New method could improve economics of sweetening natural gas

Natural gas extracted from the nation's coal beds and methane-rich geologic features must first be purged of hydrogen sulfide before it can be used as fuel. Until now, processing methods have often proved to be inefficient, requiring large amounts of heat.

But a team of Battelle researchers at the Department of Energy's Pacific Northwest National Laboratory has discovered a method that could dramatically cut the amount of heat needed during processing, reducing the amount of energy needed during a key processing step by at least 10 percent. The research team believes the discovery could ultimately lead to a more cost-effective way of tapping into extremely "sour" natural gas reserves -- those reserves that contain significant amounts of hydrogen sulfide and that may not have been economically viable to tap up to this point. Battelle operates the Pacific Northwest National Laboratory for DOE.

The researchers lay out the more efficient process and suggest how it could be applied to processing raw natural gas in the March 11 online issue of the journal Energy and Environmental Science.

Raw natural gas is purified in a process called "sweetening" before it can safely be used as a fuel. Thermal Swing Regeneration is a common industry process used for sweetening natural gas. In that process, chemical sponges called sorbents remove toxic and flammable gases, such as rotten-egg smelling hydrogen sulfide from natural gas.

The gas must first be treated with a solution of chemical sorbents that are dissolved in water. That solution must then be heated up and boiled to remove the hydrogen sulfide, in order to prepare the sorbent for future use. Once the hydrogen sulfide is boiled off, the sorbent is then cooled and ready for use again. The repeated heating and cooling requires a lot of energy and markedly reduces the efficiency of the process, scientists say.

The new, Battelle-created process called Antisolvent Swing Regeneration takes advantage of hydrogen sulfide's ability to dissolve better in some liquids than others at room temperatures. In this process, the hydrogen sulfide "swings" between different liquids during the processing at nearly room temperature, resulting in its removal, in just a few steps, from liquids that can be reused again and again.

"Because hydrogen sulfide is such a common contaminant in methane, natural gas processors could potentially use this method in the sweetening process, reducing their energy use and saving money on the cost of sorbent materials," said Phillip Koech, lead author and senior research scientist.

In the new work, Koech and colleagues tested how well they could swing hydrogen sulfide through a series of processing liquids without using water or heat. They began with a substance known as a recyclable binding organic liquid that could hold onto hydrogen sulfide without the addition of water.

First, they dissolved hydrogen sulfide in several different recyclable binding organic liquids and found that nearly all of them could hold the chemical without added water. They found one -- DMEA -- that could hold the most hydrogen sulfide. A chemical analysis suggested that hydrogen sulfide forms a salt with DMEA, turning the DMEA from an oily liquid into something more like salty water, but not water at all.

Based on the chemical characteristics of the salty DMEA, the team thought the salt could be easily disrupted and turned back into the gas hydrogen sulfide by adding a liquid hydrocarbon called an alkane. First, they mixed the hydrogen sulfide-containing DMEA with the alkane known as hexane and shook it like a bottle of salad dressing. Most of the hydrogen sulfide returned to its gaseous nature and bubbled out of the mix, leaving a soup of DMEA and hexane.

Having successfully removed the hydrogen sulfide from the DMEA, the team needed to find an alkane that would separate the hexane and the DMEA, and found one in hexadecane, which separates from DMEA in the same way that oil and vinegar drift apart in salad dressing. The team suggested the components separated due to a bit of salt left in the DMEA.

However, unlike hexane's ability to perform at room temperature, the team had to warm the DMEA-hexadecane just a little -- to about 40 degrees Celsius (104 degrees Fahrenheit), the temperature of a hot summer day -- to get the liquids to release the hydrogen sulfide. After the gas bubbled off and the two liquids separated, the team could pour off the hexadecane and re-use the left over DMEA.

Lastly, the researchers tested how well the chemicals could be re-used by recycling the hydrogen sulfide through the DMEA and hexadecane five times. The liquids retained their ability to remove the hydrogen sulfide and recover the DMEA in its initial form. The team expects DMEA will be able to pull hydrogen sulfide from natural gas using this process and they expect to scale up the process with future research.

This chemical process, called a polarity swing, occurs naturally at nearly room temperature, drastically reducing the need for heat during sweetening. Scientists estimate this method could cut the amount of energy needed to complete the sweetening process by at least 10 percent.

