This may come as a bit of a shock, but technically, not all water on Earth is composed of H.2O molecules.
Less than a century ago, the discovery of the hydrogen isotope deuterium – 2H, but often simplified to D – revealed the existence of a different kind of water with the chemical formula 2H.2O or just D2O.
Here’s how they differ. A typical hydrogen atom contains one proton in its nucleus. However, the deuterium isotope has a neutron in addition to the proton, which gives the hydrogen atom a larger mass. Therefore, water formed with this type of heavy hydrogen is usually called … heavy water.
Apart from that one important difference between H2O and D2O – which gives heavy water about 10 percent more density than regular water – these two types of water are chemically the same, although deuterium exhibits slightly different bonding behavior than regular hydrogen (which is also known as protium, by the way).
Because of that altered attachment behavior – which can affect physical chemistry when you consume deuterium in D.2O – scientists generally say it’s 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 such occasional consumption, dating back nearly a century now, there has long been a question of whether heavy water tastes the same as regular drinking water – or whether its subtle isotopic variation produces a different flavor that humans might perceive.
“There is anecdotal evidence from the 1930s that the taste of pure D2O differs from the neutral of pure H.2Oh, which is usually described 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.
But Urey and Failla [the former being Harold Urey, the scientist who discovered deuterium] answered this question in 1935 and concluded with authority that in tasting, ‘neither of us could detect the slightest difference between the taste of plain distilled water and the taste of pure heavy water’. ‘
But was that conclusion a bit premature? Ben Abu and Mason say Urey and Failla’s unequivocal opinion on the subject has effectively stifled further research in this area for much of the next century, at least in terms of human taste testing.
Tests in rats have shown that too much heavy water use can be fatal to the animals, but the evidence as to whether rats can taste the difference remains unclear.
Over the past two decades, advancements in our understanding of human taste receptors have led to a reopening of old cases like this one – and in their new research, Ben Abu, Mason and their team can finally confirm that there really is something a little different about the taste of heavy water.
Despite the fact that the two isotopes are nominally chemically identical, we have convincingly shown that humans can distinguish by taste (which is based on chemical perception) between H2O and D2Oh, with the latter having a decidedly sweet taste, ”explains senior 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 H.2O and D2Oh, and tests with mixed amounts of water showed that larger amounts of heavy water were perceived as sweeter.
However, in tests with mice, the animals did not appear to prefer drinking heavy water over plain water, although they did show a preference for sugared water – suggesting that in mice D2O does not produce the same sweet taste that humans can perceive.
Other taste tests conducted by the team suggest why this is, indicating that human taste receptivity to D2O is mediated by the taste receptor TAS1R2 / TAS1R3, which is known to respond to sweetness in both natural sugars and artificial sweeteners.
Laboratory experiments with HEK 293 cells confirmed the same and showed robust responses in cells expressing TAS1R2 / TAS1R3 when exposed to D2O.
In addition, computer modeling with molecular dynamics simulations revealed small differences in the interactions between proteins and H.2O to D2Oh, which the team says needs further study to fully explain it, but it is consistent with previous research, and provides another example of nuclear quantum effects in chemical systems, including that of water.
“Our findings indicate that the human sweet taste receptor TAS1R2 / TAS1R3 is essential for the sweetness of D2Oh, ”the authors conclude.
At the molecular level, this general behavior can be traced back to the slightly stronger hydrogen bond in D.2O to H2Oh, which is due to a nuclear quantum effect, namely difference in zero point energy … While clearly not a practical sweetener, heavy water gives a glimpse into the wide open chemical space of sweet molecules. “
The findings are reported in Communication biology