Twisted carotenoids do not support efficient intramolecular singlet fission in the orange carotenoid protein

TL;DR

This study investigates whether twisted carotenoids in the orange carotenoid protein support intramolecular singlet fission, finding that twisting alone does not facilitate this process in a minimal one-carotenoid system.

Contribution

The paper provides experimental evidence that twisted carotenoids in a simplified protein system do not support efficient intramolecular singlet fission, challenging previous hypotheses.

Findings

No long-lived triplet states observed in twisted carotenoid OCPo

Twisting alone does not induce intramolecular singlet fission in carotenoids

Intramolecular singlet fission is unlikely in minimal carotenoid systems

Abstract

Singlet exciton fission is the spin-allowed generation of two triplet electronic excited states from a singlet state. Intramolecular singlet fission has been suggested to occur on individual carotenoid molecules within protein complexes, provided the conjugated backbone is twisted out-of-plane. However, this hypothesis has only been forwarded in protein complexes containing multiple carotenoids and bacteriochlorophylls in close contact. To test the hypothesis on twisted carotenoids in a 'minimal' one-carotenoid system, we study the orange carotenoid protein (OCP). OCP exists in two forms: in its orange form (OCPo), the single bound carotenoid is twisted, whereas in its red form (OCPr), the carotenoid is planar. To enable room-temperature spectroscopy on canthaxanthin-binding OCPo and OCPr without laser-induced photoconversion, we trap them in trehalose glass. Using transient absorption spectroscopy, we show that there is no evidence of long-lived triplet generation through intramolecular singlet fission, despite the canthaxanthin twist in OCPo.