In addition to energy savings, scientists say there are other potential benefits of using Antisolvent Swing Regeneration.

"Applying ASSR to natural gas sweetening could result in a more environmentally friendly process because hexadecane is non-toxic," said David Heldebrant, corresponding author and project manager.

"We also anticipate chemical sorbents could last longer because they are not subjected to repeated heating and cooling, which degrade the sorbent."

Battelle's Independent Research and Development fund supported this work. Patents are pending on this technology and it is now available for licensing worldwide.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by DOE/Pacific Northwest National Laboratory.

Journal Reference:

Phillip K. Koech, James E. Rainbolt, Mark D. Bearden, Feng Zheng, David J. Heldebrant. Chemically selective gas sweetening without thermal-swing regeneration. Energy & Environmental Science, 2011; DOI: 10.1039/c0ee00839g

New desalination process developed using carbon nanotubes

A faster, better and cheaper desalination process enhanced by carbon nanotubes has been developed by NJIT Professor Somenath Mitra. The process creates a unique new architecture for the membrane distillation process by immobilizing carbon nanotubes in the membrane pores. Conventional approaches to desalination are thermal distillation and reverse osmosis.

"Unfortunately the current membrane distillation method is too expensive for use in countries and municipalities that need potable water," said Mitra. "Generally only industry, where waste heat is freely available, uses this process. However, we're hoping our new work will have far-reaching consequences bringing good, clean water to the people who need it."

The process is outlined by Mitra and his research team in the current issue of the American Chemistry Society's Applied Materials & Interfaces. Doctoral students Ken Gethard and Ornthida Sae-Khow worked on the project. Mitra is chairman of the department of chemistry and environmental science.

Membrane distillation is a water purification process in which heated salt water passes through a tube-like membrane, called a hollow fiber. "Think of your intestines," said Mitra. "It's designed in such a way that nutrition passes through but not the waste." Using a similar structure, membrane distillation allows only water vapor to pass through the walls of the hollow tube, but not the liquid. When the system works, potable water emerges from the net flux of water vapor which moves from the warm to the cool side. At the same time, saline or salt water passes as body waste would through the fiber.

Membrane distillation offers several advantages. It's a clean, non-toxic technology and can be carried out at 60-90oC. This temperature is significantly lower than conventional distillation which uses higher temperatures. Reverse osmosis uses relatively high pressure.

Nevertheless, membrane distillation is not trouble free. It is costly and getting the membrane to work properly and efficiently can be difficult. "The biggest challenge," said Mitra, "is finding appropriate membranes that encourage high water vapor flux but prevent salt from passing through."

Mitra's new method creates a better membrane by immobilizing carbon nanotubes in the pores. The novel architecture not only increases vapor permeation but also prevents liquid water from clogging the membrane pores. Test outcomes show dramatic increases in both reductions in salt and water production. "That's a remarkable accomplishment and one we are proud to publish," said Mitra.

Another advantage is that the new process can facilitate membrane distillation at a relatively lower temperature, higher flow rate and higher salt concentration. Compared to a plain membrane, this new distillation process demonstrates the same level of salt reduction at a 20°C lower temperature, and at a flow rate six times greater.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by New Jersey Institute of Technology.

Journal Reference:

Ken Gethard, Ornthida Sae-Khow, Somenath Mitra. Water Desalination Using Carbon-Nanotube-Enhanced Membrane Distillation. ACS Applied Materials & Interfaces, 2011; 3 (2): 110 DOI: 10.1021/am100981s

Trapping a rainbow: Researchers slow broadband light waves with nanoplasmonic structures

A team of electrical engineers and chemists at Lehigh University have experimentally verified the "rainbow" trapping effect, demonstrating that plasmonic structures can slow down light waves over a broad range of wavelengths.

The idea that a rainbow of broadband light could be slowed down or stopped using plasmonic structures has only recently been predicted in theoretical studies of metamaterials. The Lehigh experiment employed focused ion beams to mill a series of increasingly deeper, nanosized grooves into a thin sheet of silver. By focusing light along this plasmonic structure, this series of grooves or nano-gratings slowed each wavelength of optical light, essentially capturing each individual color of the visible spectrum at different points along the grating. The findings hold promise for improved data storage, optical data processing, solar cells, bio sensors and other technologies.

