  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.
