13-cis-β,β-Carotene and 15-cis-β,β-carotene

Abstract

13-cis-Beta,beta-carotene, C(40)H(56), crystallizes with a complete molecule in the asymmetric unit, whereas 15-cis-beta,beta-carotene, also C(40)H(56), has twofold symmetry about an axis through the central bond of the polyene chain. The polyene methyl groups are arranged on one side of the polyene chains for each molecule and the 6-s-cis beta end groups, with the cyclohexene rings in half-chair conformations, are twisted out of the planes of the polyene chains by angles ranging from 41.37 (17) to 52.2 (4) degrees. The molecules in each structure pack so that the arms of one occupy the cleft of the next, and there is significant pi-pi stacking of the almost-parallel polyene chains of the 15-cis isomer, which approach at distances of 3.319 (1)-3.591 (1) A.

Theoretical investigation of carotenoid ultraviolet spectra

“…BPW91/6-31G(d) method [38] level shows a small (C6OC7) s-cis isomer torsion angle of 42.9°, in relation to our B3LYP value. The experimental torsion angle for the 13-cis isomer is 47.6° [ 39], while our theoretical results are 48.2°, 48.5°, and 45.5°, for B3LYP, AM1, and PM3 methods, respectively. The 9-cis isomer torsion angles are 48.4°, 48.2°, and 38.1°, for B3LYP, AM1, and PM3 methods, respectively.…”

Two polymorphs of 20-desmethyl-β-carotene

“…The polyene chains in both polymorphs are significantly nonplanar, shown by the torsion angles along the polyene chains, as is normally the case (Mo, 1995;Bartalucci et al, 2008). The largest deviations from 180 are C11-C12-C13-C14 = À172.6 (5) for (I)-P2 1 and C8A-C9A-C10A-C11A = À173.3 (2) for (I)-P1, and the torsion angles for both polymorphs show less deviation from 180 towards the centre of the molecule, and for one half of the molecule versus the other.…”

“…Overall, the conformations of the two polymorphs are fairly similar to one another and to that of -carotene [Senge et al, 1992;Cambridge Structural Database (CSD;Allen, 2002) refcode CARTEN01], with end groups in the 6-s-cis conformation (Mo, 1995;Bartalucci et al, 2008). These are twisted out of the plane of the polyene chain by angles defined by the C5-C6-C7-C8 and C5A-C6A-C7A-C8A torsion angles, which describe the nonplanarity between the best planes through the doublebond systems of the ring and the polyene chain (Sundaralingam & Beddell, 1972;Mo, 1995;Bartalucci et al, 2008); these angles are À53.2 (8) and 47.3 (8) for (I)-P2 1 and À43.6 (3) and 56.1 (3) for (I)-P1 (Table 1), compared with a value of AE41.6 (6) for the centrosymmetric -carotene mol-ecule (Senge et al, 1992; CSD refcode CARTEN01). These values are in reasonable agreement with the calculated value for -carotene of 48.0 (Hashimoto et al, 2002), since such a twist of the end ring minimizes steric hindrance between the H atoms bonded to atoms C7 and C8 and those of the end ring methyl H atoms, and therefore is the most stable conformer.…”

“…In the (I)-P2 1 polymorph, there is no indication of thestacking interactions between polyene chains which have been seen in a number of related carotenoids (Bartalucci et al, 2007(Bartalucci et al, , 2008. However, there are some short contacts between the half-occupancy atoms C20 and C20A and atoms of adjacent polyene chains.…”

Conformational analysis of retinoic acids: Effects of steric interactions on nonplanar conjugated polyenes

“…The C5-C6 double bond of the trimethylcyclohexenyl ring of ATRA was in a non-planar cis -like conformation relative to the conjugated polyene chain (C6-C7 torsion angle of ~35°). Since this first observation 50 years ago, non-planar polyene structures about the trimethylcyclohexenyl ring have also been observed in x-ray crystal structures of 13- cis -retinoic acid[8], retinaldehydes[9–11], and β-carotenes[12–14]. Ten years after the first observation of a distortion in the polyene chain of ATRA, Stam solved the crystal structure of ATRA in a monoclinic crystal[15].…”