Abstract
Urocanic acid (2-propenoic acid, 3-[ lH-imidazol-4(5)-yl], UA)? is one of the smallest molecules to have stimulated global interest among biologists, environmentalists, photochemists, photobiologists, medicinal chemists and immunologists. Its history can be traced back more than a century to when it was first found in the urine of dogs (1). Interest in the molecule remained dormant until the middle of the century but rekindled in the late 1940s when it was detected in animal skin and sweat. This led to the proposal that UA acts as a natural sunscreen, possibly as a specific photoprotecting agent for DNA because of the overlap of the UA and DNA absorption spectra (2). Since 1983 there has been an explosive growth in UA research, primarily as a consequence of the proposal by De Fabo and Noonan (3) that the cis photoisomer (cUA) of trans-UA (tUA) could be responsible for the phenomenon of photoimmunosuppression. In a recent literature survey we found that about 25 research papers have appeared every year over the last decade involving UA. Several comprehensive reviews of the molecule's photobiology, photochemistry and photophysics have appeared (4-
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