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.