While the notion of slowing light or trapping a rainbow sounds like ad speak, finding practical ways to control photons -- the particles that makes up light -- could significantly improve the capacity of data storage systems and speed the processing of optical data.

The research required the ability to engineer a metallic surface to produce nanoscale periodic gratings with varying groove depths. This alters the optical properties of the nanopatterned metallic surface, called Surface Dispersion Engineering. The broadband surface light waves are then trapped along this plasmonic metallic surface with each wavelength trapped at a different groove depth, resulting in a trapped rainbow of light.

Through direct optical measurements, the team showed that light of different wavelengths in the 500-700nm region was "trapped" at different positions along the grating, consistent with computer simulations. To prepare the nanopattern gratings required "milling" 150nm wide rectangular grooves every 520nm along the surface of a 300-nm-thick silver sheet. While intrinsic metal loss on the surface of the metal did not permit the complete "stopping" of these plasmons, future research may look into compensating this loss in an effort to stop light altogether.

"Metamaterials, which are man-made materials with feature sizes smaller than the wavelength of light, offer novel applications in nanophotonics, photovoltaic devices, and biosensors on a chip," said Filbert J. Bartoli, principal investigator, professor and chair of the Department of Electrical and Computer Engineering. "Creating such nanoscale patterns on a metal film allows us to control and manipulate light propogation. The findings of this paper present an unambiguous experimental demonstration of rainbow trapping in plasmonic nanostructures, and represents an important step in this direction."

"This technology for slowing light at room temperature can be integrated with other materials and components, which could lead to novel platforms for optical circuits. The ability of surface plasmons to concentrate light within nanoscale dimensions makes them very promising for the development of biosensors on chip and the study of nonlinear optical interactions," said Qiaoqiang Gan, who completed this work while a doctoral candidate at Lehigh University, and is now an assistant professor in the Department of Electrical Engineering , State University of New York at Buffalo.

The study was conducted by Bartoli, Qiaoqiang Gan, Yongkang Gao and Yujie J. Ding of the Center for Optical Technologies in the Department of Electrical and Computer Engineering at Lehigh University; and Kyle Wagner and Dmitri V. Vezenov of the Department of Chemistry at Lehigh.

The study was funded by the National Science Foundation. It is published in the current issue of the Proceedings of the National Academy of Sciences.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Lehigh University, via EurekAlert!, a service of AAAS.

Journal Reference:

Qiaoqiang Gan, Yongkang Gao, Kyle Wagner, Dmitri Vezenov, Yujie J. Ding, Filbert J. Bartoli. Experimental verification of the rainbow trapping effect in adiabatic plasmonic gratings. Proceedings of the National Academy of Sciences, 2011; DOI: 10.1073/pnas.1014963108

Finding of long-sought drug target structure may expedite drug discovery

Researchers have solved the three-dimensional structure of a key biological receptor. The finding has the potential to speed drug discovery in many areas, from arthritis to respiratory disorders to wound healing, because it enables chemists to better examine and design molecules for use in experimental drugs.

The researchers are from the National Institutes of Health, collaborating with labs at The Scripps Research Institute and the University of California, San Diego. The finding is published in the March 10 edition of Science Express.

"This is an important step forward -- it was impossible until recently to know how this type of receptor is switched on by chemical signals like a tiny machine," said Dr. Kenneth A. Jacobson, chief of the Laboratory of Bioorganic Chemistry in NIH's National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) and an author on the paper. "The architecture of the activated receptor allows us to think in more detailed terms about the other half of the drug interaction. We hope that we're on the verge of a revolution that will expedite the process of crafting new drugs to treat disease."

With this finding, scientists in Jacobson's lab, including co-author Dr. Zhan-Guo Gao, will next work on testing this drug-engineering approach with similar molecules they have newly synthesized.

Jacobson and Gao are part of the NIDDK's intramural program, which enables basic scientists and clinicians of diverse skills and expertise to collaborate on solutions to some of the most difficult issues of human health. Several compounds from Jacobson's lab are currently in clinical trials as potential treatments for conditions including chronic hepatitis C, psoriasis and rheumatoid arthritis.

