Graphene Can Polarize Light

Publication in Nature Photonics from the OPERA Photonique Department : Graphene can polarize light.

Graphene, an ultra-flat monolayer of carbon atoms in a hexagonal crystal lattice, has attracted a strong wave of research interest due to its unique electrical and photonic properties.

Graphene

Image: Wikipedia

As the first two dimensional material in the world, two UK Scientists were awarded the 2010 Nobel Prize in physics since it completely changes how we look at things. Now, Dr. Han Zhang at the Service OPERA-photonique – Applied Science Faculty, ULB - in collaboration with Prof. Loh at the National University of Singapore demonstrates the world's thinnest polarizer, which relies on the coupling, guiding and polarizing of electromagnetic waves by graphene. They claim that this breakthrough will someday allow the integration on all-photonic circuits for high-speed optical communications.

Optical polarizers are elementary components of coherent and quantum optical communications by splitting the polarization state of an optical signal. Nowadays, there are rising demands for high-speed optical communications based on mobiles, calling for the miniaturization of optoelectronic devices.

However, conventional optical polarizers (sheet, prism and Brewster-angle polarizer) are expensive, bulky, and discrete and may require additional alignment.

Thanks to graphene’s ultra-broadband optical property induced by its exceptional energy band structure, as-demonstrated graphene polarizer shows very broad operation bandwidth, at least from visible to mid-infrared. By fabricating graphene polarizer, with combined advantages of low cost (down to several euros), compact footprint, ultra-fast relaxation time and broad operation range, they anticipate that this device will enable new architectures for on-chip high-speed optical communications.

In addition to the industrial potentials, this research published in Nature Photonic, on May 30th is of fundamental importance.

It tackles how light propagates along an ultra-thin two dimensional surface. By the virtue of fiber based optical channel, now we can readily uncover how graphene guides and interacts with electromagnetic waves, with polarizing effect attributed to the differential attenuation of two polarization modes.

This new conceptual finding will definitely lead to new physics, for example, localized waves or surface plasmon in graphene lattice. In the following years, researchers from the photonics, plasmonics and nano-science research communities may find in this graphene polarizer structure as a new testing ground for the ideas and methods they have been researching on their own fields, paving the way for all-carbon photonic-plasmonics devices.

Source: Université Libre de Bruxelles

A Sweet Sugary Defense Against Lethal Bacteria

Synthesising a potential vaccine candidate for an antibiotic-resistant pathogen causing infections in hospitalised patients

There is now a promising vaccine candidate for combating the pathogen which causes one of the most common and dangerous hospital infections. An international team of scientists from the Max Planck Institute of Colloids and Interfaces in Potsdam has developed a vaccine based on a carbohydrate against the Clostridium difficile bacterium, which is known to cause serious gastrointestinal diseases mainly in hospitals.

The sugar-based vaccine elicited a specific and effective immune response in mice. Moreover, the scientists have also discovered strong indications that the substance can stimulate the human immune system to form antibodies against the bacterium.

Stimulating the immune system: on the basis of a hexasaccharide, scientists from Potsdam developed a vaccine against the Clostridium difficile bacterium, which causes serious gastrointestinal diseases in hospitals.

Credit: © MPI of Colloids and Interfaces

Clostridium difficile bacterium can turn into a life-threatening condition: a highly virulent and antibiotic-resistant strain of the spore-forming pathogen Clostridium difficile bacterium appeared in the USA and certain Western European countries some eight years ago. Since then it has been posing a major risk for hospitalised patients, in particular, who are being treated with antibiotics or have a weak immune system, such as cancer or HIV patients.

Whereas no more than four per cent of healthy humans have C. difficile in their gastrointestinal system, the bacterium colonises the intestines of 20 to 40 per cent of hospitalised patients. If other bacteria in the intestinal flora are repressed by antibiotics, the rod-shaped bacterium can reproduce extremely fast. It produces toxins which cause diarrhoea and gastrointestinal inflammation, often with a lethal outcome. Surviving patients require a very costly aftercare. This new, highly virulent pathogen can produce around 20 times more toxins and significantly more spores than previously identified pathogens.

