Magnetic
fields can have a dramatic influence on the outcome of certain chemical
reactions. In general, these are reactions that involve the intermediacy
of free radicals, or molecules that by possessing an odd number
of electrons, can be viewed as "molecular magnets". Free radicals
are usually short lived (<10-3 s) and decay either by reactions
with other free radicals, or by reacting with molecules, in which case,
a new free radical is generated. In the last few years, there has been
considerable interest in the biological and health sciences in relation
to possible health risks associated with the environmental exposure to
magnetic fields related to the production, distribution and utilization
of electrical energy.1,2 It seems reasonable to assume that
any health related consequences must result from biological effects, and
that these, in turn, can only result if a magnetic field can exert influence
at a molecular level. On the basis of the premise, we have directed our
research toward the understanding of simple chemical systems that can be
influenced by modest magnetic fields.3-10. To put this in perspective,
the magnetic field from the earth is ~0.5 gauss, the field from a small
laboratory stirring magnet around 100 gauss and spectroscopic instrumentation
(such as ESR and NMR) can involve fields in the 10,000 gauss range. In
all cases, the energies associated with these field (<1 cal) are minute
in relation to bond energies, or to the thermal energy content of molecules
at room temperature. How can such a small energy influence the course of
a chemical reaction? The answer is the understanding of the radical
pair mechanism.
The magnetic properties of a free radical are related to the spin
of the residual electron. That is, molecules with an even number
of electrons usually have an identical number of electrons with spin "up"
(or north) and spin "down" (or south). Thus, while
each electron can be viewed as a magnet, these magnets are fully balanced
in the molecule. Such balance cannot be achieved in a free radical because
they contain an odd number of electrons.
Whenever two radicals come together, either by chance, or because
of the form of generation, their spins can be opposite or parallel. 
Only
in the case of opposite spins can they react to form a stable molecule
that has the proper magnetic balance. This is illustrated in Figure 1,
Path A, where the radicals are represented by circles with arrows that
remind us of the spin of these species. When this is the case, a magnetic
field can only have a minor effect on the outcome of the reaction. However,
when the spins are parallel (described as a triplet radical pair),
the situation is quite different since formation of a stable molecule requires
a change of spin, the equivalent to rotating a magnet (Figure 1, path B).
How are these magnetic field effects manifested? Figure 2 shows an
example in which a radical pair confined to a small space (provided by
a micelle) has been generated in the triplet state using laser techniques.10
The
two radicals in this case are derived from benzophenone and melatonin;
the latter is a hormone produced by the pineal gland that controls the
circadian rhythm.11 The trace in the absence of field contains
a fast and slow decay. The fast decay corresponds to those radicals that
decay by reaction with their partner in the pair, while the slow component
shows that many radicals separate and then lack a reaction partner (i.e.
they are no longer a radical pair). Those radicals that decay fast have
succeeded in changing their spin so as to achieve the magnetic balance
mentioned above.
Figure 3 illustrates the competition between the geminate decay
(fast, within the original radical pair), and separation processes that
lead to long lived radicals. An external field only influences radical
pairs, generally by slowing down processes that require a change of
spin. When a field is applied, less radicals can change their spin and
more succeed in separating from their partner. The consequence of this
is that, on average, radicals live longer and their overall concentration
increases when a field is applied.8 These effects are more pronounced
when the radicals are "encouraged" to stay together by some chemical
link or physical boundary. This boundary can be provided by a micelle (as
in our example), a bilayer membrane, or the organized structure of a cell.
Conceptually, we can rationalize these effects by stating that changing
the orientation of a magnet (spin) is more difficult when an external field
is applied. The physical phenomena underlying this process is related to
Zeeman splitting of the spin energy levels that hinders spin evolution.6,12
Over the last few years, a number of studies have shown that these
effects can occur in a wide range of chemical systems, and that they can
be caused either by static fields (as in our examples) or by oscillating
fields (e.g. 60 Hz),7 such as those produced by the
distribution and utilization of electrical energy. The model studies in
chemical systems have generally explored fields that are larger than typical
environmental fields. It remains to be established to what extent environmental
fields can influence biological processes.
Given the recognized importance of free radicals on a wide range
of biological processes, from aging to cancer, there is little doubt that
any external agent that can influence radical behavior can have health
consequences.1,13 It is likely that once controversies relating
to power lines and other environmental fields are resolved, the true potential
of magnetic fields from the perspective of therapeutic applications will
be recognized. Clearly, the ability to control free radical behavior can
be used to advantage in organic chemistry, biology and medicine.
References:
1 McLauchlan, K. (1992). Are Environmental Magnetic Fields Dangerous? Physics World. January, 41-45.
2 Hileman, B. (1993). Health Effects of Electromagnetic Fields Remain Unresolved. C & E News. November 8, 15-29.
3 Cozens, F.L. and J.C. Scaiano. (1993). A Comparative Study of Magnetic field Effects on the dynamics of Germinate and Random Radical Pair Processes in Micelles. J. Am. Chem. Soc. 115, 5204-5211.
4 Evans, C., K.U. Ingold and J.C. Scaiano. (1988). Magnetic Field Effects on the Decay of Ketyl-Aryloxy Radical Pairs in Micellar Solution. J. Phys. Chem. 92, 1257=1262.
5 Korolenko, E.C., F.L. Cozens and J.C. Scaiano. (1995). Magnetic field Effects on the dynamics of Nitroxide-Based Singlet Radical Pairs in Micelles. J. Phys. Chem. 99, 14123-14128.
6 Scaiano, J.C., E.B. Abuin and L.C. Stewart. (1982). Photochemistry of Benzophenone in Micelles. Formation and Decay of Radical Pairs. J. Am. Chem. Soc. 104, 5673-7679.
7 Scaiano, J.C., N. Mohtat, F.L. Cozens, J. McLean and A. Thansandote. (1994). Application of the Radical Pair Mechanism to Free Radicals in Organized systems: Can the Effects of 60 Hz Be Predicted From Studies Under Static fields? Bioelectromagnetics. 15, 549-554.
8 Scaiano, J.C., F.L. Cozens and J. McLean. (1994). Model for the Rationalization of Magnetic Field Effects in vivo. Application of the Radical-Pair Mechanism to Biological Systems. Photochem. Photobiol. 59, 585-589.
9 Scaiano, J.C. (1995). Influence of Combined AC-DC Magnetic Fields of Free Radicals in Organized and biological Systems. Envelopment of a Model and Application of the Radical Pair Mechanism to Radicals in Micelles. Photochem. Photobiol. 62, 818-829.
10 Scaiano, J.C. (1995). Exploratory Laser Flash Photolysis Study of Free Radical Reactions and Magnetic Field Effects in Melatonin chemistry. J. Pineal res. 19, 189-195.
11 Reiter, R.J., D.-X. Tan, B. Poeggeler, A. Menendez-Pelaez, L.-D. Chen and S. Saarela. (1994). Melatonin as a Free Radical Scavenger: Implications for Aging and Age-Related Diseases. Ann. N.Y. Acad. Sci. 719, 1-12.
12 Bittl, R., K. Schulten and N.J. Turro. (1990). Micellar Radical Pair Decay. J. Chem. Phys. 93, 8260-8269.
13 Grissom, C.B. (1995). Magnetic field Effects in Biology: A Survey of Possible Mechanisms with Emphasis on Radical-pair Recombination. Chem. Rev. 95, 3-24.
