Sensitive to oxygen: Phosphorescent iridium(III) porphyrin complexes, new tunable oxygen indicators

 Monitoring the amount of oxygen in living tissues accurately is a valuable tool in biomedical science, because it enables the elucidation of the course of metabolic processes or the detection of diseases or anomalies. Metal complexes that absorb and emit light are useful as sensors, and metal complexes of porphyrins and their derivatives are especially good candidates for such applications, as the porphyrin macrocycle can easily be modified.

Sergey M. Borisov and his co-workers at Graz University of Technology (Austria), developed new, strongly phosphorescent porphyrin complexes of iridium(III), which were applied as dyes in advanced optical oxygen-sensing materials and published them in the European Journal of Inorganic Chemistry.

Photophysical properties of porphyrin complexes of metals such as palladium or platinum have been studied before; however, there are fewer studies on iridium complexes, which are more difficult to synthesize. The absorption bands of iridium complexes are broader and are shifted to lower wavelengths in comparison to those of their platinum analogues. This enables them to be excited by visible light. Furthermore, iridium(III) is hexacoordinate, which opens up the added possibility of introducing axial ligands directly on the metal instead of modifying the porphyrin macrocycle, in contrast to the square-planar platinum(II) and palladium(II) analogues. A ?-extended iridium(III)–benzoporphyrin and four iridium(III)–octaethylporphyrin complexes with high room-temperature phosphorescence quantum yields of up to 30% were synthesized.

Axial ligands were used to change their solubility or to introduce binding groups. In this way, the complexes were rendered soluble in organic solvents, and they were incorporated into polystyrene or other polymers to yield oxygen sensors. In addition, other axial ligands, such as an imidazole ligand bearing a carboxyl group, were used to make the complexes soluble in polar solvents such as ethanol and even in aqueous buffer at physiological pH, which enabled coupling to biomolecules such as proteins, antibodies, or lipids, as demonstrated by coupling to bovine serum albumin.

The importance of these new compounds is their tunable photophysical properties and versatility, as demonstrated by their application as a water-soluble oxygen probe (by staining bovine serum albumin) and a trace oxygen sensor (by coupling to amino-modified silica gel). The obtained sensor is sensitive to small oxygen concentrations and features a highly linear calibration plot. The new dyes are particularly promising as indicators for oxygen sensors with tailor-made sensitivity.

More information: Sergey Borisov, Strongly Phosphorescent Iridium(III)–Porphyrins—New Oxygen Indicators with Tuneab­le Photophysical Properties and Functionalities, European Journal of Inorganic Chemistry, http://dx.doi.org/ … ic.201100089

Provided by Wiley (news : web)

A personality change for a catalyst

For more than 40 years, an ambition of catalysis science has been to persuade homogeneous catalysts to behave more like heterogeneous catalysts, while still maintaining their activity and exquisite selectivity. Professor Christopher W. Jones of the Georgia Institute of Technology discussed the state of current research on a class of coordination complex (metal-salen) catalysts, and how his research team is making progress in anchoring them to solid support materials. His talk on January 31 was part of the Frontiers in Catalysis Science and Engineering Seminar Series. The seminars, held at Pacific Northwest National Laboratory, allow experts to share results of studies and novel ideas.

Separation Anxiety: In homogeneous reactions, the toughest challenge is separating the catalyst from the reaction medium because the catalyst and the reactant have the same phase of matter. Solid phase heterogeneous catalysts, on the other hand, are easier to separate from the reactant, which may be a liquid or a gas. Finding the solution to the separation problem for homogeneous catalysts, while still maintaining high catalytic activity and product selectivity, is a major challenge that limits their wide-spread use in industrial applications.

The solution to this predicament may be to anchor a homogeneous catalyst, such as a typical organometallic compound, onto a solid support material. With homogeneous catalysts, you have more precise control over the local molecular structure. Unlike heterogeneous catalysts, the reaction sites can be uniform and, thus, are more selective for the desired products of a catalytic reaction. In this case, there are several problems to overcome to achieve a good reaction. First, how to anchor the catalyst to the support and get it to stay attached? Second, once attached, it must be persuaded to remain intact and behave as it did before. Finally, how long will the catalyst stay in the reactive state? It is important to perform the correct anchoring to achieve a separable, yet still active and stable catalyst.