However, a carbohydrate in the bacterial cell wall now provides the team of scientists led by Peter H. Seeberger at the Max Planck Institute of Colloids and Interfaces in Potsdam a “point of attack” for a potential vaccine. “Initial testing of the sugar-based antigen synthesised by the team has already produced very promising results”, says Peter H. Seeberger, Director at the Max Planck Institute in Potsdam.

The chemists in the team first developed a synthesis for the essential component of the antigen: the hexasaccharide. To assemble the oligosaccharide, they used four different monosaccharide building blocks. An efficient and convergent approach created the exact molecule with the required arrangement of the monosaccharides. “Synthesizing complex polysaccharides is still a challenge, not least because sugar molecules can bind in several different places”, Peter H. Seeberger says. However, the chemists were able to block other reaction sites so that they could exactly control where the original saccharides bound.

The scientists then conjugated the hexasaccharide to the CRM 197 protein, which is used in many vaccines, as sugar alone, as antigen, does not elicit an effective immune response. In order to defend itself successfully against a C. difficile infection, the immune system must also use another antigen. The chemical glycoprotein conjugate triggered a very effective immune response in two mice which were injected with the substance three times, at 2-week intervals.

“The fact that mice are producing antibodies against the carbohydrates is in itself a success”, Peter H. Seeberger says. “Not all carbohydrates trigger the production of antibodies.” Furthermore, the antibodies produced by the mice bound exclusively to the sugar. Thus, the antigen cannot cause an autoimmune disease.

Additionally, the scientists proved that the antibodies developed against the hexasaccharide are also part of the human immune response; in the stool of hospital patients infected with C. difficile, they found antibodies against the sugar.

“We can therefore expect to see that the human immune system produces antibodies against the sugar when vaccinated”, Seeberger concludes. What is more, “since the natural sugar already elicits the production of a small number of antibodies, we hope that the synthetic glycoprotein conjugate will trigger a more effective response.”

The vaccine candidate must now be subjected to further testing. First, it must be established whether it can effectively prevent infection in animals. “If these tests are successful, it will probably still take one or two years before the vaccine is tested on humans”, explains Peter H. Seeberger.

The vaccine candidate against C. difficile does not contain the only immunologically effective sugar from Seeberger's laboratory. Together with his colleagues, the chemist is developing sugar-based vaccines against numerous pathogens.

“The current work is therefore also a proof of the progress made in glycochemistry and glycobiology”, according to Seeberger, who was awarded the 2007 Körber European Science Award for his development of a sugar synthesiser.

The number of biological sugar molecules that can be produced by chemists in the laboratory is on the increase, which gives the biologists and medical scientists the opportunity to investigate their specific impacts. This fills Peter H. Seeberger with optimism: “These advances will lead to quantum leaps in related research areas, such as immunology, biology and medicine.”

Contacts and sources:

Prof. Dr. Peter H. Seeberger 
Max Planck Institute of Colloids and Interfaces, Potsdam

Citation: Matthias A. Oberli, Marie-Lyn Hecht, Pascal Bindschädler, Alexander Adibekian, Thomas Adam and Peter H. Seeberger: A Possible Oligosaccharide-Conjugate Vaccine Candidate for Clostridium difficile Is Antigenic and Immunogenic

Chemistry & Biology, 26 May 2011; DOI: 10.1016/j.chembiol.2011.03.009

New Malaria Protein Structure Upends Theory Of How Cells Grow And Move

Researchers from the Walter and Eliza Hall Institute have overturned conventional wisdom on how cell movement across all species is controlled, solving the structure of a protein that cuts power to the cell 'motor'. The protein could be a potential drug target for future malaria and anti-cancer treatments.

Researchers (from left) Dr Jake Baum. Mr Wilson Wong and Dr Jacqui Gulbis from the Walter and Eliza Hall Institute in Melbourne, Australia, have upended the theory of how cells grow and move, solving the structure of a protein that cuts power to the cell "motor". The protein could be a potential drug target for future malaria and anti-cancer treatments.

Credit: Walter and Eliza Hall Institute

By studying the structure of actin-depolymerising factor 1 (ADF1), a key protein involved in controlling the movement of malaria parasites, the researchers have demonstrated that scientists' decades-long understanding of the relationship between protein structure and cell movement is flawed.

