The reversion of OCPR to OCPO is light independent and occurs slowly in darkness. OCPO is considered the dark, stable form of the protein, and does not contribute to phycobilisome quenching. OCPR is considered to be essential for induction of the photoprotection mechanism. The photoconversion from the orange to red form has a poor light efficiency (very low quantum yield), which helps to ensure the protein's photoprotective role only functions during high light conditions; otherwise, the dissipative NPQ process could unproductively divert light energy away from photosynthesis under light-limiting conditions. As evidenced by a decreased fluorescence, OCP in its red form is capable of dissipating absorbed light energy from the phycobilisome antenna complex. According to Rakhimberdieva and coworkers, about 30-40% of the energy absorbed by phycobilisomes does not reach the reaction centers when the carotenoid-induced NPQ is active. The exact mechanism and quenching site in both the carotenoid as well as the phycobilisome still remain uncertain. The linker polypeptide ApcE in the allophycocyanin (APC) core of the phycobilisomes is known to be important, but is not the site of quenching. Several lines of evidence suggest that it is the 660 nm fluorescence emission band of the APC core which is quenched by OCPR. The temperature dependence of the rate of fluorescence quenching is similar to that of soluble protein folding, supporting the hypothesis that OCPO slightly unfolds when it converts to OCPR. Contributed by the auxiliary function of carotenoids as quenchers of singlet oxygen, a photoprotective role of OCP has also been demonstrated under strong orange-red light, which are conditions where OCP cannot be photoactivated for its energy-quenching role. This is significant because all oxygenic phototrophs have a particular risk of oxidative damage initiated by singlet oxygen (1O2), which is produced when their own light-harvesting pigments act as photosensitizers. The three-dimensional protein structure of OCP (in the OCPO form) was solved in 2003, before its photoprotective role had been elucidated. The 35 kDa protein contains two structural domains: an all-α-helical N-terminal domain (NTD) consisting of two interleaved 4-helix bundles, and a mixed α/β C-terminal domain (CTD).