Electronic absorption and fluorescence spectra of urocanic acid are st
udied in aqueous solution at different pH. The observed results are ex
plained using molecular orbital geometry optimization calculations for
the ground states of a number of tautomers of urocanic acid including
those of its anion, followed by calculation of electronic transitions
. The isomers of urocanic acid which would strongly absorb UV-B radiat
ion have thus been identified. Further, the possible mechanisms of iso
merization of urocanic acid from the trans- to cis-form which is relat
ed to its role in immunosuppression and UV-B induced skin cancer have
been suggested.