2 research outputs found
Excited States of the Inactive and Active Forms of the Orange Carotenoid Protein
The orange carotenoid protein (OCP)
is a crucial player in the process of nonphotochemical quenching in
a large number of cyanobacteria. This water-soluble protein binds
one pigment only, the keto carotenoid 3ā²-hydroxyechinenone,
and needs to be photoactivated by strong (blue-green) light in order
to induce energy dissipation within or from the phycobilisome, the
main light harvesting system of these organisms. We performed transient-absorption
spectroscopy on OCP samples frozen in the inactive and active forms
at 77 K. By making use of target analysis we determined the excited
state properties of the active form. Our results show that OCP photoactivation
modifies the carotenoid excited state energy landscape. More specifically
the photoactivated OCP is characterized by one state with predominantly
ICT character (ICT/S<sub>1</sub>) and a lifetime of 2.3 ps, and another
state with mainly S<sub>1</sub> character (S<sub>1</sub>/ICT) with
a lifetime of 7.6 ps. We also show that the kinetic model is fully
consistent with the RT data obtained earlier (Berera et al., <i>J. Phys. Chem.</i> <i>B</i> <b>2012</b>, <i>116</i>, 2568ā2574). We propose that this ICT/S<sub>1</sub> state acts as the quencher in the OCP mediated nonphotochemical
quenching
The Photophysics of the Orange Carotenoid Protein, a Light-Powered Molecular Switch
To cope with the deleterious effects of excess illumination,
photosynthetic
organisms have developed photoprotective mechanisms that dissipate
the 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), which
binds the carotenoid 3ā²-hydroxyechinenone as its only pigment.
While the spectroscopic properties of the inactive form of OCP have
been described, the nature of the excited states in the active form
still awaits elucidation. We applied transient absorption spectroscopy
to the dark and the light activated forms of OCP to study and compare
the excited state dynamics of both forms. We show that excitation
of the photoactivated OCP leads to the population of new carotenoid
excited states. One of these states populated shortly after excitation
is characterized by a very pronounced charge transfer character and
a lifetime of about 0.6 ps. When the illuminated sample is exposed
to a dark relaxation period, it responds to excitation as the original
dark sample, showing that photoactivation and decay of the photoactivated
state are fully reversible. Thus OCP functions as a light-powered
molecular switch that modulates its spectroscopic properties as a
response to specific changes in light environment. We discuss the
importance of this switch in cyanobacteria photoprotection and propose
a mechanism wherein the red state of OCP echinenone acts as an energy
dissipator via its charge transfer state