The Photophysics of the
Orange Carotenoid Protein,
a Light-Powered Molecular Switch
- Publication date
- Publisher
Abstract
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 3′-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