Assessment of S* in the Orange Carotenoid Protein

TL;DR

This study investigates whether the carotenoid singlet excited state (S*) is necessary for the photoconversion of the orange carotenoid protein, finding it is not essential and likely results from ground-state heterogeneity.

Contribution

The paper demonstrates that S* is not required for OCP photoconversion and clarifies its origin as ground-state heterogeneity, challenging previous assumptions.

Findings

S* is only observed for pumps below 495 nm.

Photoconversion occurs even when S* is not formed.

S* likely arises from ground-state heterogeneity.

Abstract

The orange carotenoid protein (OCP) is the water-soluble mediator of non-photochemical quenching in cyanobacteria, a crucial photoprotective mechanism in response to excess illumination. OCP converts from a dark-adapted inactive state (OCPo) to an active quenching conformation (OCPr) under high-light conditions, resulting in a concomitant redshift in the absorption of the bound carotenoid. Here, we test whether a long-lived carotenoid singlet excited state (S*) is required for this photoconversion. We measured pump wavelength-dependent transient absorption of OCPo trapped in trehalose-sucrose glass films. We found that initial OCP photoproducts are still formed despite the glass preventing completion to OCPr, and that S* is only apparent for <495 nm pumps. By comparison to the pump wavelength-dependence of the OCPo to OCPr conversion in buffer, we show that S* is not required for photoconversion, and that S* likely arises from ground-state heterogeneity within OCPo.