Magnetic isotope effects in biology: a marker for radical pair reactions and electromagnetic field effects?

To explore the possible usefulness of magnetic isotope effects, especially to detect processes affected by magnetic fields.

The theoretical model of Brocklehurst and McLauchlan is used to estimate the effects of isotopic substitution (e.g. 13C for 12C) on the yields of chemical reactions involving radical pairs.

It is demonstrated that in some circumstances isotope effects on the yields from initially singlet pairs (bond breaking to give radicals) could be very large and easily detected. The effects on radical pairs formed by random encounters are necessarily much smaller but may be detectable. Illustrative calculations are presented for carbon, hydrogen and oxygen isotopes; deuterium substitution also produces very large mass effects but its study could be useful to support work on carbon.

Effects may be small but worth looking for: distinguishing them from mass effects will be aided by measuring distributions within product molecules. In laboratory experiments (especially tomography) a combination of isotopic substitution and applied fields is a potentially powerful technique.