in designing energy strategies.

In chemistry, the simple approach that assigns a positive charge to a hydrogen atom and a negative charge to an oxygen atom can be very powerful. This simple model can work when it comes to understanding the forces that move molecules around in water. In other cases, it doesn't work.

When taking measurements with certain instruments, the simple model matches experimental results quite well. But, when using other techniques, the model differs wildly from what's measured. Theoretical chemists at PNNL and LLNL were the first to figure out what was happening.

They found that to measure electrical properties occurring at the molecular scale, where the length scales are measured in billionths of a meter, the models need to consider that the protons are in the nucleus and the electrons are everywhere else.

"It comes down to understanding where in the molecule you are making the measurements," said Dr. Shawn Kathmann, a chemical physicist at PNNL who worked on the study.

Complex descriptions of matter, referred to as ab initio electronic structure calculations, that focus on identifying electrons location and electron holography experiments showed that the conflict was caused by where you were measuring the surface potential in the molecules. If you determined the surface potential right next to the protons, you got one answer. If you determined the potential in the void between molecules, you get a different answer. And, finally, if you took measurements close to the electrons, you got still another answer.

"When you treat the electrons and protons appropriately, you get more accurate results," said Kathmann.

This research is part of ongoing work at PNNL to fundamentally understand the forces inside water and other molecules. The goal is to push past the existing knowledge frontiers regarding ions and interfaces. The team is working on developing models that more accurately and appropriately represent electrons. They are also striving to isolate the effects of electrons in driving matter at interfaces as well as the electrical stresses inside aqueous electrolytes.

More information: Kathmann SM, et al. 2011. "Understanding the Surface Potential of Water." The Journal of Physical Chemistry B 115, 4369-4377. DOI: 10.1021/jp1116036

Provided by Pacific Northwest National Laboratory (news : web)

Atomic-scale structures of ribosome could help improve antibiotics

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This "action shot" reveals the motion of the small ribosomal subunit, depicted by difference vectors, during ratcheting. Credit: Jamie Cate

(PhysOrg.com) -- It sounds like hype from a late-night infomercial: It can twist and bend without breaking! And wait, there's more: It could someday help you fend off disease!

But in this case it's true, thanks to scientists from several institutions including the U.S. Department of Energy's Lawrence Berkeley National Laboratory. They derived atomic-scale resolution structures of the cell's protein-making machine, the ribosome, at key stages of its job.

The structures, developed primarily at Berkeley Lab's Advanced Light Source, reveal that the ribosome's ability to rotate an incredible amount without falling apart is due to the never-before-seen springiness of molecular widgets that hold it together.

The structures also provide an atom-by-atom map of the ribosome when it's fully rotated during the final phase of protein synthesis. Many antibiotics target the ribosomes of disease-causing microbes at precisely this stage. The high-resolution structures could allow scientists to develop antibiotics that better target this cellular Achilles' heel, perhaps leading to drugs that are less susceptible to resistance.

"Parts of the ribosome are much more flexible than we previously thought. In addition, now that we have a fully rotated ribosomal structure, scientists may be able to develop new antibiotics that are not as sensitive to ribosomal mutations. This could help mitigate the huge problem of multidrug resistance," says Jamie Cate, a staff scientist in Berkeley Lab's Physical Biosciences Division and an associate professor of biochemistry, molecular biology, and chemistry at the University of California at Berkeley.

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Cate conducted the research with a team that includes scientists from Cornell University and Duke University. Their research is published in the May 20 issue of the journal Science.

The ribosome works like a protein assembly line. Its smaller subunit moves along messenger RNA, which contributes genetic information from the cell's DNA. The smaller subunit also binds to transfer RNA, which connect the genetic code on one end with amino acids on the other. The amino acids are stitched together into proteins by the larger subunit, which also binds to the transfer RNA. In this way, the two ribosomal subunits come together to create proteins that conduct the heavy lifting in the cells of all living things, from bacteria to trees to humans.

Scientists have used biochemistry and low-resolution electron microscopy to map much of the ribosome's structural changes throughout its protein-making cycle. But key steps remained unclear, such as a ratchet-like motion of the small ribosomal subunit relative to the large subunit as it moves in one direction along the messenger RNA to make a protein. These parts rotate relative to another, but scientists didn't know how this large-scale twisting motion worked in molecular detail — or why it didn't simply wrench the entire ribosome apart.

To find out, the scientists turned to the Advanced Light Source, a synchrotron located at Berkeley Lab that generates intense x-rays to probe the fundamental properties of molecules. Using beamline 8.3.1 and the SIBYLS beamlines, they determined the structure of Escherichia coli ribosomes in two key states for the first time at an atomic-scale resolution. In the first state, transfer RNA is bound to the two subunits in a configuration that occurs after the ribosome has made and released a protein. In the second state, the ribosome's subunits are fully rotated, which occurs when the subunits are recycled and ready to make another protein. The scientists used x-ray crystallography to piece together these structures at a resolution of approximately 3.2 Angstroms (one Angstrom is a ten-billionth of a meter, about the radius of the smallest atoms).

The resulting structures, which are two to three times higher resolution than previous images of the ribosome at these states, capture the inner-workings of the ribosome like never before. They reveal that the ribosome machine contains molecular-scale compression springs and torsion springs made of RNA. These molecular springs keep the ribosome's subunits tethered together even as they rotate with respect to each other.

"This is first time we've seen the ribosome at the endpoint of this motion at this resolution," says Cate. "And the question is, when you have these big motions, why doesn't the ribosome fall apart. We found that the ribosome has RNA springs that adjust their shape and allow it to stay together during these large-scale motions."

The structures also provide a new way to compete in the arms race between antibiotics and the microbes they're designed to knock out.

"The ribosome is one of the major targets of antibiotics, and we've identified elements of its rotation that can be targeted by new or modified antibiotics," says Cate. "This kind of precision could be especially powerful in the age of personalized medicine. Scientists could figure out at a genetic level why someone isn't responding to an antibiotic, and then possibly switch to a more effective antibiotic that better targets the microbe that's causing their disease."

More information: The research is described in a paper entitled “Structures of the bacterial ribosome in classical and hybrid states of tRNA binding” that is published in the May 20 issue of the journal Science.

Provided by Lawrence Berkeley National Laboratory (news : web)