MEDICAL AND ENVIRONMENTAL EFFECTS OF UV RADIATION - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 38

photon. Figure 1 shows an energy level diagram, with the ground singlet state (S 0 ),
lowest excited singlet state (S 1 ) and lowest excited triplet state (T 1 ). In this diagram, the
vertical axis corresponds to increasing energy.
In absorption, a photon of at least the same energy spacing as the S 0 -S 1 energy
gap is absorbed (A) by the biomolecule, raising the molecule to the lowest excited
singlet. The energy may be transferred non-radiatively (shown by curved lines; by
rotation, translation, etc, without photon emission) to the T 1 ; this is termed intersystem
crossing (isc). Both the S 1 and T 1 may relax by non-radiative emission (nre) to the
ground state. The energy may also be emitted promptly (within picoseconds to
nanoseconds) from the S 1 by emission of a photon, i.e., producing fluorescence (F). If
the molecule has undergone intersystem crossing to the lowest excited triplet, it may
emit energy via photon emission, termed phosphorescence (P). Since the latter process
involves a change in spin state, it is a forbidden'' transition, and thus may be

significantly delayed—from microseconds to as much as seconds, minutes, or even
hours—with respect to the original absorption event.
The excited molecule may dissipate the absorbed energy by undergoing
photochemistry (PC). Photochemical reactions may occur from either the S 1 or the T 1 .
In some molecules different reactions may result from photochemistry from specific
excited states. The occurrence of photochemistry in an excited molecule competes with
but does not preclude energy dissipation by light emission and by non-radiative decay.
This brief summary of photon absorption and energy dissipation points out two
important points: first, only a photon with the same energy as the energy difference that
matches or exceeds the gap between S 0 and S 1 will be absorbed. (For this simplified
discussion, vibrational and rotational states of molecules are ignored.) For a more
complete discussion, the reader is referred to classic photochemistry texts such as
Calvert and Pitts [8]). This is known as the First Law of Photochemistry, which is
sometimes stated as "No photochemical reaction without light absorption.'' Second,
light absorption occurs by quantum events, i.e., by absorption of discreet photons, not of
an energy continuum. This is critical in analysis of action spectra, in which we often
want to compare an action spectrum (See Section V) with the absorption spectra of
candidate target molecules.

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3. Potentially Important Target Biomolecules

UV radiation reaching the surface of the earth includes wavelengths in the range 290-
400 nm. Since the First Law of Photochemistry (see above) tells us that a photon must
be absorbed for photochemistry to occur, we can ask which biomolecules absorb
radiation in this wavelength range. Proteins, ribonucleic acid (RNA), and
deoxyribonucleic acid (DNA) all absorb in range 290-400 nm.
Many studies show that proteins can be inactivated by UV radiation (for an
excellent summary, see Setlow and Pollard [9]). Since absorption must precede
photochemistry, amino acids with substantial molar extinction coefficients (H) for UV
absorption would be good candidates for absorbing photons in the ultraviolet range that
could lead to protein inactivation by UV radiation. The aromatic amino acids,
tryptophan and tyrosine, indeed have Hs in the range of 1000 10,000 at ~ 280 nm.

However, they have rather low quantum yields (~ 0.005) for photochemical reactions.
Nonetheless, proteins with aromatic amino acids (but little cystine, see below) are

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