A Study of N‐Butyl‐(all‐trans‐retinylidene)amine and Its Protonated Species by ¹H‐ and ¹³C‐NMR. Spectroscopy

Abstract

SummaryThe Schiff base of all-trans-retinal was investigated in organic solution by 'H-and 13C-NMR. at high field. Complete assignment of the 'H-NMR. peaks of N-butyl-(all-trans-retiny1idene)amine (2) and the N-butyl-(all-trans-retiny1idene)-ammonium ion (3) was achieved by INDOR (internuclear double resonance). The vicinal proton coupling constants of the polyene chain show that the n-bond orders remain unchanged in N-butyl-(all-trans-retiny1idene)amine relative to all-transretinal (l), but change towards larger n-delocalization in the N-butyl-(all-transretiny1idene)ammonium ion. At -6 1 a only one isomer of N-butyl-(all-trans-retinylidene)ammonium was observed. This was shown to be trans at the imine linkage and independent of the solvent. The trifluoroacetic acid counter-ion can approach the positive charge of the N-atom in the weakly polar solvent dichloromethane but not in the leveling solvent methanol. In dichloromethane the nature of the 1:l complex is a H-bonded (0-... H-Nf) ion-pair whose rate of breaking and forming is rapid at RT. Strong stabilization of the ion-pair resulted from homoconjugation with a second molecule of trifluoroacetic acid. Excess of acid efficiently diminished the isomerization rate at the C, N-bond.Introduction. -The visual response in vertebrates is initiated by the transisomerization of the pigment 1 1-cis-retinal triggering a series of conformational changes in the apoprotein opsin [l] [2]. In conjunction with an investigation of the conformation of rhodopsin in detergent micelles we have recently performed a NMR. study of the conformation and dynamics of all-trans-retinal in organic solution and after incorporation into aqueous dodecyldimethylamine oxide micelles [3]. The ordered structure of the amphiphatic detergent or lipid molecules may better simulate the actual environment of the retinal chromophore [4]. Since the linkage to the retinal in rhodopsin is generally agreed to be via the primary amino group of a lysine [5] [6] we have also included in our NMR. studies the Schiff s base of all-trans-retinal.

Solid state NMR study of [epsilon-13C]Lys-bacteriorhodopsin: Schiff base photoisomerization

“…Thus, both the 14-'3C and the E-'3C chemical shifts are even further downfield in M than in bR568. This is consistent with the known deshielding effects of Schiff base deprotonation on both of these carbons (21,22) and with the idea that the M intermediate contains a C=N anti chromophore (2,9).…”

“…The signals at 110.5 and 122 ppm have previously been assigned to the 14-'3C of retinal in bR555 and bR568, respectively (3). The remaining signals at 48 and 55 ppm are assigned to the E-'3C label in lys-216, based on the chemical shifts observed in protonated retinal Schiff bases (21,22). The 13C chemical shifts in Fig.…”

Protonation of a conjugated Schiff base by weak to strong acids: a 13C and 15N nuclear magnetic resonance study

“…This pattern is obtained for all acids used and in both solvents (CDC13 and CD30D). It has already been noted by Harbison et al (5b), Inoue et al (13), and by Pattaroni and Lautenvein (14), who report a similar behavior for all-trans-retinylidene n-buty lamine (NRBA) and strong acids. It can be rationalized by the fact that all odd-numbered carbon atoms have a partial positive charge while those that are even numbered are in a nonresonance position.…”

“…Table 1 shows that the chemical shifts are all different since, in that solvent, there is no levelling effect. However, the chemical shifts are linearly correlated with the pKas only for the carboxylic acids, while for the hydrohalides it is the size of the counterion that regulates the deshielding of HBA (14,17). Since it has been demonstrated by ir (18b) and by nrnr (10, 14) that TFA was protonating HBA completely (loo%), the same procedure as described before can be used.…”

Model Compounds for Rhodospin and Bacteriorhodopsin: Synthesis, and 1H‐ and 13C‐NMR Study

“…The complete assignment of the proton spectra (Tubk 2) is easily carried out with the aid of J(H,H) and by comparison with previous models [18]. The differentiation between H-C(11) and H-C(12) in A-1 B compounds and between H-C(7) and H-C(8) in A-2 B compounds is based on the existence of the hoinoallylic coupling constant of H-C (7) or H-C(11) with 3H-C(5').…”