# Biological activity of the isotopes of the same element

I have a question regarding biological activity of different isotopes of the same elements, notably $$\ce{^206Pb}$$ and $$\ce{^208Pb}$$. Lead is known to be toxic, such that it can replace calcium from bones and iron from hemoglobin for example. Would one isotope behave differently than another in such cases?

When it comes to hydrogen/deuterium chemical activities, some difference could arise as one isotope is twice as large as another. Would this be the case for heavier elements, such as lead?

• Your reasoning seems ok if radionuclides are not considered, of course. For heavy water, if I remember, there is a nice Wikipedia article. Mar 17 '21 at 9:31
• There are interesting but subtle isotope effects for many important elements common in biological systems (eg oxygen and carbon). These are widely used to determine the origin or local environmental temperature of old specimens. Mar 17 '21 at 12:32
• This earlier Q&A (Biological Consequences of Asteroid Mining—Death by Isotope?) might be of interest.
– ron
Mar 17 '21 at 15:16
• There is quite a bit of literature about lithium-6 versus lithium-7 for psychiatric use. Here is a review article: pubmed.ncbi.nlm.nih.gov/3530801 Also this: pubmed.ncbi.nlm.nih.gov/32084493 Mar 17 '21 at 15:38

I would not expect significant differences in the chemical properties (reactivity) of these lead isotopes, the differences in atomic weight being as you point out very small. There are reports that claim an effect. Ref 1 for instance makes such a claim, but there is no plausible mechanism presented to explain the reported isotopic discrimination. Double check what peer reviewers think (see e.g. the response in Ref 2 to the results presented in Ref 1).

References

1. Wu J, Liu D, Xie Q, Wang J (2012) Biological Fractionation of Lead Isotopes in Sprague-Dawley Rats Lead Poisoned via the Respiratory Tract. PLOS ONE 7(12): e52462. https://doi.org/10.1371/journal.pone.0052462

2. Gulson, Brian and Kamenov, George D. and Manton, William and Rabinowitz, Michael (2018) Concerns about Quadrupole ICP-MS Lead Isotopic Data and Interpretations in the Environment and Health Fields. International Journal of Environmental Research and Public Health 15(4): 723. https://doi.org/10.3390/ijerph15040723

I'm assuming we can neglect differences due to radioactivity/radioisotopes in biological systems.

## Hydrogen vs Deuterium

The kinetic isotope effect - the difference in reaction rates and reactivity for different isotopes of the same element - is well known for hydrogen and its two stable isotopes protium and deuterium. For example levels of deuteration above 25% have shown adverse effects in a variety of animals. [1]

## Heavy Elements

But stable isotopes of elements as heavy as carbon can have measurably different effects in biological systems.

### Carbon

In photosynthesis, lighter $$\ce{^12C}$$ is incorporated into plants faster than $$\ce{^13C}$$ creating a difference in the abundance of $$\ce{^13C}$$ in biological sources compared to other enviromental sources. Furthermore different methods of photosynthesis - i.e. the $$\ce{C_3}$$ and $$\ce{C_4}$$ pathways found in different groups of plants - lead to different discrepencies in $$\ce{^13C}$$ abundance ($$\delta^{13}\ce{C}$$) which can provide information about the diets and nutrient sources of the animals that eat them.