"Discoveries like this, with the potential to lead to future treatments in a wide variety of areas, are why NIH funds basic science," said NIDDK Director Dr. Griffin P. Rodgers. "By understanding the body at its smallest components, we can learn how to improve whole-body health."

A receptor is a protein that receives and sends signals to other molecules. The three-dimensional structure of the solved receptor also contains an agonist -- a chemical command signal from outside the cell -- in this case, an adenosine molecule. Similar to the function of a telephone receiver, the receptor acts as a sensor, picking up the message from the agonist and transmitting its information, which begins processes inside the cell.

The researchers discovered that a previously known agonist molecule would bind to its receptor target in a way that stabilizes the protein for crystallization. Once crystallized, the structure can be seen by bombarding it with X-rays. The agonist solidifies the protein by connecting to multiple parts of the receptor with its molecular arms, in the process initiating the function of the entire structure. This adenosine receptor, called A2A, counteracts inflammation and responds to organs in distress. It belongs to the G-protein coupled receptor family, which is involved in processes necessary for many drugs currently in use to take effect. These findings may lead to new drugs for many diseases.

The research was also supported by the National Cancer Institute and the National Institute of General Medical Sciences, both components of the NIH.

"Long-term NIH technology investments in structural biology, including the Protein Structure Initiative, have brought diverse teams of investigators together and yielded powerful methods like the ones used in this study," said NIGMS Director Dr. Jeremy M. Berg. "Receptors must undergo substantial changes in shape in order to function, and revealing these molecular dances in such great detail is an impressive accomplishment."

The NIDDK, a component of the National Institutes of Health (NIH), conducts and supports research on diabetes and other endocrine and metabolic diseases; digestive diseases, nutrition and obesity; and kidney, urologic and hematologic diseases. Spanning the full spectrum of medicine and afflicting people of all ages and ethnic groups, these diseases encompass some of the most common, severe and disabling conditions affecting Americans.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by NIH/National Institute of Diabetes and Digestive and Kidney Diseases.

Journal Reference:

F. Xu, H. Wu, V. Katritch, G. W. Han, K. A. Jacobson, Z.-G. Gao, V. Cherezov, R. C. Stevens. Structure of an Agonist-Bound Human A2A Adenosine Receptor. Science, 2011; DOI: 10.1126/science.1202793

Researchers find novel role for calcium channels in pacemaker cell function

Pacemaker cells in the sinoatrial node control heart rate, but what controls the ticking of these pacemaker cells? New research by Angelo Torrente and his colleagues of the M.E. Mangoni group's, reveals, for the first time, a critical functional interaction between Cav1.3 calcium ion (Ca2+) channels and ryanodine-receptor (RyR) mediated Ca2+ signaling.

The study also sheds light on a long-standing debate regarding the relative contributions of the 'funny current' generated by ion channels and the RyR dependent spontaneous diastolic Ca2+ release theory in determining heart rate.

The investigation by the research team compared pacemaker cells in normal mice with mutants that lacked the L-type Cav1.3 channels to contrast how they handled calcium. They found that the absence of Cav1.3 channels in sinoatrial node (SAN) cells reduced the frequency of Ca2+ transients, which determine the rate of cardiac muscle contraction. The Cav1.3 channels were also found to be important regulators of ryanodine-receptor dependent local calcium release in the diastolic pacemaker phase. Overall, their results show that local calcium release in SAN cells is tightly controlled by the Cav1.3 channels.

Defects in calcium channels controlling heart muscle function are known to cause heart failure, and this study reveals that Cav1.3 mutant mice also suffer from bradycardia and other cardiac arrhythmias.

"Our results clarify the role of Cav1.3 channels in pacemaker generation, and are a step towards using it as a target for drug therapy to treat heart dysfunction related to the sinoatrial node", says A. Torrente of CNRS in Montpellier, France, who was the lead author on the study.

Not only Cav1.3 channels are critical to the heart pacemaker cell function, they appear to be important to several other cellular mechanisms as well. In both humans and mice, Cav1.3 mutations have been linked to sinoatrial node dysfunction and deafness (or SANDD) syndrome. Cav1.3 channels are believed to play a role in pancreatic ß-cell stimulation, and they may also serve as pacemaker channels in the central nervous system, playing a pathophysiological role in Parkinson's disease.

