Int. J. Environ. Res. Public Health 2010, 7 3.2. Key Features of a Radical Pair Magnetoreceptor 944 As mentioned above, radical pair reactions were first proposed as a magnetoreceptor by Schulten and coauthors. The unique property of the radical pairs is that their chemical fate is largely controlled by weak (in the microTesla range) magnetic fields via their spin correlation. It was shown that the magnetic field effects on chemical reactions are much stronger in viscous solution, as in micelles. Figure 3. Reaction scheme for a radical pair reaction with magnetic field-dependent reaction products. The radical pair is generated by an electron transfer from a donor molecule D to an acceptor molecule A. An external magnetic field affects interconversion between singlet and triplet states of the radical pair. In these conditions, an applied weak magnetic field will result in an increased transient conversion of the radical pair into the triplet state, causing triplet products to be formed more rapidly and in higher yield (S—singlet state of the radical pair, T—triplet state of the pair). (Reproduced from [32] by permission of the publisher, with minor modification). Magnetic field effects in reactions of spin-correlated pairs are related to singlet-triplet transitions in these pairs, which can switch between the singlet and triplet channels of the reaction. These transitions usually take the form of dynamic oscillations (beats) between the singlet and triplet states of a pair, with the frequencies depending on both the strength of external magnetic field (as a consequence of Zeeman effect) and specific parameters of the radicals, such as g-factors and hyperfine coupling constants [41]. Through their effects on the evolution of the overall spin in a radical pair, hyperfine and Zeeman interactions (and consequently external magnetic field strength) may control the reaction yields into different reaction channels. The dependence of a reaction yield on the external magnetic field strength is called a MARY spectrum (MARY—Magnetically Affected Reaction Yields) [42]. Several models describing magnetic field influence on kinetics of enzymatic reactions that involve free radical-dependent chemistry have been elaborated. These models demonstrate that even subtle alterations in radical pair recombination kinetics induced by weak magnetic fields might lead to measurable effects on enzyme activity [43,44]. Study of magnetic field effects on radical pair reactions that may have biological consequences is an important task [45].