Heavy water, light uranium: One sweet contrast

Before going any further, it’s worth acknowledging that the terms “heavy” and “light” are relative, and an atom of the new ultralight isotope of uranium, with 122 neutrons and an atomic weight of 214, is still over 10 times more massive than a molecule of heavy water, with an atomic weight of 20.

Sweetness in moderation: Heavy water (D₂O) is stable and naturally occurring. It differs from ordinary water (H₂O) through the substitution of deuterium (so-called heavy hydrogen) for hydrogen. Because it is less likely to absorb neutrons than H₂O, purified heavy water is used as a moderator and coolant in some nuclear power reactor designs, most notably in Canada’s fleet of CANDU pressurized heavy-water reactors.

Deuterium was discovered in 1931 by Harold Urey, who received a Nobel Prize for his work. Soon after, scientists began to compare anecdotal reports after tasting heavy water. In 1935, Science published a short letter by Urey stating unequivocally after a blind taste test by two subjects (one of which was Urey himself) that “pure deuterium oxide has the same taste as ordinary distilled water.” Recent research, however, has proven otherwise.

Pavel Jungwirth and Phil Mason, of the Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences (IOCB Prague), led a research team that used molecular dynamics simulations, cell-based experiments, mouse models, and human subjects to show conclusively that heavy water tastes sweet to humans (but not, incidentally, to mice). In their research, published in Communications Biology, they concluded that the effect was mediated by the human sweet taste receptor TAS1R2/TAS1R3.

“Despite the fact that the two isotopes are nominally chemically identical, we have shown conclusively that humans can distinguish by taste (which is based on chemical sensing) between H₂O and D₂O, with the latter having a distinct sweet taste,” said Jungwirth, in an article published on April 7 by IOCB Prague. “Our study thus resolves an old controversy concerning the sweet taste of heavy water using state-of-the-art experimental and computer modeling approaches, demonstrating that a small nuclear quantum effect can have a pronounced influence on such a basic biological function as taste recognition.”

The researchers also found that heavy water can make chemical sweeteners taste even sweeter, but it has no future in the beverage industry because in large concentrations, heavy water is toxic to animals and plants.

The sweet taste receptor responsible for the perceived sweetness is located not only in the human tongue but also in other tissues, and since heavy water is used in some medical procedures, the researchers believe that their findings could have clinical applications.

Ultralight uranium: A research team led by Zai-Guo Gan at the Chinese Academy of Sciences has created a new uranium isotope in a “fusion-evaporation” reaction by firing a beam of argon at a tungsten target and monitoring the output. The research was carried out at the Heavy Ion Research Facility in Lanzhou and published in the journal Physical Review Letters on April 14.

Naturally occurring uranium typically contains either 143 neutrons (fissile uranium-235) or 146 neutrons (uranium-238). The newly confirmed isotope has just 122 neutrons, one fewer than the previous record for the element.

The researchers identified two previously discovered light uranium isotopes—uranium-216 and uranium-218—as well as the novel uranium-214, which has a half-life of 0.5 millisecond. Their findings could reportedly contribute to an understanding of alpha decay—the emission of an alpha particle consisting of two protons and two neutrons. The researchers observed that uranium-214 and uranium-216 decay more easily than do light isotopes of other elements. According to a summary published in Physics on April 14, “Measurements from the three observed uranium isotopes suggest that they experience an enhanced proton-neutron interaction compared with isotopes of other elements.”