"A better understanding of these channels in SAN could help us to comprehend the mechanism of calcium release in many other tissues and disease conditions as well", says Torrente.

More information: The presentation, "CAV1.3 L-TYPE CALCIUM CHANNELS-MEDIATED RYANODINE RECEPTOR DEPENDENT CALCIUM RELEASE CONTROLS HEART RATE" is at 10:30 a.m. on Wednesday, March 9, 2011 in Hall C of the Baltimore Convention Center. ABSTRACT: http://tinyurl.com/4pkjxgk

Provided by American Institute of Physics

Light-sensing receptor plays role in temperature sensation: study

A light-sensing receptor that's packed inside the eye's photoreceptor cells has an altogether surprising role in cells elsewhere in the body, Johns Hopkins scientists have discovered. Using fruit flies, they showed that this protein, called rhodopsin, also is critical for sensing temperature.

A report on the work appears March 11 in Science.

“For decades, this well-known molecule — one of the most-studied sensory receptors — was thought to function exclusively in the eye as a light receptor, but now we have found that fly larvae and possibly other organisms use it to distinguish between slight temperature differences,” says Craig Montell, Ph.D., a professor of biological chemistry and member of the Center for Sensory Biology in the Institute for Basic Biomedical Sciences. “And it makes you wonder about what was the more ancient role for rhodopsin — was it used originally for light or temperature detection?”

The Hopkins team identified rhodopsin while investigating the process that results in the activation of a temperature-sensor protein known as a TRPA1, one of many so-called “trip” channels abundant on sensory cells that receive communication from the outside world. Montell discovered earlier that TRPA1 enables fly larvae to detect tiny changes in the range of temperature that’s optimal for their survival. However, unlike TRP channels that function in avoiding hot and cold temperatures, TRPA1 was not directly turned on by changes in temperature in the comfortable temperature range, which extends from 18 to 24 degrees centigrade (equivalent to about 64 to 75 degrees Fahrenheit).

The team set out to determine what receptor responds to the temperature in order to set off the signaling cascade that results in TRPA1 activation.

A reasonable place to start looking for likely suspects, Montell said, was the large family of G-protein coupled receptors, because they are cell-surface molecules known to activate TRP channels. Still, the researchers were faced with more than a hundred possible gene candidates, each coding for a different G-protein coupled receptor in flies: If it was a GPCR, then which one?

“There were no precedents for a GPCR functioning in thermosensation, leaving us wonder where to start,” Montell says. “We considered rhodopsin, even though it was thought to be required exclusively for light reception, because some of the other proteins that we showed previously to function in thermosensation were required in photoreceptor cells.”

Using larvae missing the gene that codes for rhodopsin, the team conducted a series of tests to compare their behaviors with normal (wild-type) animals. The researchers released about 75 larvae on a plate with two temperature zones; half of the plate was kept at their favorite temperature of 18 degrees C, and the other at an alternative temperature, ranging from 14 to 32 degrees C. After 10 minutes, the researchers counted the number of larvae that had crawled to the 18-degree side, and the number on the side with the alternative temperature. They discovered that in contrast to the wild-type larvae, which preferred 18 degrees over any other temperature, the larvae lacking rhodopsin couldn’t discriminate temperatures in comfortable range, just like the larvae lacking TRPA1. However, the rhodopsin mutant larvae were able to choose 18 degrees over temperatures that were too hot or cold.

“The genetics and the behavior show that rhodopsin is required for thermosensation,” Montell says. “Larvae that contain mutations disrupting rhodopsin are profoundly defective in their ability to sense temperatures, but only in the comfortable range. The simplest interpretation of these results is that rhodopsin is activated by temperature and this in turn, activates TRPA1. However, we cannot exclude that there is an additional accessory protein required for rhodopsin to act as a thermosensor.”

This rhodopsin which functioned in “feeling” temperature was required in a new type of thermosensory neuron in the body wall of fruit fly larvae as well as in neurons in the head region of the animals.

Montell says this new thermosensing role for rhodopsin has absolutely nothing to do with light. Wild-type fly larvae kept in a dark box were able to choose the preferred 18 degrees centigrade over 24 degrees C.

