Two-Dimensional Electronic Spectroscopy Characterization of Fucoxanthin-Chlorophyll Protein Reveals Excitonic Carotenoid-Chlorophyll Interactions.

Fucoxanthin Chlorophyll Protein (FCP) is a Light Harvesting Complex found in diatoms and brown algae. It is particularly interesting for its efficiency in capturing the blue-green part of the light spectrum due to the presence of specific chromophores (fucoxanthin, chlorophyll a, and chlorophyll c). Recently, the crystallographic structure of FCP was solved, revealing the 3D arrangement of the pigments in the protein scaffold. While this information is helpful for interpreting the spectroscopic features of FCP, it has also raised new questions about the potential interactions between fucoxanthin and chlorophyll c. These interactions were suggested by their spatial closeness but have never been experimentally observed. To investigate this possible interaction mechanism, in this work, two-dimensional electronic spectroscopy (2DES) has been applied to study the ultrafast relaxation dynamics of FCP. The experiments captured an instantaneous delocalization of the excitation among fucoxanthin and chlorophyll c, suggesting the presence of a non-negligible coupling between the chromophores.

The Energy Transfer Yield between Carotenoids and Chlorophylls in Peridinin Chlorophyll a Protein Is Robust against Mutations.

The energy transfer (ET) from carotenoids (Cars) to chlorophylls (Chls) in photosynthetic complexes occurs with almost unitary efficiency thanks to the synergistic action of multiple finely tuned channels whose photophysics and dynamics are not fully elucidated yet. We investigated the energy flow from the Car peridinin (Per) to Chl a in the peridinin chlorophyll a protein (PCP) from marine algae Amphidinium carterae by using two-dimensional electronic spectroscopy (2DES) with a 10 fs temporal resolution. Recently debated hypotheses regarding the S2-to-S1 relaxation of the Car via a conical intersection and the involvement of possible intermediate states in the ET were examined. The comparison with an N89L mutant carrying the Per donor in a lower-polarity environment helped us unveil relevant details on the mechanisms through which excitation was transferred: the ET yield was conserved even when a mutation perturbed the optimization of the system thanks to the coexistence of multiple channels exploited during the process.

Solvent-Dependent Characterization of Fucoxanthin through 2D Electronic Spectroscopy Reveals New Details on the Intramolecular Charge-Transfer State Dynamics.

The electronic state manifolds of carotenoids and their relaxation dynamics are the object of intense investigation because most of the subtle details regulating their photophysics are still unknown. In order to contribute to this quest, here, we present a solvent-dependent 2D Electronic Spectroscopy (2DES) characterization of fucoxanthin, a carbonyl carotenoid involved in the light-harvesting process of brown algae. The 2DES technique allows probing its ultrafast relaxation dynamics in the first 1000 fs after photoexcitation with a 10 fs time resolution. The obtained results help shed light on the dynamics of the first electronic state manifold and, in particular, on an intramolecular charge-transfer state (ICT), whose photophysical properties are particularly elusive given its (almost) dark nature.