A comprehensive investigation of the mechanism and photophysics of isomerization of a protonated and unprotonated Schiff base of 11-cis-retinal

The photochemical event in rhodopsins

“…Both Birge and Becker agree, however, that the mixing of the two states is essential for promoting cis-trans photoisomerization. According to Becker the pK, of the Schiff base and the negative charge on the nitrogen increase upon excitation with no twist; the charge density decreases upon twisting to 90" (68). Contrary to previous observations they found that the quantum yield (+I) for the protonated 11-cis Schiff base is independent from the excitation wavelength from 355 to 532 nm.…”

“…Only in non-polar solvents does protonation increase the quantum yield. Becker and Freedman (68) interpret these variations in quantum yield by increased B,+-A,-mixing as a function of increased polarity of the environment. According to their results A , does not especially promote isomerization; it would seem rather that B,+ facilitates it more.…”

“…After photon absorption the excited molecule may have a great amount of vibrational energy and may start twisting (68,69) making it possible for the proton to move away from the nitrogen despite its increased basicity. This would remove some of the extra stabilization provided by the proton bridge and give the green light to the Schiff base to isomerize.…”

Application of the monomer reactivity ratios to the kinetic-model discrimination and the solvent-effect determination for the styrene/acrylonitrile monomer system

“…The penultimate unit effect appeared to increase approximately in the following order: acetonitrile Ͻ DMF Ͻ MEK Ͻ toluene. Therefore, it was inversely proportional to the polarity (expressed in Debye unit) of the solvent used (3.92, 23 3.82, 24 2.77, 25 and 0.36, 26 respectively). These findings led to the conclusion that the stronger interactions between the solvent and monomers were less when the penultimate unit effect was pronounced.…”

Clay Mimics Color Tuning in Visual Pigments

“…In solution, the cationic chromophore and the anion present are free and their short separation gives the energy state corresponding to a l max at 430-460 nm. [6] In contrast, the position of counterion in the protein is controlled so as to change the interaction, leading to maxima suited for acquisition of visible light (400-700 nm; l max from 425 to 560 nm). In bovine rhodopsin, the oxygen atoms of Glu113 are located at a 3-4-distance from the Schiff base nitrogen atom (Figure 1 a), [4] and l max is shifted to 498 nm.…”