Figure 3. Potential energy profiles of the key electronic states in cryptochrome.

The energy for oxidized flavin is shown in green, for radical pair state [FAD^·− + W400^·+] in red and for radical pair state [FAD^·− + W377^·+] in blue. Solid symbols represent calculated energies, while lines represent schematic potential energy surfaces. The energies were computed using the CASSCF method (a) and the perturbation theory-based XMCQDPT-2 method (b). Open circles denote the energies calculated for the optimized [FAD^·− + W400^·+] radical pair after the rearrangement of the W377 residue shown in Fig. 4. Δ₁ is the energy difference between the RP2 and the RP1 state calculated for the RP1-optimized geometry and Δ₂ is the energy difference between the RP1 and the RP2 state calculated for the RP2-optimized geometry; Δ₁₂ is the energy difference between geometry optimized RP2 state and geometry optimized RP1 state. The values of the energies Δ₁, Δ₂, Δ₁₂, according to the data in Tab. 1, are 1.52 eV, 0.50 eV, 0.36 eV, respectively, for the CASSCF method and 1.32 eV, 0.38 eV, 0.26 eV, respectively, for the XMCQDPT-2 method.