c21 JWBS043-Rogers September 13, 2010 11:30 Printer Name: Yet to Come
QUANTUM YIELDS 347This chain reaction is damaging to biological membranes, especially cell walls, and
has been implicated in both carcinogenesis and the aging process. Within living
systems, the unsaturated fatty acids are most vulnerable to autooxidation.
Living biological systems (as contrasted to dead ones) are protected from this
autooxidation process through the action of small amounts of exogenousantioxidants,
including vitamin E. Antioxidants form very reactive free radicals themselves. These
radicals interfere with the chain oxygenation cycle in the propagation steps by reacting
with the radical ROO•. This terminates the chain and limits damage to the organism.
Vitamin E is the name given to the group of fourtocopherols. The tocopherols are
aromatic alcohols ArOH, which react with peroxyl radicals.ROO•+ArOH→ROOH+ArO•The phenol radical ArO•is stabilized by delocalization of electrons over the aromatic
framework. Because of their stability, they persist in the system and eventually react
with a peroxyl radical to complete the chain-breaking mechanismArO•+ROO•→products21.2.2 Ozone Depletion
Neither O nor O 2 absorbs radiation in the 200- to 300-nm (ultraviolet) region, but
ozone O 3 absorbs at about 250 nm, shielding the earth’s surface and creatures living
on it from mutagenic DNA alteration by impact of ultraviolet photons. Among the
many photochemical reactions occurring in the earth’s stratosphere is a sequence in
which the middle two reactions below are in balance, maintaining a low but fairly
constant ozone level.O 2 +hν(<200 nm)→2O
O 3 +hν(UV)→O 2 +O
O+O 2 +M←→O 3 +M
O+O 3 →2O 2M is a molecule, possibly N 2 , that does not enter into the reaction but exchanges
energy through collisions with those that do.
Other reactions interfere with this cycle, hence they interfere with the stratospheric
ozone level. An example is the chlorine cycle involving Cl 2 O 2 derived ultimately from
man-made chlorofluorocarbons. Stratospheric chlorine has increased by a factor of
about 7 in the past 50 years, and a decrease in stratospheric ozone has been found
(primarily for reasons of atmospheric turbulence) over Antarctica. This is the source
of the polar “ozone hole.” There is no reason to suppose that ozone depletion, though
it is most easily monitored over the poles, is limited to the polar stratosphere.