A New Spectral Window on Retinal Protein Photochemistry

A VIS pump/hyperspectral NIR probe study of all-trans-retinal protonated Schiff base (RPSB) in ethanol is presented. Upon irradiation, a short-lived absorption band covers the recorded range of lambda = 1-2 mu m. It decays to reveal the tail of S-1 emission at lambda < 1.3 mu m, along with a residual absorption at longer wavelengths, both of which decay with the known kinetics of internal conversion to S-0. The existence of this hitherto unrecorded excited-state absorption deep in the NIR will require a revision of current models for RPSB electronic structure. The phenomenological similarity of these observations with ultrafast NIR studies of carotenoids raises the question of whether three, and not two, electronic states participate in RPSB photochemistry as well. The relevance of these observations to retinal protein photochemistry is discussed.