THz Characterization of All-Trans and 9-cis Retinal, Experiment, and Modeling

Two conformations of the retinal molecule have been studied in order to characterize the molecule´s THz transmission spectra in both the ground and metastable states. When subjected to an adequate external excitation the retinal molecule can experience a change in conformation and associated THz transmission spectra. In an attempt to accomplish this characterization, the FTIR system was modified to include a simple off axis excitation source inside of the systems sample chamber. Measurements were made of the retinal molecule´s THz spectra both with and without external excitation of the molecule. The results gathered were then compared with the results obtained from simulation. Data obtained from two retinal isomers reveal more spectral features at frequencies ~11 - 15 cm^-1 than were predicted for these conformations. The most likely explanation for this is that the material is actually a mixture of several metastable conformations. There is correlation between simulated and measured THz spectra in the ground state at a frequency of 14 cm^-1 for all-trans retinal. The strongest vibrational mode frequency predicted for the 9-cis conformation through modeling was 22 cm^-1, which correlates quite well with the experimental line at 21.3 cm^-1 in the ground state. When the 9-cis samples were exposed to UV illumination there was a noticeable change in the absorption spectra and this line at 21.3 cm^-1 almost disappeared which can be related to the transformation of a 9-cis into a more stable all-trans retinal. The absorption spectra of all-trans retinal that is the most stable conformation showed very weak features in experimental spectra, with some of them changing under illumination.