Fig. 4. Light-induced ¹³C retinal carbon and SB chemical shift changes.

(A) Chemical shift differences between dark state (G) and photointermediates K, L, and O are plotted for carbon atoms C10 to C15, C20, and SB. The gray bars illustrate the effect of the all-trans 13-cis isomerization as observed for the model compounds N-retinylidenebutylamine (C10 to C15) and retinal (C20) (44, 45). The ¹⁵N chemical shift of KR2 in the dark and intermediate states is compared to GPR (37), BR (28), and ChR2 (36). (B) Chemical shift difference between the K- and L-state. (C) Illustration of chemical shift changes observed at retinal carbons and SB nitrogen in the dark state [Protein Data Bank (PDB): 6REW], K-state (PDB: 6TK5), L-state (PDB: 6TK4), and O-state (PDB: 6XYT). The yellow and blue spheres indicate shielding and deshielding at the nuclei, respectively.