UV synchrotron radiation linear dichroism spectroscopy of the anti-psoriatic drug anthralin

Anthralin (1,8-dihydroxyanthrone, 1,8-dihydroxy-9(10H)-anthracenone), also known as dithranol and cignolin, is one of the most efficient drugs in the treatment of psoriasis and other skin diseases. The precise mode of biochemical action is not fully understood, but the activity of the drug is increased by the influence of UV radiation. In the present investigation, the UV absorption of anthralin is studied by synchrotron radiation linear dichroism (SRLD) spectroscopy on molecular samples partially aligned in stretched polyethylene, covering the near and vacuum UV regions with wavenumbers ranging from 23,000 to 58,000 cm–1(430–170 nm). The observed polarization spectra are well predicted by quantum chemical calculations using time-dependent density functional theory (TD–DFT). About a dozen spectral features are assigned to computed electronic transitions. The calculations support interpretation of the anomalous fluorescence of anthralin as a result of barrier-less excited state intramolecular proton transfer (ESIPT) to the tautomer 8,9-dihydroxy-1(10H)-anthracenone.