Is orange carotenoid protein photoactivation a single-photon process?

Area of Science:

• Biophysics
• Photochemistry
• Cyanobacterial Biology

Background:

• The orange carotenoid protein (OCP) is crucial for photoprotection in cyanobacteria.
• OCP photoactivation involves a cascade of intermediates leading to the active OCP^R form.
• Existing models struggle to explain both single-pulse and continuous light irradiation data.

Purpose of the Study:

• To investigate the photoactivation mechanism of *Synechocystis* OCP under biologically relevant conditions.
• To reconcile discrepancies between single-photon absorption models and experimental observations.
• To elucidate the role of photon flux density in OCP photoactivation.

Main Methods:

• Utilized stationary irradiation light with biologically relevant photon flux density.
• Employed nanosecond laser pulse excitation for OCP photoactivation studies.
• Analyzed photoactivation kinetics under continuous and pulsed light conditions.

Main Results:

• OCP^R formation quantum yield increases with photon flux density.
• A simple single-photon absorption model is insufficient to describe OCP photoactivation kinetics.
• Evidence suggests a two-photon absorption mechanism involving a red intermediate.

Conclusions:

• OCP photoactivation is not solely initiated by single-photon absorption.
• A red intermediate with a ~100 ms lifetime is essential for OCP^R formation.
• The findings necessitate a revised model for OCP photoactivation, incorporating multi-photon events.