The indirect activation of the TRPA1 channel via a signaling cascade that requires rhodopsin most likely represents “a quality of life issue” for the larvae, Montell muses. It allows to them to give up avoiding temperatures that are slightly less preferred than 18 degrees and to adapt if they can’t find their favorite temperature in their thermal landscape. Direct activation of TRP channels by noxious temperatures is more about survival.

Provided by Johns Hopkins University (news : web)

Researchers find drug that stops progression of Parkinson's disease in mice

In a major breakthrough in the battle against Parkinson's disease, researchers at the University of Colorado School of Medicine have discovered a drug that stops the progression of the degenerative illness in mice and is now being tested in humans.

"Drugs currently used to treat Parkinson's disease just treat symptoms; they do not stop the disease from getting worse," said senior author Curt Freed, MD, who heads the division of Clinical Pharmacology and Toxicology at the CU School of Medicine. "We've now discovered that we can prevent the progression of the disease by turning on a protective gene in the brain."

The results have been published online in the Journal of Biological Chemistry.

Lead author Wenbo Zhou, PhD, Assistant Professor of Medicine, and Freed, a national pioneer in Parkinson's research, have found that the drug phenylbutyrate turns on a gene that can protect dopamine neurons in Parkinson's disease. The gene, called DJ-1, can increase production of antioxidants like glutathione to reduce the debilitating effects of excess oxygen in brain cells. In addition, activating DJ-1 helps cells eliminate abnormal proteins that otherwise accumulate and kill brain cells. Dopamine neurons are particularly susceptible to too much oxygen and abnormal protein deposits. Parkinson's disease is caused by dying midbrain dopamine neurons.

Zhou and Freed have studied the DJ-1 gene since 2003 when a European group discovered that mutations in DJ-1 could cause Parkinson's disease. The Colorado scientists immediately started work to see why the gene was so important and have published a series of papers on the subject since 2005. But to convert their findings into a practical treatment for Parkinson's disease, they needed to find a drug to turn on the DJ-1 gene.

"We know some drugs can turn on genes. For example, steroids like testosterone act on genes in muscle cells to create muscle bulk," said Freed.

After testing many drugs, the team found that phenylbutyrate could activate DJ-1 and keep dopamine neurons from dying. Next, they put the drug in the drinking water of mice genetically programmed to get Parkinson's disease as they aged.

Aging mice receiving the drug were able to move normally, had no decline in mental function, and their brains did not accumulate the protein that causes Parkinson's. By contrast, older animals that did not get the drug saw a steady decline in their ability to move as their brains were damaged by abnormal proteins.

The researchers began giving phenylbutyrate to people in 2009, to test the safety of the drug in Parkinson patients.

Zhou and Freed will publish the human results in the coming months.

"We look forward to a future when Parkinson patients will be able to take a pill that will turn on the DJ-1 gene and stop the progressive disability associated with the illness," Freed said. "Right now, when you get the diagnosis of Parkinson's, you can expect to have a steady decline in the ability to move. While drugs like L-DOPA are very important for generating dopamine in the brain and making movement possible, these drugs have little impact on the ongoing deterioration of the patients' own brain cells."

Over one million people in the United State have the disease which usually strikes those in their 50s and 60s. Patients have a decline in their ability to walk, talk, and write because of slow movement and rigid muscles. They develop tremors and reflexes slow down. The current treatment of Parkinson's is based on drugs that increase dopamine production in the brain.

Freed is a national leader in transplanting dopamine cells into the human brain to relieve symptoms. He and his neurosurgical colleague Robert Breeze, MD, have done the operation in 61 patients, more than any other group in the world. The procedure can replace the need for drugs but even cell transplants do not prevent the progression of the disease.

Freed and Zhou are now looking for other drugs that might turn on the DJ-1 gene. One drawback of phenylbutyrate is that patients must take very large doses, 16 grams per day or 32 large tablets taken at frequent intervals. While the drug is approved by the FDA for treating a rare genetic disease in infants, whether it can stop Parkinson's in people remains to be seen.

But Zhou and Freed believe the discovery offers new hope for those suffering from Parkinson's disease.

"If we can say to someone that as of today we can stop your disease from getting worse, that would be a truly significant achievement," Freed said.

More information: http://www.jbc.org/content/early/2011/03/03/jbc.M110.211029.full.pdf+html