This can be a bit shocking, but technically not all water on earth consists of H2O molecules.
Less than a century ago, the discovery of the hydrogen isotope deuterium – 2H, but often simplified to D – reveals the existence of another type of water with the chemical formula 2H2O or simply 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 the one important difference between H2O and D2O – which gives heavy water about 10 percent greater density than ordinary water – these two types of water are chemically the same, although deuterium shows slightly different binding behaviors than ordinary hydrogen (also known as protium).
Due to the altered binding behavior – which can affect body chemistry if you take deuterium in D2O – scientists 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 are often administered to participants in scientific experiments.
Due to such accidental consumption, which now goes back almost a century, there has been a long question as to whether heavy water tastes the same as ordinary drinking water – or whether the subtle isotopic variation produces a different taste that people might notice.
“There is anecdotal evidence from the 1930s that the taste of pure D2O is different from the neutral of pure H2Oh, what is mostly described as’ sweet ‘,’ explains an international team of researchers led by first writers and biochemists Natalie Ben Abu and Philip E. Mason in a new study.
‘Urey and Failla, however [the former being Harold Urey, the scientist who discovered deuterium] addressed this question in 1935 and came to the authoritative conclusion that ‘none of us could taste the slightest difference between the taste of ordinary distilled water and the taste of pure heavy water’. ‘
But was the conclusion a bit premature? Ben Abu and Mason say that Urey and Failla’s unequivocal views on the subject have effectively stifled further research in this area for most of the next century, at least in terms of human taste tests.
Tests in rats have shown that consuming too much heavy water can be fatal to the animals, but evidence whether rats can taste the difference remains unclear.
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 their team can finally confirm that there really is something different about the taste of heavy water.
“Despite the fact that the two isotopes are chemically identical, we have unequivocally shown that humans can distinguish between H according to taste (based on chemical observation)2O and D2Oh, with the latter a distinctly 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 H2O and D2Oh, and tests with mixed amounts of water have shown that larger amounts of heavy water are considered sweeter.
However, in tests with mice, the animals do not appear to drink heavy water over normal water, although they have a preference for sugar water, indicating that D in mice2Oh does not evoke the same sweet taste that people can experience.
Other taste tests done by the team indicate why this is so, indicating that human taste is receptive 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, with robust responses in TAS1R2 / TAS1R3 expressing cells when exposed to D.2O.
In addition, computational modeling with molecular dynamics simulations revealed slight differences in the interactions between proteins and H2O versus D2Oh, which according to the team needs to be studied further to explain in full, but it is consistent with previous research, and provides another example of nuclear quantum effects in chemical systems, including those of water.
“Our findings point to the human sweet taste receptor TAS1R2 / TAS1R3 as essential for sweetness of D2Oh, “concludes the authors.
At the molecular level, this general behavior can be traced back to the slightly stronger hydrogen bond in D2O vs H2Oh, which is due to a nuclear quantum effect, namely difference in zero point energy … Although it is not a practical sweetener, heavy water offers a glimpse into the wide chemical space of sweet molecules. ‘
The findings are presented in Communication Biology.