Time-resolved photophysics of orange carotenoid protein trapped in trehalose-sucrose glass films

This thesis presents studies on the photophysics of the orange carotenoid protein (OCP), a natural single-carotenoid-binding protein found in cyanobacteria, here binding canthaxanthin. We initially studied OCP to test a hypothesis forwarded in studies of light-harvesting complexes (LHCs) from purple photosynthetic bacteria. A protein-induced carotenoid twist in some LHCs putatively sensitises intramolecular singlet fission (SF), thus yielding the relatively long-lived (microsecond) triplet states observed. SF has promise in boosting solar photovoltaic efficiency, among other applications with a presently unmet requirement of microsecond triplet lifetimes. OCP is among the simplest systems for a direct test of the twist-induced intramolecular SF hypothesis. To prevent artefacts associated with its native photoprotective function involving a light-induced conformational change, we trap OCP in a trehalose-sucrose glass matrix, which we demonstrate yields identical initial photophysics to conventional buffer measurements. We found that OCP-twisted canthaxanthin does not yield the triplet population promised by the aforementioned studies. Indeed, a follow-up study inspired by this work (included as an appendix here) shows that SF in those LHCs proceeds on adjacent carotenoid and bacteriochlorophyll molecules, and significantly contributes to solar energy harvesting. We then tested a recent suggestion in the OCP literature that the light-induced photoconversion in OCP is triggered by a long-lived carotenoid singlet-like feature, dubbed S*. By examination of the pump wavelength dependence on the OCP photophysics in films and in buffer, we show that S* does not correlate with the photoconversion yield, ruling out S* as a sole trigger of the process. Dark-adapted OCP was found to have significant ground-state heterogeneity, with S* arising from it. We follow up with a hypothesis that the trigger is instead through a <1% yield of intersystem crossing triplets, though we lack a direct measurement, owing to difficulty in measuring the low yield of photoproducts.