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Research Roundup

Surfactant Curtails Nanotube Clumping in Water
Scientists have long touted carbon nanotubes as a futuristic means of delivering drugs, fortifying brittle materials and conducting current in miniaturized circuits. But attempts to introduce actual nanotubes into these roles have often been stopped in their tracks by the slender filaments' stubborn and unhelpful tendency to clump together in solution.

Now scientists at Penn have found that a readily available chemical, a surfactant called sodium dodecylbenzene sulfonate (NaDDBS), disperses nanotubes in water with remarkable efficiency. The discovery, described in a paper published in January in the journal Nanoletters, represents an important step towards wider applications of nanotubes.

"Scientists have suggested many possible applications for carbon nanotubes, but tube aggregation in solution has obstructed progress," said lead author Dr. Mohammad F. Islam, a postdoctoral researcher in Penn's Department of Physics and Astronomy. "This new approach improves our ability to manipulate single tubes. Single nanotubes can now participate in controlled self-assembly, form fibers and composites, and serve as microfluidic sensors in water."

When Dr. Islam and senior author Dr. Arjun G. Yodh, professor of physics, added NaDDBS to a cocktail of water and nanotubes, the surfactant adhered weakly to the nanotubes, preventing the tubes from clinging to one another. Dr. Islam, Dr. Yodh and colleagues determined that NaDDBS increased the concentration of single carbon nanotubes in water up to 100-fold. Even at high concentrations, roughly 63 percent of nanotubes in aqueous solution remained unbound.

"Sodium dodecylbenzene sulfonate is pretty non-invasive, so we expect that the nanotubes' unique electronic, thermal, optical and mechanical properties will be preserved in suspension," said Dr. Yodh. "An added bonus of our complete solubilization approach is that it is gentle. Mixing this particular surfactant with nanotubes and water in a low-power, high-frequency sonicator, as we did, resulted in very little breakage of the nanotubes, which has been a problem with other treatments."

Carbon nanotubes tend to cling together because they are subject to substantial van der Waals attractions. While researchers have explored numerous surfactants to counter this attraction, Dr. Islam and Dr. Yodh suggest that NaDDBS's benzene ring, together with its long alkane tail and charge group, conspire to produce an unusual molecular arrangement on the nanotube surface that reduces aggregation.

Drs. Islam and Yodh were joined on the Nanoletters paper by co-authors Dr. Enrique Rojas, Dr. D.M. Bergey and Dr. Alan T. "Charlie" Johnson, all of the Department of Physics and Astronomy and LRSM. The research was funded by the NSF, NASA and the Petroleum Research Fund.

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Dogs Fed a Reduced-Calorie Diet Live Longer
A 14-year study of canine diet and health has found that dogs fed a calorie-restricted diet live a median 1.8 years longer than dogs allowed to eat more and are slower to develop chronic diseases such as osteoarthritis.

The findings add to the growing body of evidence that caloric restriction in a wide range of species significantly boosts longevity. Dogs are the only large mammals--and the closest human relatives-- for which a diet-restriction study has been completed. Similar studies involving primates are ongoing.

The results, from scientists at Penn's School of Veterinary Medicine, Nestle Purina PetCare Company, University of Illinois, Cornell University and Michigan State University, were the subject of a September symposium in St. Louis. Partial results were published last May in the Journal of the American Veterinary Medical Association.

The study involved 48 labrador retrievers from seven litters. Littermates were paired, with one dog fed 25 percent fewer calories than its sibling starting at 8 weeks of age. The researchers found a median life span of 13 years among dogs whose food intake was reduced, while dogs in the group fed a diet higher in calories were uniformly overweight and had a median life span of 11.2 years.

"Impressive as they are, the life span figures are only part of the story," said Dr. Gail K. Smith, professor of orthopedic surgery at Penn and chair of the Department of Clinical Studies at the School of Veterinary Medicine. "The study also showed that lean body conformation forestalls some chronic illnesses, most notably osteoarthritis, and that diet can either mitigate or exacerbate the expression of genetic diseases.

"This study should reinforce for dog owners the importance of keeping their dogs lean, with palpable ribs and an obvious waistline," Dr. Smith said. "Avoid giving dogs too many high-calorie treats and consider a brand of balanced dog food formulated to be low in caloric content while providing a sense of satiety."