Deactivation: The Achilles Heel of Catalysis: Many prior studies have not determined catalyst performance on reasonable timeframes which accurately measure the stability of a newly anchored homogeneous catalyst system. If the time period is too long, it will not match real-world conditions. The results of such studies may not be meaningful.

"Even a pretty bad catalyst can carry out a batch catalytic reaction to completeness over a very long time period," said Dr. Charles Peden, Laboratory Fellow and Interim Director of the Institute for Interfacial Catalysis at PNNL.

Professor Jones and his team are working on detailed studies of how their anchored metal-salen catalysts are deactivating, using reaction conditions where meaningful catalysis kinetics measurements can be made. By understanding how and when the catalyst deactivates, better catalysts can be designed from the beginning to prevent those specific deactivation mechanisms.

"Catalysis is an entirely kinetic phenomenon. Kinetics should always be accurately reported in catalysis," said Jones.

Knowing whether the catalyst is stable on the support is key. And that gets back to correct anchoring to get a separable catalyst. Anchoring homogeneous catalysts on solids is an expensive process, so stable catalysts are needed to make them economically viable.

"If the catalyst deactivates severely, the cost to create the material is not justified. Deactivation is the Achilles heel of these catalysts," said Jones.

Which is why, to date, there are few good examples of practical applications for anchored homogeneous catalysts. Creating stable and separable catalysts that continue to carry out their designed catalytic reactions remains the ultimate goal.

More information: http://iic.pnl.gov/seminars/

Provided by Pacific Northwest National Laboratory (news : web)

Researchers hunt for green catalysts

L. Keith Woo is searching for cleaner, greener chemical reactions. Woo, an Iowa State University professor of chemistry and an associate of the U.S. Department of Energy's Ames Laboratory, has studied catalysts and the chemical reactions they affect for more than 25 years. And these days, his focus is on green catalysis.

That, he said, is the search for catalysts that lead to more efficient chemical reactions. That could mean they promote reactions at lower pressures and temperatures. Or it could mean they promote reactions that create less waste. Or it could mean finding safer, cleaner alternatives to toxic or hazardous conditions, such as using water in place of organic solvents.

"We're trying to design, discover and optimize materials that will produce chemical reactions in a way that the energy barrier is lowered," Woo said. "We're doing fundamental, basic catalytic work."

And much of that work is inspired by biology.

In one project, Woo and his research group are studying how iron porphyrins (the heme in the hemoglobin of red blood cells) can be used for various catalytic applications. Iron porphyrins are the active sites in a variety of the enzymes that create reactions and processes within a cell. Most of the iron porphyrin reactions involve oxidation and electron transfer reactions.

Because the iron porphyrins of biology have evolved into highly specialized catalysts, Woo and his research group are studying how they can be used synthetically with the goal of developing catalysts that influence a broader range of reactions.

"We've found porphyrins are capable of doing many reactions -- often as well, or better, or cheaper than other catalysts," Woo said.

Another project is using combinatorial techniques to accelerate the development, production and optimization of catalysts. Woo and his research group are using molecular biology to quickly screen a massive library of DNA molecules for catalyst identification and development. The goal is to create water-soluble catalysts for organic reactions.

"Combinatorial approaches such as these have been applied to drug design, but their use in transition metal catalyst development is in its infancy," Woo wrote in a summary of the project.

A third project is looking for catalysts that allow greener production of lactams, which are compounds used in the production of solvents, nylons and other polymers. Commercial lactam production traditionally uses harsh reagents and conditions, such as sulfuric acid and high temperatures, and also creates significant wastes.

Woo, in collaboration with Robert Angelici, a Distinguished Professor Emeritus of Chemistry, has found a gold-based catalyst that eliminates the need for the acid and high pressure and also eliminates the wastes. The Iowa State Research Foundation Inc. is seeking a patent on the technology.

