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Quantum Compass of Migratory Birds

FIGURE 4.12 Model for the cCRY4 photocycle and calculated absolute absorbance spectra of photointermediates

(Mitsui et al., 2015). (a) Short duration (0.5 minutes) irradiation with blue light (453 nm, 1 mW/cm²) reduced the

FADox chromophore to the FADH^• form. Extended blue light or red light irradiation further reduced FADH^• to

FADH⁻. FADH^• is likely oxidized to FADox directly in dark, while FADH⁻ is likely oxidized to FADox via FADH^•

during dark incubation. (b) Absolute absorbance spectra of cCRY4 photointermediates estimated by calculation.

Te dark-adapted samples are considered to be mainly composed of FADox with a small amount of residual FADH^•.

Te sample, afer long irradiation with white light (128 minutes, 1 mW/cm²), is presumed to be composed of only

FADH⁻ forms. Spectral changes induced by the initial 0.5 minutes blue light and the following 8 minutes red light

irradiations are assumed to correspond to conversions from FADox to FADH^• and FADH^• to FADH⁻, respectively.

Te absorbance spectra were estimated as follows. (i) All spectra were normalized by their optical density and pre­

sumed to have absorbances at the peak of the dark-adapted spectrum such that the FADox form (447 nm) was 1.0.

Absorbances at putative isosbestic points between the FADH^• and FADH⁻ forms or the FADox and FADH⁻ forms

were used. (ii) When the putative photobleaching rates of FADox were changed from 20% to 50%, putative abso­

lute absorption spectra for FADH^• were calculated and the photobleaching rate was determined to be 31%, placing

the isosbestic point between FADH^• and FADH⁻ at 446 nm. (iii) Te determined absolute absorption spectrum of

FADH^• was diminished from “dark” or “white 128 min” in the spectrum with changing putative contents of the

contaminating FADH^• form. Mitsui et al. (2015) inferred the FADH^• content in the FADox and FADH⁻ forms to

be 3%, assuming that FADox had no absorbance in the longer wavelength region (> 520 nm). (Reproduced with

permission from Mitsui et al., 2015, Copyright 2015, American Chemical Society.)

Moreover, using spectroscopic methods, Hochstoeger et al. (2020) showed that pigeon (Columba

livia) cryptochrome ClCRY4 (Zoltowski et al., 2019) is photoreduced efciently and forms long-lived

spin-correlated radical pairs via a tetrad of tryptophan residues. Hochstoeger et al. (2020) reported that

ClCRY4 is broadly and stably expressed within the retina but enriched at synapses in the outer plexiform

layer in a repetitive manner. A proteomic survey for retinal-specifc ClCRY4 interactors identifed mol­

ecules that are involved in receptor signaling, including glutamate receptor-interacting protein 2, which

colocalizes with ClCRY4. Teir data support a model whereby ClCRY4 acts as a UV-blue photorecep­

tor and/or a light-dependent magnetosensor by modulating glutamatergic synapses between horizontal

cells and cones (Hochstoeger et al., 2020).