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 that the conjugated backbone is
twisted out of plane. However, this hypothesis has been forwarded
only 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 a 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.