And, in a fourth project, Woo is working to understand the chemistry behind the chemical reactions that create bio-oil from the fast pyrolysis of biomass. Fast pyrolysis quickly heats biomass (such as corn stalks and leaves) in the absence of oxygen to produce a liquid bio-oil that can be used to manufacture fuels and chemicals.

Woo's projects are supported by grants from the National Science Foundation, the U.S. Department of Energy, Iowa State's Institute for Physical Research and Technology, Iowa State's Bioeconomy Institute, and the National Science Foundation Engineering Research Center for Biorenewable Chemicals based at Iowa State. Woo's research team includes post-doctoral researcher Wenya Lu and doctoral students B.J. Anding, Taiwo Dairo, Erik Klobukowski and Gina Roberts.

Sit down with Woo and he'll call up slide after slide of the chemical equations that describe chemical reactions.

And before long he's describing how catalysts are discovered these days.

"The traditional way to develop catalysts was very Edisonian -- one experiment at a time," Woo said. "It was all by trial and error."

Now, with high-throughput approaches, Woo said his research group is able to quickly test a reaction using one hundred trillion different catalysts.

And that, Woo said, is "helping us find less expensive and more environmentally friendly materials and conditions to perform these catalytic reactions."

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Iowa State University.

Natural clay as a potential host rock for nuclear waste repositories

 Scientists at Johannes Gutenberg University Mainz (JGU), Germany, have studied natural claystone in the laboratory for more than four years in order to determine how the radioactive elements plutonium and neptunium react with this rock.

The experiments have been performed as part of a Germany-wide project sponsored by the Federal Ministry of Economics and Technology (BMWi) to find a suitable geological repository for radioactive waste. Geological configurations that could play a role for the permanent disposal of nuclear waste include not only salt and granite formations, but also claystone. The results obtained by the team of nuclear chemists under the supervision of Professor Tobias Reich confirm that natural clay has certain useful properties that counteract the migration of radioactive materials. Professor Reich, director of the Institute of Nuclear Chemistry at Mainz University, summarizes the results obtained so far as follows: "It does seem that clay could be suitable as host rock, although we still need to wait for the outcome of long-term investigations."

The cylinders of clay used by the Mainz team of nuclear chemists have come a long way: Cores of Opalinus Clay were obtained by drilling in the Underground Rock Laboratory Mont Terri in the Swiss Jura mountains. This clay formation was deposited some 180 million years ago. In Switzerland, Opalinus Clay is being considered as a rock formation for the storage of radioactive waste. The bore cores were first transferred to the Institute for Nuclear Waste Disposal (INE) in Karlsruhe, Germany, where they were cut in small round disks with a thickness of 11 millimeters. At the Institute of Nuclear Chemistry in Mainz, these clay disks were packed in diffusion cells and contacted with pore water containing radioactive neptunium or plutonium. Other samples of clay were transferred to test tubes where a suspension of this material with pore water and the radioactive elements was agitated and centrifuged and then analyzed by highly sensitive mass spectrometers in order to determine the sorption characteristics of the clay material. Afterwards, the samples were transported to the particle accelerators in Grenoble, Karlsruhe, and at the Paul Scherrer Institute in Switzerland, where they were investigated by beams of synchrotron radiation with a width of only 0.0015 millimeters. "This provides us with detailed information on the distribution of the elements, and where and how the elements are sorbed on the clay material," explains Reich.

The batch experiments show that radioactive plutonium in the oxidation state IV is nearly totally sorbed on Opalinus Clay, leaving almost no plutonium in the aqueous solution. In the case of neptunium(V), the corresponding ratio is 60:40. However, if neptunium is reduced to neptunium(IV) by iron minerals present in the clay, a near 100 percent sorption of neptunium on the clay is observed. The diffusion experiments using "radioactive" water have demonstrated that water passes through a clay cylinder with a thickness of 1.1 centimeters within a week. Neptunium, on the other hand, hardly diffuses through the clay, and even after a month it is still almost where it started.

