Infrared Signatures of Phycobilins within the Phycocyanin 645 Complex

Abstract

Aquatic photosynthetic organisms evolved to use a variety of light frequencies to perform photosynthesis. Phycobiliprotein phycocyanin 645 (PC645) is a light-harvesting complex in cryptophyte algae able to transfer the absorbed green solar light to other antennas with over 99% efficiency. The infrared signatures of the phycobilin pigments embedded in PC645 are difficult to access and could provide useful information to understand the mechanism behind the high efficiency of energy transfer in PC645. We use visible-pump IR-probe and two-dimensional electronic vibrational spectroscopy to study the dynamical evolution and assign the fingerprint mid-infrared signatures to each pigment in PC645. Here, we report the pigment-specific vibrational markers that enable us to track the spatial flow of excitation energy between the phycobilin pigment pairs. We speculate that two high-frequency modes (1588 and 1596 cm–1) are involved in the vibronic coupling leading to fast (<ps) and direct energy transfer from the highest to lowest exciton, bypassing the intermediate excitons.

Molecular and Supramolecular Materials: From Light-Harvesting to Quantum Information Science and Technology

“…The dihydrobiliverdins (DBVs) are strongly coupled to form a molecular exciton dimer and serve as the donor, while the phycocyanobilins (PCBs) are the terminal acceptors. , As a result, excitation is delocalized and the lower exciton state of the donor is in closer resonance with the acceptor n = 1 excited state, facilitating EET. These intermolecular nonclassical effects have been observed via broadband transient absorption spectroscopy, two-dimensional photon echo, two-dimensional electronic spectroscopy (2DES), and two-dimensional electronic vibrational spectroscopy, and there is a proposed vibronic coupling mechanism for energy transfer in PC645 . The delocalization across donor chromophores, vibronic coupling mechanism, and related rapid EET rates for closed conformation PBPs demonstrate that intermolecular interactions within LH systems, even at the simple dimer level, make the ultrafast dynamics of light harvesting nontrivial.…”