The reason for these adverse effects is that replacing hydrogen with its heavier isotope deuterium slows down the rate of any chemical reaction in which the chemical bond to the hydrogen atom is broken. This includes a great many chemical reactions occurring in biological systems, and not just those involving water; the hydrogen atoms from water end up in a number of other biomolecules, so any process involving these hydrogen atoms will also be slowed down. Thus the heavy water acts like a brake on a large number of metabolic processes.
The amount by which an isotopic substitution like this slows down a chemical reaction is called a kinetic isotope effect. Such effects are a major tool in the study of chemical reactions, including enzymatic reactions. Deuterium isotope effects can be as large as 6 or 7, which means that the reaction rate is 6 or 7 times slower when deuterium is substituted for hydrogen. In rare cases where a quantum mechanical effect called tunneling occurs in the reaction, deuterium isotope effects of 20 or more have been observed.
The major reason for the difference in the rates of the chemical reactions involving the two isotopes of hydrogen is the difference in their masses. Deuterium atoms have an atomic mass of 2, which is double that of normal hydrogen. Of course other atoms have isotopes also, and your comment that for isotopes of other elements these effects would not be present is perceptive but not quite completely correct. Isotope effects do occur with the heavier elements but they are much smaller. For instance if we replaced the oxygen of water (which is normally oxygen 16) with oxygen 18 we end up with a water molecule having the same mass as in the heavy water discussed above, but in which the isotope effects on its reactions would be very small. This is because changing the oxygen atomï¿½s mass from 16 to 18 is a much smaller fractional change than the doubling of mass of hydrogen when we go from hydrogen to deuterium. Oxygen-18 isotope effects are never more than about 1.07, or 7 % slower with the heavier isotope.
Answered by: Philip Carlson, Chemistry Undergrad Student
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