Photophysical, photochemical, and thermodynamic properties of shikimic acid derivatives: calycin and rhizocarpic acid (lichens)

Photophysical and photochemical parameters of the lichen metabolites calycin and rhizocarpic acid were determined. Experiments were carried out in micellar solutions of 3% Brij 35, at pH 2 and 12, and in acetonitrile. Both metabolites absorb in the UV-A and UV-B regions, and emit fluorescence in the visible region of the solar spectrum. Shifts were not observed in the absorption spectra, at pH 2 and 12. The low φc, between 10−5 and 10−2, shows that both compounds are photostable in the experimental conditions. For rhizocarpic acid, two values of pKa were obtained: 5.1 corresponding to the hydroxyl group, and 9.0 corresponding to the protonated nitrogen. Calycin presents only one value of pKa: 4.9, that is attributed to the hydroxyl group. l-(+)-Gluconic-γ-lactonic acid was used as a reference model; the compound showed greater photoinstability, demonstrating that the photodegradation observed occurs mainly in the oxolane carbonylic ring.