A ¹³C‐NMR. Study of cis‐trans Isomeric Vitamins A, Carotenoids and Related Compounds

Abstract

The 1H-decoupled 1sC-NMR. spectra of 35 all-trans, 17 mono-cis vitamin A compounds (acetates, alcohols, aldehydes, acids and esters) and of one 11,13-di-cis compound (11,13di-cis rctinol) are reported. Included in this investigation are desmeth yl-, dcsmethylethyl, and aryl-vitamin A analogues and others as well as 30 reference compounds of smaller molecular weight. Furthermore, the 13C-NMR. spectra of 23 p-apo-and other carotenoids were studicd. A complete assignment of the signals of all 106 compounds to the specific carbon atoms was achieved by extensivc application of lanthanide shift reagents, mainly Yb(dpm)s, by CW-offset and selective 1H-decoupling experiments, by comparison of the shifts of related compounds, and in three cases by utilization of specifically deuteriatcd compounds (11,15,. Thc chemical shift differences between the cis-and trans-vitamin A compounds and the applicability of the shift reagents for the assignment of the W-NMR. spcctra arc discussed.

Synthesis of 8-, 9-, 12-, and 13-mono-13C-retinal

“…The 300-MHz 'H nmr spectra of la,b are identical to those described by Patel (25) except for the additional '"-'H coupling constants as shown in Table 1. The 75.5-MHz "C nmr spectra of la,b display the enriched signals and are further identical to those described by Englert (16) except for the additional 1qC-'3C coupling constants as shown in Table 2; ms: l a and l b m / z 285 (M'): level of "C enrichment calculated from the mass spectra: 93%.…”

“…They display one strong single line due to the labelled carbon atom at the chemical shift value known from the natural abundance I3C nmr spectrum of unmodified retinal (16 Table 2.…”

Dark-adapted bacteriorhodopsin contains 13-cis, 15-syn and all-trans, 15-anti retinal Schiff bases.

“…As shown in Table 1, the shift difference between the all-trans and 13-cis protonated Schiff bases in solution is a mere 1.4 ppm at the C-14 position, in accordance with Englert (25), who reported changes of 1-2 ppm for seven retinal derivatives on 13-cis isomerization. Similarly, isomerization about the C-9, C-10 and C-11, C-12 double bonds causes only small changes in the shifts of the carbons of the isomerized bond itself (25)(26)(27), about an order of magnitude too small to account for the change observed here. The nature of the Schiff base counterion might also be expected to affect the isotropic shift of protonated Schiff bases at the C-14 position.…”

“…Specifically, Englert (25) shows upfield shifts of -6-8 ppm at C-12 on going from all-trans to 13-cis isomers in seven retinal derivatives, and at C-8 in the case of C-9, C-10 isomerization. Recently, we showed that the C-12 position of N-(13-cis-retinylidene)propyliminium trifluoroacetate has a shift 9.1 ppm upfield of the all-trans-derivative (15), a shift actually larger than any observed by Englert.…”

Physico-chemical and synthetic studies on carotenoids

“…data on fifteen carotenoid end groups and several others have been recorded since (Ref. 2). A major problem with this technique is the rather large samples required, although modern instruments using super conducting magnets have reduced the quantity required.…”