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The hypothesis of GLIKMAN and ZABRODA (Biochemistry [USSR] 84,, 239 [1969]) that the primary electron donor during photoreduction of manganese(III) in Mn(III)-hydroxychlorin compounds in oxygen free aqueous alkaline solutions is the axially bound OH- ion was tested with Mn(III)-2-a-hydroxyethyl-isochlorin e4. It has been shown that
1) the primary generation of OH radicals upon irradiation of the complex is highly improbable,
2) light is not essential for the reduction reaction,
3) the kinetics of photoreduction of the Mn(III)-compound in 2 N NaOH clearly is not compatible with OH radical formation.
During photooxidation of polycyclic aromatic hydrocarbons (PAH) products can be formed which develop chemiluminescence on treatment with bases. Flash photolysis experiments show that this is the case only after previous formation of cation radicals, e.g. in the presence of CCl4 as solvent or of e-acceptors in aprotic solvents. These radicals react with oxygen to peroxy-radicals which can combine to several kinds of peroxides. Primary and secondary peroxides are the sources of chemiluminescent activity.
Chemiluminescent peroxides can also be obtained by irradiation of PA H carbonyl com pounds in protic solvents under nitrogen. It is assumed that two excited CO groups combine exceptionally with their O-atom s thus creating a peroxide bond. 24 aromatic aldehydes, ketones, dicarboxylic acid anhydrides and coumarines develop chemiluminescence after illumination with wavelengths ≥ 320 nm with intensities varying 4 magnitudes of order.
The sensitivity of the photochemiluminescent method is sufficient to detect amounts of PA H and their CO derivatives in the ppb to ppm range.
Cryptochrome 1a, located in the UV/violet-sensitive cones in the avian retina, is discussed as receptor molecule for the magnetic compass of birds. Our previous immunohistochemical studies of chicken retinae with an antiserum that labelled only activated cryptochrome 1a had shown activation of cryptochrome 1a under 373 nm UV, 424 nm blue, 502 nm turquoise and 565 nm green light. Green light, however, does not allow the first step of photoreduction of oxidized cryptochromes to the semiquinone. As the chickens had been kept under ‘white’ light before, we suggested that there was a supply of the semiquinone present at the beginning of the exposure to green light, which could be further reduced and then re-oxidized. To test this hypothesis, we exposed chickens to various wavelengths (1) for 30 min after being kept in daylight, (2) for 30 min after a 30 min pre-exposure to total darkness, and (3) for 1 h after being kept in daylight. In the first case, we found activated cryptochrome 1a under UV, blue, turquoise and green light; in the second two cases we found activated cryptochrome 1a only under UV to turquoise light, where the complete redox cycle of cryptochrome can run, but not under green light. This observation is in agreement with the hypothesis that activated cryptochrome 1a is found as long as there is some of the semiquinone left, but not when the supply is depleted. It supports the idea that the crucial radical pair for magnetoreception is generated during re-oxidation.