The first of these is trans-urocanic acid (trans-UA), which forms in your skin upon deamination of the amino acid histidine. It absorbs in the same wavelength region as DNA, so it was thought to act as a natural sunscreen to protect DNA from UV damage (it was even added to topical sunscreens for awhile!). But UV absorption by trans-UA also results in trans to cis isomerization, and cis-UA has been shown to exhibit immunosuppressive activity in mice, which could actually help CAUSE skin cancer. Urocanic acid also has some interesting photochemistry. Upon excitation on the low energy end of its absorption, its photoisomerization quantum yield is maximal. Upon excitation at the absorption maximum of trans-UA, almost NO photoisomerization occurs. This wavelength-dependent photochemistry is odd in a molecule; usually the isomerization rate increases until the photostationary state is achieved.

Trans-coumaric acid (trans-CA) is the chromophore in the photosensory protein photoactive yellow protein. Photosensory proteins convert light energy from the ambient environment into a cellular response. Upon absorption of a blue photon, the photoisomerization of trans-CA triggers a photocycle in PYP that causes a negative phototactic response to blue light in certain purple bacteria.

The photophysics of these molecules in the gas phase is extremely similar. In both cases, isomerization occurs in the S1 electronic excited state, which is p-p* in nature, upon overcoming an energy barrier of 4 and 3.4 kcal/mol in trans-UA and trans-CA respectively. Both molecules have a broad feature at high energies in their excitation spectra that is attributed to absorption to the S2 electronic excited state. Upon excitation to the S2 state, internal conversion to S1 occurs, and isomerization occurs to a much lesser extent than from direct excitation to the S1 state. The presence of this second close-lying excited state provides the reason for the rare wavelength-dependent photochemistry in trans-urocanic acid. In PYP, this seco nd excited state has not been observed and may aid in understanding some of the controversies in the field.