ABSTRACT

CONTENTS 6.1 Introduction ....................................................................................................................... 157 6.2 Theoretical Background................................................................................................... 158

6.2.1 Hyperfine Interaction-Induced Singlet-to-Triplet Conversion...................... 159 6.2.2 High-Field Regime: Spin Rephrasing through the Dg Mechanism.............. 159 6.2.3 Low-Field Effect: B < 1mT ................................................................................. 160 6.2.4 Free Radicals in Radio Frequency Fields.......................................................... 160

6.3 General Characteristics of the Free Radical Mechanism............................................ 160 6.3.1 Experimental Discrimination of Free Radical Models.................................... 161

6.4 Free Radicals in Biology .................................................................................................. 161 6.4.1 Biological Transduction Mechanisms................................................................ 161 6.4.2 Role of Freely Diffusing Radicals....................................................................... 162 6.4.3 Animal Navigation Models Based on Free Radicals ...................................... 163 6.4.4 Coenzyme B12-Dependent Reactions................................................................. 163 6.4.5 Other Experimental Observations...................................................................... 163

6.5 Conclusion ......................................................................................................................... 164 References ................................................................................................................................... 164

The physical chemistry of spin-correlated free radical pairs offers several mechanisms explaining how magnetic fields may influence biochemical processes. The mechanisms are classified on the basis of the dominating contribution to spin interconversion, and they cover a wide range of field strengths. Of particular interest is what is called the low-field mechanism, which has been extensively developed over the last decade and is now capable of explaining biological effects induced by magnetic fields well below 1mT. The principal mechanism behind the free radical mechanism was discovered in the

physical problem of magnetic field dependence on positronium decay (Deutsch and Brown, 1952; Halpern, 1954). However, the development of the radical pair mechanism in chemistry has its roots in the work of Kaptein and Oosterhoff (1969), Closs (1969), and Brocklehurst (1969).