MEDICAL AND ENVIRONMENTAL EFFECTS OF UV RADIATION - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 43

For analytical action spectra designed to elucidate the absorbing moiety
(chromophore) or mechanism of reaction, Jagger has an excellent description of the
criteria that must be met for obtaining valid data [28]. These are:
x The quantum yield must be the same at all wavelengths.
x The absorption spectrum of the chromophore must be the same in vivo as in vitro.
x Intracellular screening, such as absorption or scattering within a cell, must be either
negligible or constant at all wavelengths.
x The incident radiation is not entirely absorbed by the sample at any wavelength of
interest.
x Reciprocity of time and UV exposure holds under the conditions of the
determinations.
For DNA in dilute solution and short path length, it is easy to fulfill these
criteria. The data in Figure 4 show an action spectrum for induction of cyclobutyl
pyrimidine dimers in purified DNA in dilute solution [29]. The action spectrum in the
UVC (220-290 nm) and UVB (290-320 nm) resembles the absorption spectrum of
DNA. The high sensitivity of the gel electrophoresis/number average molecular length
analysis method, which can range over some 5 or 6 orders of magnitude, allows the
quantitation of very low frequencies of dimers induced by UVA (320-400 nm).
Measurement of analytical action spectra in biological systems, especially in
higher organisms, is fraught with difficulties. Consideration of the criteria of Jagger
[28] outlined above indicates that many are difficult to fulfill in complex systems.
Some of these apparent "complications'' can be useful in providing critical information
Figure 5. Action spectra for pyrimidine dimer induction in T7 dna in solution, in dna in alfalfa seedlings, and
in human skin in situ.
on the biological system. For example, the action spectrum in a complex organism for a
DNA damage that results from direct photon absorption in DNA provides information
on the internal absorption/shielding/scattering that protects DNA from damage by
incident photons. Figure 5 shows action spectra for formation of cyclobutyl pyrimidine