Heavy water (D2O) has fascinated scientists since the discovery of deuterium (1931) by U.S. Nobel laureate Urey. In 1935 D2O was found to be indistinguishable from ordinary water (H2O) in terms of taste, which had a significant impact on the ongoing debate on the subject.
According to a recent study by scientists from the Hebrew University of Jerusalem, the Czech Institute of Organic Chemistry and Biochemistry, and the Leibniz-Institute for Food Systems Biology at the Technical University of Munich, highly purified heavy water (D2O) has a noticeably sweeter taste than normal water of the same purity.
What is heavy water?
Heavy water (or deuterium oxide, D2O) is water in which the hydrogen atoms in ordinary water have been replaced by deuterium atoms to form D2O.
There are three naturally occurring isotopes of hydrogen (protium, deuterium, and tritium), which are three distinct forms of hydrogen atoms that differ in the number of neutrons in the nucleus. Normal hydrogen atoms contain one proton and no neutron, while deuterium atoms contain one proton and one neutron.
The physical and chemical properties of the two waters are similar, but not identical. Deuterium is about twice as heavy as hydrogen, and its density is 10 percent greater. This is why ice cubes made with heavy water float and float in it, but sink in ordinary water.
The differences in the properties of D2O and H2O, such as pH or melting and boiling points, are very small and are due to changes in zero-point vibrations that lead to slightly stronger hydrogen bonding in D2O than in H2O.
Although heavy water is not radioactive, it is not completely safe to drink. The problem is the greater mass of heavy water compared to ordinary water. Essentially, the difference in mass slows down the biochemical reactions that use water.
Prokaryotic organisms, such as bacteria, can function in heavy water, albeit with a slightly impeded growth rate. Plant and animal cells, on the other hand, are more complex, so too much heavy water causes disease or death. Heavy water disrupts mitosis, a type of cell division used to repair damage and grow new cells. The mitotic spindles of cells containing too much heavy water simply cannot divide the cell evenly to form two identical new cells.
Replacing 20% of normal cell water with heavy water is survivable for humans and other mammals (though not recommended). At about 25% deuteration (replacement) of body water, there is a detrimental effect on cell division and overall metabolism. In contrast, 50% deuterium in water causes death in animals and humans. Heavy water poisoning is similar to radiation poisoning or cytotoxic poisoning during chemotherapy.
What does heavy water taste like?
Although the two isotopes of water are chemically identical, and taste perception depends on chemical properties, scientists from the Czech Republic, Israel and Germany have shown in their study that unlike ordinary water, heavy water tastes sweet to humans but not to mice.
The researchers addressed the taste of heavy water in a series of sensory experiments in humans, behavioral experiments in mice, sweet taste cell tests and computational modeling.
Their results show that people perceive heavy water as clearly distinguishable from ordinary water based on its taste. According to the study authors, people in the experiment describe heavy water as mildly sweet and significantly sweeter than ordinary water. Moreover, D2O increased the perceived sweetness of other sweeteners at low concentrations. The findings also show that deuterated water induces sweet taste through activation of the human TAS1R2/TAS1R3 receptor. In contrast, taste experiments in mice show that these animals do not prefer D2O over H2O.
Heavy water, while not a practical sweetener, provides insight into the wide-open chemical space of sweet molecules. Heavy water can be used in medical procedures. The sweet taste receptor is located not only on the tongue, but also in other tissues of the human body. And the discovery that heavy water can elicit taste receptor responses provides important information for clinicians and their patients.