Coherent anti-Stokes vibrational Raman spectra of artificial rhodopsin pigments containing ring structures blocking 11-cis isomerization

The structural changes in the retinal chromophore that underlie the initial picosecond processes in the room temperature rhodopsin (RhRT) photo-reaction (i.e., photoRT and bathoRT intermediates) are examined through the vibrational spectra of four artificial Rh pigments (new vibrational spectra from three artificial Rh pigments are presented here). Each of these Rh pigments contains a retinal in which isomerization around the C11C12 bond, thought to be the primary reaction coordinate in the RhRT photo-reaction, is blocked. Specifically, vibrational spectra from the ground electronic states of Rh7.10 (containing a 7-membered carbon ring spanning the C10–C11C12–C13 bonds in the retinal), Rh7.10/8D (Rh7.10 containing a deuterium at C8), Rh7.10/ND (Rh7.10 containing a deuterium at the Schiff-base nitrogen) and Rh8.10 (containing an 8-membered carbon ring spanning the C10–C11C12–C13 bonds in the retinal) are recorded by picosecond resonance coherent anti-Stokes Raman spectroscopy (PR/CARS). These PR/CARS data are analyzed with the aid of vibrational mode, assignments originating with native Rh used to identify the structural motions that can be associated with specific vibrational features. These PR/CARS assignments provide the spectroscopic information required to interpret the structural changes in the retinal chromophore, observed via picosecond time-resolved CARS measurements (reported elsewhere), following optical excitation of these same artificial Rh pigments. The relationships between structural changes in these artificial Rh pigments and those occurring in native Rh are also discussed.