This may sound a little surprising, but technically speaking, not all of Earth’s water is made up of H2Or molecules.
Less than a century ago, the discovery of the isotope hydrogen deuterium – 2H, but often simplified to D, revealed the existence of another type of water with the chemical formula 2H2Or or simply D2O.
This is how they differ. A typical hydrogen atom contains a proton inside its nucleus. The isotope of deuterium, however, has a neutron in addition to the proton, giving the hydrogen atom a larger mass. Therefore, water formed with this type of heavy hydrogen is often called … heavy water.
Apart from this key difference between H2O and D.2Or, that it provides heavy water with 10% more density than normal water, these two types of water are chemically the same, although deuterium exhibits a slightly different bonding behavior than normal hydrogen ( by the way, also known as proti).
Due to this altered binding behavior, it can affect body chemistry if deuterium is ingested in D.2Scientists generally say that it is not a good idea to drink heavy water, at least not in high doses.
However, small amounts are considered harmless to humans and, in fact, are often administered to participants in scientific experiments.
Because of this incidental consumption, which dates back almost a century, there has long been a question about whether heavy water tastes the same as normal drinking water or whether its subtle variation isotopic produces a different taste that people may perceive.
“There is anecdotal evidence from the 1930s that indicates that the taste of pure D2Or is different from the pure H neutral2Or, being described primarily as “sweet,” explains an international team of researchers led by early authors and biochemists Natalie Ben Abu and Philip E. Mason in a new study.
“However, Urey and Failla [the former being Harold Urey, the scientist who discovered deuterium] he addressed this question in 1935, concluding with authority that in tasting “none of us could detect the slightest difference between the taste of ordinary distilled water and the taste of pure heavy water.” “
But was this conclusion a little premature? Ben Abu and Mason say that Urey and Failla’s unequivocal opinion on the subject effectively stifled research in this area for much of the next century, at least in terms of evidence of human tastes.
Tests on rats have shown that excessive water consumption can be fatal to animals, but tests on whether rats can prove the difference are not yet clear.
In the last two decades or so, advances in our understanding of human taste receptors have led to the reopening of old cases like this, and in their new research, Ben Abu, Mason, and his team can finally confirm that they really there is something a little different about the taste of heavy water.
“Although the two isotopes are nominally chemically identical, we have conclusively shown that humans can distinguish by taste (which is based on chemical detection) between H2O and D.2Or, with that last clear sweet taste, ”explains author and physical chemist Pavel Jungwirth of the Czech Academy of Sciences.
In a taste test experiment with 28 participants, most people were able to distinguish between H2O and D.2Or, and tests with mixed amounts of water revealed that higher proportions of heavy water were perceived as a sweeter taste.
However, in tests with mice, the animals did not appear to prefer to drink heavy water over normal water, although they did show a preference for sugary water, suggesting that in mice, D2Or it doesn’t cause the same sweet taste that people can perceive.
Other taste tests performed by the team suggest why this is so, indicating that the receptivity of human taste to D2Or it is mediated by the TAS1R2 / TAS1R3 taste receptor, which is known to respond to sweetness in both natural sugars and artificial sweeteners.
Laboratory experiments with HEK 293 cells confirmed the same, showing robust responses to TAS1R2 / TAS1R3-expressing cells when exposed to D2O.
In addition, computational modeling with molecular dynamics simulations revealed slight differences in the interactions between proteins and H2O versus D.2Or, that the team says it needs additional studies to fully explain it, but according to previous research, and provides another example of nuclear quantum effects in chemical systems, including water.
“Our findings indicate that the TAS1R2 / TAS1R3 human sweet taste receptor is essential for the sweetness of D2Or, ”the authors conclude.
“At the molecular level, this general behavior can be traced back to the slightly stronger hydrogen bond of D2Or against H2Or, that it is due to a nuclear quantum effect, that is, to the difference of zero point energy … Although it is clearly not a practical sweetener, heavy water hints at the open chemical space of the sweet molecules. “
The findings are reported in Biology of Communications.