Figure 1.

(a) Simplified models of the circadian clock feedback loop in Drosophila. CLOCK (CLK) and CYCLE (CYC) promote the tim and per genes. PER and TIM first accumulate in the cytoplasm and then enter into the nucleus to block their gene transcription. Upon light, absorption CRY binds to TIM, and this results in the degradation of TIM^67,68. (b) Flavinsemiquinone, ${\text{FADH}}^{\mathbf{·}}$, and superoxide ${\text{O}}_2^{\mathbf{·}-}$ radical pair in CRY, considered in the RPM model in the present work. The radical pair undergoes interconversion between singlet and triplet states.