Dr Jake Baum and Mr Wilson Wong from the institute's Infection and Immunity division and Dr Jacqui Gulbis from the Structural Biology division, in collaboration with Dr Dave Kovar from the University of Chicago, US, led the research, which appears in today's edition of the Proceedings of the National Academy of Sciences USA.

Dr Baum said actin-depolymerising factors (ADFs) and their genetic regulators have long been known to be involved in controlling cell movement, including the movement of malaria parasites and movement of cancer cells through the body. Anti-cancer treatments that exploit this knowledge are under development.

A protein diagram shows the structure of the malaria parasite protein ADF1 (actin-depolymerising factor 1) (left) compared to a human ADF (right). The noticeable lack of the 'finger' in malaria parasite ADF1 led researchers from the Walter and Eliza Hall Institute in Melbourne, Australia, to upend the conventional theory of how the protein controlled cell movement.

Credit: Dr Jake Baum and Mr Wilson Wong, Walter and Eliza Hall Institute.
"ADFs help the cell to recycle actin, a protein which controls critical functions such as cell motility, muscle contraction, and cell division and signaling," Dr Baum said. "Actin has unusual properties, being able to spontaneously form polymers which are used by cells to engage internal molecular motors – much like a clutch does in the engine of your car. A suite of accessory proteins control how the clutch is engaged, including those that dismantle or 'cut' these polymers, such as ADF1.

"For many years research in yeast, plants and humans has suggested that the ability of ADFs to dismantle actin polymers – effectively disengaging the clutch – required a small molecular 'finger' to break the actin in two," Dr Baum said. "However, when we looked at the malaria ADF1 protein, we were surprised to discover that it lacked this molecular 'finger', yet remarkably was still able to cut the polymers. We discovered that a previously overlooked part of the protein, effectively the 'knuckle' of the finger-like protrusion, was responsible for dismantling the actin; we then discovered this 'hidden' domain was present across all ADFs."

Mr Wong said that the Australian Synchrotron was critical in providing the extraordinary detail that helped the team pinpoint the protein 'knuckle'. "This is the first time a 3D image of the ADF protein has been captured in such detail from any cell type," Mr Wong said. "Imaging the protein structure at such high resolution was critical in proving beyond question the segment of the protein responsible for cutting actin polymers. Obtaining that image would have been impossible without the synchrotron facilities."

A protein diagram shows the structure of the malaria parasite protein ADF1 (actin-depolymerising factor 1). The noticeable lack of the 'finger' led researchers from the Walter and Eliza Hall Institute in Melbourne, Australia, to upend the conventional theory of how the protein controlled cell movement.

Credit: Dr Jake Baum and Mr Wilson Wong, Walter and Eliza Hall Institute.
Dr Baum said the new knowledge will give researchers a much clearer understanding of one of the fundamental steps governing how cells across all species grow, divide and, importantly, move. "Knowing that this one small segment of the protein is singularly responsible for ADF1 function means that we need to focus on an entirely new target not only for developing anti-malarial treatments, but also other diseases where potential treatments target actin, such as anti-cancer therapeutics," Dr Baum said. "Malaria researchers are normally used to following insights from other biological systems; this is a case of the exception proving the rule: where the malaria parasite, being so unusual, reveals how all other ADFs across nature work."

More than 250 million people contract malaria each year, and almost one million people, mostly children, die from the disease. The malaria parasite has developed resistance to most of the therapeutic agents available for treating the disease, so identifying novel ways of targeting the parasite is crucial.

Dr Baum said that the discovery could lead to development of drugs entirely geared toward preventing malaria infection, without adverse effects on human cells. "One of the primary goals of the global fight against malaria is to develop novel drugs that prevent infection and transmission in all hosts, to break the malaria cycle," Dr Baum said. "There is a very real possibility that, in the future, drugs could be developed that 'jam' this molecular 'clutch', meaning the malaria parasite cannot move and continue to infect cells in any of its conventional hosts, which would be a huge breakthrough for the field."

This project was funded by the National Health and Medical Research Council (NHMRC).

Contacts and sources:

Liz Williams
Walter and Eliza Hall Institute