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Author(s): Alexander V. Ruban [1, 6]; Rudi Berera [2, 6]; Cristian Ilioaia [3, 4]; Ivo H. M. van Stokkum [2]; John T. M. Kennis [2]; Andrew A. Pascal [3]; Herbert [...]

Analysis on a range of artificial light-harvesting dyads built fro zinc phthalocyanine (Pc) covalently attached to carotenoids with 9,10 or 11 conjugated carbon-carbon double bonds is performed to determine the energy transfer and excited-state deactivation pathway following excitation of the strongly allowed carotenoid [S.sub.2] state as a function of the conjugation length. A new unseen energy relaxation pathways and a unique type of collective carotenoid-phthalocyanine excited state in carotenophthaloyanine dyads are observed.

The orange carotenoid protein (OCP)is a crucial player in the process of nonphotochemical quenching ina large number of cyanobacteria. This water-soluble protein bindsone pigment only, the keto carotenoid 3′-hydroxyechinenone,and needs to be photoactivated by strong (blue-green) light in orderto induce energy dissipation within or from the phycobilisome, themain light harvesting system of these organisms. We performed transient-absorptionspectroscopy on OCP samples frozen in the inactive and active formsat 77 K. By making use of target analysis we determined the excitedstate properties of the active form. Our results show that OCP photoactivationmodifies the carotenoid excited state energy landscape. More specificallythe photoactivated OCP is characterized by one state with predominantlyICT character (ICT/S1) and a lifetime of 2.3 ps, and anotherstate with mainly S1character (S1/ICT) witha lifetime of 7.6 ps. We also show that the kinetic model is fullyconsistent with the RT data obtained earlier (Berera et al., J. Phys. Chem.B2012, 116, 2568–2574). We propose that this ICT/S1state acts as the quencher in the OCP mediated nonphotochemicalquenching. [ABSTRACT FROM AUTHOR]

To cope with the deleterious effects of excess illumination,photosyntheticorganisms have developed photoprotective mechanisms that dissipatethe absorbed excess energy as heat from the antenna system. In cyanobacteria,a crucial step in the process is the activation, by blue-green light,of a soluble protein, known as orange carotenoid protein (OCP), whichbinds the carotenoid 3′-hydroxyechinenone as its only pigment.While the spectroscopic properties of the inactive form of OCP havebeen described, the nature of the excited states in the active formstill awaits elucidation. We applied transient absorption spectroscopyto the dark and the light activated forms of OCP to study and comparethe excited state dynamics of both forms. We show that excitationof the photoactivated OCP leads to the population of new carotenoidexcited states. One of these states populated shortly after excitationis characterized by a very pronounced charge transfer character anda lifetime of about 0.6 ps. When the illuminated sample is exposedto a dark relaxation period, it responds to excitation as the originaldark sample, showing that photoactivation and decay of the photoactivatedstate are fully reversible. Thus OCP functions as a light-poweredmolecular switch that modulates its spectroscopic properties as aresponse to specific changes in light environment. We discuss theimportance of this switch in cyanobacteria photoprotection and proposea mechanism wherein the red state of OCP echinenone acts as an energydissipator via its charge transfer state. [ABSTRACT FROM AUTHOR]