A Spectroscopic Study of the Epidermal Ultraviolet Chromophore
trans-Urocanic Acid
Kerry M. Hanson, Bulang Li, and John D. Simon
Photoacoustic calorimetry and steady-state spectroscopic techniques are used to
investigate the wavelength-dependent photoreactivity of trans-urocanic acid under
representative physiological conditions in vitro. The maxima of the emission and
excitation spectra and the value of the reaction enthalpy as determined from photoacoustic
measurements, varied as a function of the excitation wavelength. From these data,
we are able to confirm that the photolysis of trans-UA at 264 nm (the peak of
the absorption spectrum where isomerization is inefficient) generates a long-lived
electronically excited triplet state that lies approximately 230 kJ/mole above
the ground state. Isomerization does not occur from this triplet electronic state,
but bimolecular energy transfer to O2, generating singlet oxygendoes occur. Excitation
of trans-UA at 308 nm (in the tail of the absorption spectrum), does not lead
to triplet state formation. Instead, excitation at this wavelength lead to isomerization.
Using measured isomerization quantum yields at this photolysis wavelength , we
are able to determine that the ground state of cis-UA lies about 40 kJ/mole above
that of the trans isomer. Photoacoustic data recorded following the photolysis
of cis-UA at 264 nm and 308 nm show the same trends, supporting the conclusion
that the wavelength-dependent chemistry is not due to the presence of multiple
ground-state rotamers. The data indicate that the broad structureless absorption
spectrum of trans-UA is comprised of overlapping transitions due to at least two
distinct electronic states. The different reactivities of these two states result
in the wavelength-dependent isomerization yields that have been measured for trans-UA
in the UV-B (280 -320 nm) range.
The Journal of the American Chemical Society, volume 119, p. 2715 (1997)