Self-Assembled Aggregates of the Carotenoid Zeaxanthin:  Time-Resolved Study of Excited States

Abstract

In this study, we present a way of controlling the formation of the two types of zeaxanthin aggregates in hydrated ethanol: J-zeaxanthin (head-to-tail aggregate, characteristic absorption band at 530 nm) and H-zeaxanthin (card-pack aggregate, characteristic absorption band at 400 nm). To control whether J- or H- zeaxanthin is formed, three parameters are important: (1) pH, that is, the ability to form a hydrogen bond; (2) the initial concentration of zeaxanthin, that is, the distance between zeaxanthin molecules; and (3) the ratio of ethanol/water. To create H-aggregates, the ability to form hydrogen bonds is crucial, while J-aggregates are preferentially formed when hydrogen-bond formation is prevented. Further, the formation of J-aggregates requires a high initial zeaxanthin concentration and a high ethanol/water ratio, while H-aggregates are formed under the opposite conditions. Time-resolved experiments revealed that excitation of the 530-nm band of J-zeaxanthin produces a different relaxation pattern than excitation at 485 and 400 nm, showing that the 530-nm band is not a vibrational band of the S2 state but a separate excited state formed by J-type aggregation. The excited-state dynamics of zeaxanthin aggregates are affected by annihilation that occurs in both J- and H-aggregates. In H-aggregates, the dominant annihilation component is on the subpicosecond time scale, while the main annihilation component for the J-aggregate is 5 ps. The S(1) lifetimes of aggregates are longer than in solution, yielding 20 and 30 ps for H- and J-zeaxanthin, respectively. In addition, H-type aggregation promotes a new relaxation channel that forms the zeaxanthin triplet state.

Origin of Absorption Changes Associated with Photoprotective Energy Dissipation in the Absence of Zeaxanthin

“…To produce such large red shifts, a dramatic change in xanthophyll environment would be required, such as the appearance of a static dipole or point charge near the pigment responsible (41,42). An alternative explanation for a large red shift would be an excitonic splitting of the S 2 excited state of the xanthophyll (46). This so-called Davydov splitting is a result of strong Coulombic coupling between the excited states of closely associated molecules.…”

“…Because zeaxanthin occupies a relatively peripheral position within the LHCII trimer (44), aggregation could result in close associations between xanthophylls belonging to neighboring trimers. Indeed, it has been shown that the formation of excitonically coupled J-aggregates of zeaxanthin (co-linear or "top-to-tail" chains of molecules) in water-ethanol mixtures gives rise to a red shift in the 0-0 transition to 535 nm (34,45,46), whereas lutein, antheraxanthin, and violaxanthin J-type aggregates possess red-shifted bands between 500 and 530 nm (45). In the following study, we specifically address the origin of ⌬A 525 .…”

β-Carotene to bacteriochlorophyll c energy transfer in self-assembled aggregates mimicking chlorosomes

“…The absorption spectrum taken before precipitation shows a significant blue shift (compared to that of β-carotene in THF) that is reminiscent of formation of H-aggregates [51]. In contrast, β-carotene incorporated into aggregates From experiments with BChl c aggregates with quinones [31] we know that the changes in the Soret band upon aggregation are less pronounced than those in the Q y band.…”

“…Effect of environment can hardly be responsible for such a change, because the S 1 lifetime of β-carotene is essentially insensitive to solvent properties [54]. It is known that aggregation of carotenoids induces changes in the S 1 lifetime and the decay becomes non-exponential due to annihilation [51]. Although the 6 ps lifetime observed here is comparable with the 5 ps component detected in J-aggregates of zeaxanthin [51], a carotenoid with spectroscopic properties nearly identical to those of β-carotene, the absence of annihilation in the intensity-dependent kinetics shown in Figure 5 makes aggregation of β-carotene as the reason for the S 1 lifetime shortening very unlikely.…”

“…It is known that aggregation of carotenoids induces changes in the S 1 lifetime and the decay becomes non-exponential due to annihilation [51]. Although the 6 ps lifetime observed here is comparable with the 5 ps component detected in J-aggregates of zeaxanthin [51], a carotenoid with spectroscopic properties nearly identical to those of β-carotene, the absence of annihilation in the intensity-dependent kinetics shown in Figure 5 makes aggregation of β-carotene as the reason for the S 1 lifetime shortening very unlikely. Moreover, J-aggregates also contain a substantial time component of ~30 ps [51] that has not been observed in our experiments.…”