Chiral Amplification in Nature: Studying Cell‐Extracted Chiral Carotenoid Microcrystals via the Resonance Raman Optical Activity of Model Systems

Abstract

Carotenoid microcrystals, extracted from cells of carrot roots and consisting of 95 % of achiral β‐carotene, exhibit a very intense chiroptical (ECD and ROA) signal. The preferential chirality of crystalline aggregates that consist mostly of achiral building blocks is a newly observed phenomenon in nature, and may be related to asymmetric information transfer from the chiral seeds (small amount of α‐carotene or lutein) present in carrot cells. To confirm this hypothesis, we synthesized several model aggregates from various achiral and chiral carotenoids. Because of the sergeant‐and‐soldier behavior, a small number of chiral sergeants (α‐carotene or astaxanthin) force the achiral soldier molecules (β‐ or 11,11′‐[D2]‐β‐carotene) to jointly form supramolecular assemblies of induced chirality. The chiral amplification observed in these model systems confirmed that chiral microcrystals appearing in nature might consist predominantly of achiral building blocks and their supramolecular chirality might result from the co‐crystallization of chiral and achiral analogues.

Organization of Carotenoid Aggregates in Membranes Studied Selectively using Resonance Raman Optical Activity

“…α‐carotene embedded in DPPC liposomes forms J ‐aggregates as shown by the batochromic shift of the π→ π* absorption band compared with the monomer. [ 3c,13 ] The maxima due to the vibrational structure are observed at 417, 449, 482, and 523 nm (for the monomer in acetone at 423, 448, and 475 nm). Aggregation is confirmed by the appearance of the very intense exciton ECD signal with the vibrational structure (bands at ≈378, 421, 448, 477, 513, and 537 nm) observed for all used temperatures (vide infra).…”

“…Similarly, as for ECD, the ROA signal corresponds to the exciton coupling of electric dipole transitions of monomers in the aggregate. [ 3c ] Due to these features, the ROA of aggregates can be recorded selectively in the pool of both monomers and aggregates, contrarily from the Raman signal that is derived from those two components.…”

“…For chiral carotenoids (e.g., zeaxanthin, lutein, α‐carotene, astaxanthin, and many others) assembled supramolecular structures show supramolecular chirality. [ 3 ] Due to exciton coupling, supramolecular chirality is a source of a very intense chiroptical signal for vibrational optical activity (VOA), hence vibrational circular dichroism (VCD) and ROA provide sometimes 10 2 increase of the signal compared to the monomers. [ 4 ] Another mechanism of VOA enhancement is related to the interaction of electronic excited states with the vibrational ones.…”

“…[ 8 ] It means that ECD intensity is directly interrelated with RROA which is derived also from the excitonic coupling of the electric dipole transition moments of the monomers in the assembly. [ 3c ] Due to the intense ECD signal, RROA of carotenoid aggregates is measurable even at low concentrations of 10 −5 –10 −6 mol∙dm −3 . [ 3a,c ] Resonance enhancement of otherwise weak ROA signal is extensively studied in various contexts, [ 9 ] among others, as a strategy to record the signal of samples in low concentrations.…”

“…[ 3c ] Due to the intense ECD signal, RROA of carotenoid aggregates is measurable even at low concentrations of 10 −5 –10 −6 mol∙dm −3 . [ 3a,c ] Resonance enhancement of otherwise weak ROA signal is extensively studied in various contexts, [ 9 ] among others, as a strategy to record the signal of samples in low concentrations.…”