Thin, millimeter-wide sections of the clay disks also show which chemical reactions of the radioactive elements occur as they pass through the clay material: Plutonium(VI) is reduced during its passage through the clay cylinder and is recovered as plutonium(IV). "This is a great advantage, as plutonium(IV) stays where it is put." Professor Reich and his research team also discovered what is responsible for the sorption of the radioactive substances: It is predominantly the clay minerals, while the iron minerals responsible for the reduction process are only of minor importance in this connection.

Opalinus Clay, which occurs not only in Switzerland but also in Southern Germany, thus appears to be an excellent candidate for further investigation on the migration of long-lived radionuclides -- neptunium, for example, has a half-life of 2.14 million years. The Mainz team of nuclear chemists had previously reported similar results using the clay mineral kaolinite from the United States of America. "In the meantime we have developed the necessary equipments and have defined the experimental processes required," Reich summarizes the outcome of the experiments with Opalinus Clay. Over the coming three years, he and his team plan to investigate the behavior of clay in the presence of higher salt concentrations.

The studies are being conducted as part of a project initiated by the German Federal Ministry of Economics and Technology in 1995 to find a suitable location for a nuclear waste repository. Eight research institutions are participating in the joint project entitled "Interaction and transport of actinides in natural claystone, under consideration of humic substances and organic clay materials" to investigate to what extent Opalinus Clay could be a suitable host rock for the permanent disposal of highly radioactive waste.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Johannes Gutenberg University Mainz.

Materials identified that may deliver more 'bounce'

ScienceDaily (Mar. 9, 2011) — Rutgers researchers have identified a class of high-strength metal alloys that show potential to make springs, sensors and switches smaller and more responsive.

The alloys could be used in springier blood vessel stents, sensitive microphones, powerful loudspeakers, and components that boost the performance of medical imaging equipment, security systems and clean-burning gasoline and diesel engines.

While these nanostructured metal alloys are not new -- they are used in turbine blades and other parts demanding strength under extreme conditions -- the Rutgers researchers are pioneers at investigating these new properties.

"We have been doing theoretical studies on these materials, and our computer modeling suggests they will be super-responsive," said Armen Khachaturyan, professor of Materials Science and Engineering in the Rutgers School of Engineering. He and postdoctoral researcher Weifeng Rao believe these materials can be a hundred times more responsive than today's materials in the same applications.

Writing in the March 11 issue of the journal Physical Review Letters, the researchers describe how this class of metals with embedded nanoparticles can be highly elastic, or "springy," and can convert electrical and magnetic energy into movement or vice-versa. Materials that exhibit these properties are known among scientists and engineers as "functional" materials.

One class of functional materials generates an electrical voltage when the material is bent or compressed. Conversely, when the material is exposed to an electric field, it will deform. Known as piezoelectric materials, they are used in ultrasound instruments; audio components such as microphones, speakers and even venerable record players; autofocus motors in some camera lenses; spray nozzles in inkjet printer cartridges; and several types of electronic components.

In another class of functional materials, changes in magnetic fields deform the material and vice-versa. These magnetorestrictive materials have been used in naval sonar systems, pumps, precision optical equipment, medical and industrial ultrasonic devices, and vibration and noise control systems.

The materials that Khachaturyan and Rao are investigating are technically known as "decomposed two-phase nanostructured alloys." They form by cooling metals that were exposed to high temperatures at which the nanosized particles of one crystal structure, or phase, are embedded into another type of phase. The resulting structure makes it possible to deform the metal under an applied stress while allowing the metal to snap back into place when the stress is removed.

These nanostructured alloys might be more effective than traditional metals in applications such blood vessel stents, which have to be flexible but can't lose their "springiness." In the piezoelectric and magnetorestrictive components, the alloy's potential to snap back into shape after deforming -- a property known as non-hysteresis -- could improve energy efficiency over traditional materials that require energy input to restore their original shapes.

In addition to potentially showing responses far greater than traditional materials, the new materials may be tunable; that is, they may exhibit smaller or larger shape changes and output force based on varying mechanical, electrical or magnetic input and the material processing.

