The photophysics of cryptophyte light-harvesting

Recent studies of the optical properties and the critical role of phycobiliproteins in the absorption of green light for photosynthesis in cryptophyte algae (Rhodomonas CS24 and Chroomonas CCMP270) are reviewed. Investigations of two different isolated proteins, phycoerythrin 545 (PE545) and phycocyanin 645 (PC645), whose crystal structures are known to 0.97 and 1.4 Å resolution respectively, are described. Steady-state spectroscopic measurements, including polarization anisotropy and circular dichroism, are used in combination with ultrafast transient grating and transient absorption techniques to elucidate a detailed picture of resonance energy transfer within the light-harvesting proteins. Quantum chemical calculations are employed to estimate phycobilin excited states, and generate transition density cubes which are used to calculate accurately the electronic coupling between the chromophores in PE545 and PC645. Energy transfer dynamics are examined using the generalized Förster theory. Kinetic models for energy transfer dynamics in both proteins are presented for comparison. Investigations of energy transfer from phycoerythrin 545 to chlorophyll-containing light harvesting complexes and photosystems in the intact algae Rhodomonas CS24 and Chroomonas CCMP270 are also reported.