Structure related aggregation behavior of carotenoids and carotenoid esters

• Aggregates of 12 carotenoids in hydrated polar solvents & thin films were compared.
• Structural features were clearly related to J- or H-type aggregation behavior.
• Strongly coupled H-type aggregation of γ-carotene was observed for the first time.
• First reports on β-cryptoxanthin and (9Z)-β-carotene aggregates are presented.
• First reports on aggregates of zeaxanthin mono- and diesters are provided.

Carotenoids are lipophilic natural pigments inherently aggregating in hydrophilic environments. Such molecular self-assembly is crucial for the proper functioning of biological systems. Although several carotenoid aggregates have been investigated in recent years, our study provided further insights into the influence of specific structural modifications. For this purpose, the aggregation of 12 structurally-related carotenoids was studied in hydrated polar solvents, thin films, and precipitates from saturated solutions by UV/vis and circular dichroism spectroscopy, light microscopy, point-dipole approximation and computational modeling. Regarding the acyclic ψ,ψ-carotene (lycopene), the monocyclic β,ψ-carotene (γ-carotene), and the bicyclic β,β-carotene (β-carotene), the replacement of acyclic ψ-end groups by β-rings was demonstrated to consecutively modulate spectroscopic properties and the intermolecular distance within the aggregate. The presence of at least one open chain ψ-end, as found in γ-carotene and lycopene, fostered the formation of strongly coupled H-aggregates, whereas β,β-carotene prevailed as J-aggregate. While the insertion of one hydroxyl function to the β,β-carotenoid molecule (β-cryptoxanthin) similarly yielded J-aggregates, the presence of two hydroxyl functions (zeaxanthin) led to tightly-packed H-aggregates due to the formation of intermolecular hydrogen bonds. When blocking at least one of these hydrogen bonding sites, as studied with short-, middle-, and long-chain acyl zeaxanthins, H-type switched to J-type aggregation, irrespective of the chain length of the acyl moiety. Moreover, (Z)-isomerization was shown to prevent an ordered aggregation of carotenoid molecules. In brief, the optical properties and the aggregate structure of several most frequently occurring carotenoids are highly influenced by typical biosynthetic reactions, such as cyclization, isomerization, hydroxylation and esterification.

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