The researchers hope to test the results of their computer simulations on actual metals in the near future.

The Rutgers team collaborated with Manfred Wittig, professor of Materials Science and Engineering at the University of Maryland. Their research was funded by the National Science Foundation and the U.S. Department of Energy.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Rutgers University.

Journal Reference:

Wei-Feng Rao, Manfred Wuttig, and Armen G. Khachaturyan. Giant Nonhysteretic Responses of Two-Phase Nanostructured Alloys. Phys. Rev. Lett., (in press)

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

Synthetic biology: Novel kind of fluorescent protein developed

 Proteins are the most important functional biomolecules in nature with numerous applications in life science research, biotechnology and medicine. So how can they be modified in the most effective way to attain certain desired properties? In the past, the modifications were usually carried out either chemically or via genetic engineering.

The team of Professor Arne Skerra from the TUM Chair of Biological Chemistry has now developed a more elegant combined solution: By extending the otherwise universal genetic code, the scientists are able to coerce bacterial cells to produce tailored proteins with synthetic functional groups. To put their idea to the test, they set out to crack a particularly hard nut: The scientists wanted to incorporate a non-natural amino acid at a specific site into a widely used natural protein.

In bioresearch this protein is commonly known as "GFP" (= green fluorescent protein). It emits a bright green glow and stems originally from a jellyfish that uses the protein to make itself visible in the darkness of the deep sea. The team chose a pale lavender coumarin pigment, serving as side chain of a non-natural amino acid, as the synthetic group. The scientists "fed" this artificial amino acid to a laboratory culture of Escherichia coli bacteria -- the microorganism workhorses of genetic engineering, whose natural siblings are also found in the human intestine. Since the team had transferred the modified genetic blueprints for the GFP to the bacteria -- including the necessary biosynthesis machinery -- it incorporated the coumarin amino acid at a very specific site into the fluorescent protein.

This spot in the GFP was carefully chosen, explains Professor Skerra: "We positioned the synthetic amino acid at a very close distance from the fluorescence center of the natural protein." The scientists employed the principle of the so-called Foerster resonance energy transfer, or FRET for short. Under favorable conditions, this process of physical energy transfer, named after the German physical chemist Theodor Foerster, allows energy to be conveyed from one stimulated pigment to another in a radiation-less manner.

It was precisely this FRET effect that the scientists implemented very elegantly in the new fluorescent protein. They defined the distance between the imported chemical pigment and the biological blue-green (cyan, to be more precise) pigment of the jellyfish protein in such a way that the interplay between the two dyes resulted in a completely novel kind of fluorescent chimeric biomolecule. Because of the extreme proximity of the two luminescent groups the pale lavender of the synthetic amino acid can no longer be detected; instead, the typical blue-green color of the fluorescent protein dominates. "What is special here, and different from the natural GFP, is that, thanks to the synthetically incorporated amino acid, the fluorescence can be excited with a commercially available black-light lamp in place of an expensive dedicated LASER apparatus," explains Sebastian Kuhn, who conducted these groundbreaking experiments as part of his doctoral thesis.

According to Skerra, the design principle of the novel bio-molecule, which is characterized by a particularly large and hard to achieve wavelength difference between excitation and emitted light, should open numerous interesting applications: "We have now demonstrated that the technology works. Our strategy will enable the preparation of customized fluorescent proteins in various colors for manifold future purposes." This research project was financially supported by the German Research Foundation (DFG) as part of the Excellence Cluster "Munich Center for Integrated Protein Science" (CIPS-M).

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Technische Universitaet Muenchen.

Journal Reference:

Sebastian M. Kuhn, Marina Rubini, Michael A. Mu¨ller, Arne Skerra. Biosynthesis of a Fluorescent Protein with Extreme Pseudo-Stokes Shift by Introducing a Genetically Encoded Non-Natural Amino Acid outside the Fluorophore. Journal of the American Chemical Society, 2011; : 110225040126039 DOI: 10.1021/ja1099787

New method for studying molecule reactions a breakthrough in organic chemistry

Good chemists are passive-aggressive -- they manipulate molecules without actually touching them. In a feat of manipulating substances at the nanoscale, UCLA researchers and colleagues demonstrated a method for isolating two molecules together on a substrate and controlling how those two molecules react when excited with ultraviolet light, making detailed observations both before and after the reaction.

