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In non-hadronic axion models, which have a tree-level axion-electron interaction, the Sun produces a strong axion flux by bremsstrahlung, Compton scattering, and axiorecombination, the "BCA processes." Based on a new calculation of this flux, including for the first time axio-recombination, we derive limits on the axion-electron Yukawa coupling gae and axion-photon interaction strength ga using the CAST phase-I data (vacuum phase). For ma <~ 10 meV/c2 we find ga gae < 8.1 × 10−23 GeV−1 at 95% CL. We stress that a next-generation axion helioscope such as the proposed IAXO could push this sensitivity into a range beyond stellar energy-loss limits and test the hypothesis that white-dwarf cooling is dominated by axion emission.
We analyzed a eukaryotically encoded rubredoxin from the cryptomonad Guillardia theta and identified additional domains at the N- and C-termini in comparison to known prokaryotic paralogous molecules. The cryptophytic N-terminal extension was shown to be a transit peptide for intracellular targeting of the protein to the plastid, whereas a C-terminal domain represents a membrane anchor. Rubredoxin was identified in all tested phototrophic eukaryotes. Presumably facilitated by its C-terminal extension, nucleomorph-encoded rubredoxin (nmRub) is associated with the thylakoid membrane. Association with photosystem II (PSII) was demonstrated by co-localization of nmRub and PSII membrane particles and PSII core complexes and confirmed by comparative electron paramagnetic resonance measurements. The midpoint potential of nmRub was determined as +125 mV, which is the highest redox potential of all known rubredoxins. Therefore, nmRub provides a striking example of the ability of the protein environment to tune the redox potentials of metal sites, allowing for evolutionary adaption in specific electron transport systems, as for example that coupled to the PSII pathway.