Carbon‐13 NMR spectra of all‐trans‐N‐retinylidene‐n‐butylamine as an analogue of the Schiff's base linkage‐compound in visual pigment

Carbon‐13 NMR spectra have been obtained for all‐trans‐N‐retinylidene‐n‐butylamine as an analogue of the Schiff's base linkage compound in visual pigment. The chemical shift changes on going from all‐trans‐retinal1 to all‐trans‐N‐retinylidene‐n‐butylamine are observed. These changes indicate the collapse of the bond alternation in conjugated polyene carbons, whereas N‐protonation in all‐trans‐N‐retinylidene‐n‐butylamine does not affect the bond alternation.