Letting go

Nearly one third of the world’s population, from Australia to Germany to Argentina, is infected with the single-celled parasite Toxoplasma gondii, usually from eating undercooked, contaminated meat. (The U.S. infection rate is 22.5 percent.) Most people don’t experience symptoms other than a minor flu-like illness, but for pregnant women, and individuals with a weak immune system, toxoplasmosis can be disastrous. In the January 13, 2012, issue of Science, Boston College biologists Marc-Jan Gubbels and Gabor Marth reported their finding that a protein called DOC2, produced by a T. gondii gene, appears to play an essential role in the parasite’s reproductive process.

According to Gubbels, whose research has focused on the cell biology of T. gondii, the parasite reproduces only inside host cells. After it has penetrated a cell membrane and replicated, its offspring eventually burst the cell and enter tiny intracellular spaces, where they search for new host cells. The parasite’s mobility, Gubbels says, is made possible by organelles inside T. gondii that are called micronemes.

Located on one end of the parasite, these micronemes release a sort of biological “glue” in the presence of DOC2, enabling T. gondii to “grip” cell membranes as it moves between and into host cells. In the absence of DOC2, Gubbels says, the micronemes don’t secrete this sticky substance, and as a result, the parasite can’t gain traction. It simply writhes in one place, unable to move toward, or attach to, the membrane of a new host cell.

Marth and Gubbels’s team discovered DOC2’s role in T. gondii reproduction by observing mutated strains of the parasite with video microscopy. “We introduced a chemical that creates random mutations in the parasite’s DNA,” says Marth, a computational biologist. When he and Gubbels noticed a strain that that was unable to move from cell to cell, Marth analyzed its DNA for genes that might be responsible for the change. Using computer programs he developed, he compared the whole genomic DNA of the mutant T. gondii strain with that of normal (“wild-type”) parasites, revealing coding errors in a gene responsible for creating DOC2 proteins.

Gubbels and Marth note that the DOC2 gene is also present in the DNA of Plasmodium falciparum, a similar microbe that causes malaria, leading the scientists to speculate that this parasite may also need the protein to reproduce. If that’s true, and if scientists can develop a chemical safe for humans and able to block the activity of DOC2, the researchers reason, it might be possible to treat the disease by stopping the parasite’s reproductive cycle.

Splitting up

In a paper featured in the March 28 issue of the Journal of the American Chemical Society, Dunwei Wang, associate professor of chemistry, and a team of researchers in his Merkert Center laboratory published the results of what they call “proof-of-concept” work in developing materials to enhance the conversion of solar energy to hydrogen fuel by splitting water.

Their goal was to take a nontoxic, chemically stable semiconductor—they chose hematite (Fe2O3), an abundant form of iron oxide—and improve its material construction at the molecular scale so that it could facilitate the efficient separation of water into hydrogen and oxygen. (It’s possible to split water using other substances such as cadmium selenide, but these tend to be both highly expensive to produce and toxic).

When solar energy strikes a hematite crystal in water, it creates a weak negative charge by exciting electrons in the hematite. This movement creates “holes”—areas where electrons used to be—that attract electrons from passing water molecules. In the process, the water is broken into its two components: oxygen, which bubbles off as a gas, and hydrogen ions, which remain in the water.

In pure hematite this process lasts only a few seconds—ions quickly build up in the water, causing a chemical imbalance that halts the reaction. Producing a sustained reaction that can create hydrogen gas instead of ions requires a crystal able to generate a more powerful negative charge than pure hematite (roughly -0.4 volts, Wang says, is needed).

Using a method called atomic layer deposition (ALD), which allows the buildup of materials in precise, molecule-thick coats, Wang’s team constructed a crystal “layer cake” of two different types of hematite. Its bottom portion is a 20 nanometer-thick stratum of plain hematite, and its top layer, only 5 nanometers thick, is made of hematite that the researchers “doped,” or embedded, with magnesium atoms, which permit the creation of additional electron “holes.”

Wang says that by using ALD, he created a junction between the two layers that changed the electronic properties of the entire crystal, resulting in a greater overall negative charge. But exactly how this occurs at the atomic level is still unclear, he says.

For now, the team’s crystal, which is about the size of a postage stamp, can generate only half of the -0.4 volts needed to create hydrogen gas. Still, Wang says, “By using that layer [of magnesium-doped hematite], we’ve shown that we’re pushing the system in a direction where it will eventually be able to produce hydrogen.”

David Levin is a science writer based in Boston.

Read more by David Levin