Their research is published March 10 in the journal Science.

"This is one step in measuring and understanding the interactions between light and molecules, which we hope will eventually lead to more efficient conversion of sunlight to electrical and other usable forms of energy," said lead study author Paul S. Weiss, a distinguished professor of chemistry and biochemistry who holds UCLA's Fred Kavli Chair in Nanosystems Sciences. "Here, we used the energy from the light to induce a chemical reaction in a way that would not happen for molecules free to move in solution; they were held in place by their attachment to a surface and by the unreactive matrix of molecules around them."

Weiss is also director of UCLA's California NanoSystems Institute (CNSI) and a professor of materials science and engineering at the UCLA Henry Samueli School of Engineering and Applied Science.

Controlling exactly how molecules combine in order to study the resulting reactions is called regioselectivity. It is important because there are a variety of ways that molecules can combine, with varying chemical products. One way to direct a reaction is to isolate molecules and to hold them together to get regioselective reactions; this is the strategy used by enzymes in many biochemical reactions.

"The specialized scanning tunneling microscope used for these studies can also measure the absorption of light and charge separation in molecules designed for solar cells," Weiss said. "This gives us a new way to optimize these molecules, in collaboration with synthetic chemists. This is what first brought us together with our collaborators at the University of Washington, led by Prof. Alex Jen."

Alex K-Y. Jen holds the Boeing-Johnson Chair at the University of Washington, where he is a professor of materials science and engineering and of chemistry. The theoretical aspects of the study were led by Kendall Houk, a UCLA professor of chemistry and biochemistry who holds the Saul Winstein Chair in Organic Chemistry. Houk is a CNSI researcher.

The study's first author, Moonhee Kim, a graduate student in Weiss' lab, managed to isolate and control the reactions of pairs of molecules by creating nanostructures tailored to allow only two molecules fit in place. The molecules used in the study are photosensitive and are used in organic solar cells; similar techniques could be used to study a wide variety of molecules. Manipulating the way molecules in organic solar cells come together may also ultimately lead to greater efficiency.

To isolate the two molecules and align them in the desired -- but unnatural -- way, Kim utilized a concept similar to that of toddler's toys that feature cutouts in which only certain shapes will fit.

She created a defect, or cutout, in a self-assembled monolayer, or SAM, a single layer of molecules on a flat surface -- in this case, gold. The defect in the SAM was sized so that only two organic reactant molecules would fit and would only attach with the desired alignment. As a guide to attach the molecules to the SAM in the correct orientation, sulfur was attached to the bottoms of the molecules, as sulfur binds readily to gold.

"The standard procedure for this type of chemistry is to combine a bunch of molecules in solution and let them react together, but through random combinations, only 3 percent of molecules might react in this way," UCLA's Houk said. "Our method is much more targeted. Instead of doing one measurement on thousands of molecules, we are doing a range of measurements on just two molecules."

After the molecules were isolated and trapped on the substrate, they still needed to be excited with light to react. In this case, the energy was supplied by ultraviolet light, which triggered the reaction. The researchers were able to verify the proper alignment and the reaction of the molecules using the special microscope developed by Kim and Weiss.

The work was funded by the U.S. Department of Energy, the National Science Foundation, the Air Force Office of Scientific Research and the Kavli Foundation.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by University of California - Los Angeles.

Journal Reference:

Moonhee Kim, J. Nathan Hohman, Yang Cao, Kendall N. Houk, Hong Ma, Alex K.-Y. Jen, and Paul S. Weiss. Creating Favorable Geometries for Directing Organic Photoreactions in Alkanethiolate Monolayers. Science, 11 March 2011: 1312-1315.[ DOI: 10.1126/science.1200830