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HPC helps scientists reconsider DDT

Speed read
  • Insecticide DDT effective against malaria but harms environment
  • High-performance computing uncovers new crystalline forms
  • New forms prove more effective against insects leading to lighter environmental impact

DDT has a complicated history.

The chemical compound dichloro-diphenyl-trichloroethane was first synthesized in 1874. After its effectiveness as an insecticide was discovered in the 1940s, it soon saturated forests, fields, and swamps in an effort to eradicate everything from fire ants to malaria-carrying mosquitoes.

But DDT didn't discriminate — it also killed fish, birds, and threatened the survival of iconic species like the American bald eagle and peregrine falcon.<strong>Safe spray. </strong> Applying DDT insecticide along the banks of Tennessee River for malaria control. Computer simulations have found a new crystal form of the compound which could mean a more efficient, reduced application of the chemicals.

Recognizing the threat, scientist Rachel Carson published Silent Spring in 1962. Her landmark study revealed the harmful consequences of widespread aerial spraying of DDT and jump-started the modern environmental movement in the US.

The Federal Environmental Pesticide Control Act of 1972 banned the use of DDT in the US, and threatened species have since begun to rebound. But the World Health Organization still recommends the use of DDT in rotation with other pesticides as a prophylaxis against malaria.

Still endemic in Africa and southeast Asia, malaria infects over 200 million people each year and kills more than 400,000. More than two-thirds of those deaths are children under five.

“One has to balance the human cost against the environmental impact of using insecticides like DDT,” says New York University (NYU) chemist Bart Kahr, whose lab has discovered a new form of DDT that may have a lower environmental impact.

Less is more

DDT is a neurotoxin that kills insects by inhibiting certain proteins in cell membranes. An insect must walk directly across the crystal surface of DDT (e.g., on a leaf) and absorb the poison through their footpads in order to be effective.

All other animals—birds, fish, humans—receive DDT through the food chain. For example, fish ingest DDT runoff from large-scale spraying, and it’s stored in their fatty tissue. Birds eat the fish, and larger birds eat smaller birds.

Top predators such as eagles and falcons absorb the highest concentrations of the compound, which lead to weaker eggshells and population decline.

<strong>DDT</strong> works its way through the food chain, with the highest concentration ending up in the top predators such as eagles and falcons. The eggs of these predators weaken and pupulations suffer, placing the bald eagle, pictured here, at risk. Courtesy Dave Menke; US Fish and Wildlife Service.

But the newly discovered crystal form of DDT is more lethal to insects than the original, and it kills more quickly. The new form could be used to develop more efficient contact insecticides.

Mosquitoes would have less of a chance of developing resistance, and there would be less excess DDT to poison the food chain.

“If DDT is still going to be used to battle malaria,” says Kahr, “then the less we apply, the better it will be for the environment.”

Crystal gazing

Previously, DDT was thought to exist in only one form.

But recent advances in crystal structure prediction using high performance computing have allowed chemists to hypothesize possible new arrangements of its molecules.

Despite the molecules in both forms being nearly indistinguishable, the crucial difference lies in how the molecules are arranged with respect to one another.

“Imagine that, in one form, the crystals are like a pair of hands facing one another,” says Kahr. “While in the other the molecules are arranged such that one’s palm is facing the back of the other hand.”

Kahr’s lab collaborates with Qiang Zhu, assistant professor of physics and astronomy at the University of Nevada, Las Vegas (UNLV), to predict new crystal forms.

Zhu accessed the Stampede supercomputer via the Extreme Science and Educational Discovery Network (XSEDE) network, to run USPEX, an evolutionary algorithm that explores the potential space of molecular structures.

For generations it has been a real challenge to predict the solid state structure or structures of any molecule,” says Kahr. “And now, finally, scientists like Zhu are beginning to make real headway.”

Rewriting history

Despite excitement about the technical breakthrough of a new crystal form of DDT, Kahr remains cautious about his discovery.

<strong>Scientist, author, activist, hero. </strong> Rachel Carson published her landmark study in 1962. Silent Spring revealed the harmful consequences of widespread aerial spraying of DDT and jump-started the modern environmental movement in the US. Courtesy Rachel Carson.

So much so that the issue of Angewandte Chemie in which his results were published also includes a critical essay examining the legacy of Rachel Carson and subsequent attempts by the chemical industry to diminish the dangers of DDT in order to roll back environmental regulation.

“I grew up in the 1960s and 70s,” says Kahr. “Rachel Carson was a genuine American hero. It’s shocking to see how the internet has taken a heroic, poetic scientist and transformed her into a figure on par with Hitler or Stalin — it’s outrageous.”

History is complicated—and so is most of what you read on the internet. But every day in universities and research centers around the globe, scientists like Kahr and Zhu continue to make new discoveries and attempt to see our world more clearly.

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