Carbon-13 NMR spectra have been observed for 11- and 13-cis-retinals and their Schiff base linkage compounds with butylamine as a model compound of visual chromophore. The chemical shift changes on going from retinal to Schiff base. The changes in chemical shifts of polyene carbons were discussed in correlation with the changes of π-electron densities.
Carbon-13 NMR and UV absorption spectra have been obtained for a//-Z/-anj-7V-retinylidene-«-butylamine and its acid-addition salts as an analogue of the Schiffs base linkage compound in visual pigment. Carbon-13 chemical shift changes on going from aZZ-rrans-ZV-retinylidene-n-butylamine to its acid-addition salts are observed. The -electron density and -bond order of polyene chain were calculated for these compounds by CNDO/2-MO method. The change in carbon-13 chemical shift was found to correlate quite well with that in -electron density. It was deduced that the large red shift of UV absorption maximum by N protonation of a/Z-Zz-tinj-ZV-retinylidene-zi-butylamine may be mainly due to the decrease of the degree of bond alternation and the delocalization of the positive charge in conjugated polyene chain. The effect of solvent (methanol and carbon tetrachloride) on the UV bathochromic shift and carbon-13 chemical shift was also discussed briefly.
Abstract-Carbon-13 NMR spectra of all-trans retinal, and vitamin A, were measured by the pulse Fourier transform method in CCI,. All peaks in these spectra were assigned from considerations of chemical shifts, half proton-decoupled spectra, spin-lattice relaxation times and induced chemical shift by the addition of shift reagent. The carbon-13 NMR spectrum was also measured for all-trans retinal, which had been exposed to the sunlight for three hours, and the induced isomer was proposed to be 1 1-s-cis retinal,.
Abstract-Carbon-1 3 NMR spectra have been obtained for alltrans-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-retinal, to all-trans-N-retinylidenen-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.
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