"Dogs in the calorie-restricted group didn't require treatment for osteoarthritis until a mean age of 13.3 years, fully three years later than the dogs in the control group," Smith said. "Because osteoarthritis is painful, this deferral represents a substantial boost in quality of life."

Dr. Smith was joined in the study, funded and conducted by Nestle Purina PetCare, by Darryl N. Biery at Penn; Richard D. Kealy, Dennis F. Lawler and Joan M. Ballam at Nestle Purina; Elizabeth H. Greeley and Mariangela Segre at Illinois; George Lust at Cornell; and Howard D. Stowe at Michigan State.

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"Jumping Genes" May Aid in Discovery of Gene Function
Researchers at Penn's School of Medicine have bred a mouse to model human L1 retrotransposons, the so-called "jumping genes." Retrotransposons are small stretches of DNA that are copied from one location in the genome and inserted elsewhere, typically during the genesis of sperm and egg cells. The L1 variety of retrotransposons, in particular, are responsible for about one third of the human genome.

The mouse model of L1 retrotransposition is expected to increase our understanding of the nature of jumping genes and their implication in disease. According to the Penn researchers, the mouse model may also prove to be a useful tool for studying how a gene functions by knocking it out through L1 insertion. Their report was in the December, 2002 issue of Nature Genetics.

"There are about a half million L1 sequences in the human genome, of which 80 to 100 remain an active source of mutation," said Dr. Haig H. Kazazian, Jr., chair of the Department of Genetics and senior author in the study. "This animal model will help us better understand how this happens, as well as provide a useful tool for discovering the function of known genes."

In humans, retrotransposons cause mutations in germ line cells, such as sperm, which continually divide and multiply. Like an errant bit of computer code that gets reproduced and spread online, retrotransposons are adept at being copied from one location and placed elsewhere in the chromosomes. When retrotransposons are inserted into important genes, they can cause disease, such as hemophilia and muscular dystrophy. On the other hand, retrotransposons have been around for 500 to 600 million years, and have contributed a lot to evolutionary change.

For some time, researchers have been trying to understand how retrotransposons affect the genome and, in addition, what science may learn from the techniques they employ. According to Dr. Kazazian and his colleagues, the mouse model displays high-frequency chromosome to chromosome retrotransposition of human L1s, which behave in exactly the same way as they do in humans. While the current tissue culture model works well, it does not mimic the way retrotransposons jump in chromosomes.

The researchers believe that by understanding the mechanics of retrotransposition, they might be able to use similar techniques for genetic therapies in humans. They also hope to learn more about the basic mysteries behind retrotransposition, such as why L1 retrotransposons only seem to effect the germ line and not any other type of cell in the body.

Funding for this research was provided by grants from the NIH.

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Optimism in HIV-Positive Patients May Lead to Risky Moves
New study findings suggest that HIV-positive patients who believe they will live for many years are more likely to miss medication doses and to not practice safe sex than their peers who are less hopeful. Optimism can often help patients cope with a medical condition; but these findings indicate that in the context of HIV, there can be negative consequences to a positive outlook, according to Dr. William C. Holmes, assistant professor of medicine and epidemiology, and medical student Joseph L. Pace.

The authors surveyed 220 HIV-positive people about their backgrounds, disease history, attitude about their illness and health behaviors. The investigators found that people were more likely to use negative words about being HIV-positive when they were first diagnosed than at the time they completed the surveys. Most of the patients said they thought they would live for many years, and 27 percent said they expected to reach old age. White respondents, those with less education, and patients with relatively low levels of CD4 cells were less likely to hold out hope for the future.

Those patients who said they were relatively optimistic about the future were twice as likely as those with relatively pessimistic outlooks to sometimes forget to take their medications, and they were almost twice as likely to report not practicing safe sex. About 26 percent of optimists and 13 percent of pessimists occasionally forgot to take their medications; 57 percent of optimists and 29 percent of pessimists said they did not always practice safe sex.

The full report, "HIV-Seropositive Individuals' Optimistic Beliefs About Prognosis and Relation to Medication and Safe Sex Adherence" was published in the September issue of the Journal of General Internal Medicine.

See past Research Roundup

  Almanac, Vol. 49, No. 21, February 11, 2003