The chemistry of sites that specifically bind Rb in Chlorella pyrenoidosa has been investigated by changing or modifying specific chemical groups or bonds in the cell and observing changes in binding capacity. Boiling the cells in water or in 70 per cent ethanol did not affect binding capacities of the sites. These results suggest that the integrity of the sites is independent of both hydrogen bonds and hydrophobic bonds, and that the sites, therefore, do not consist of a protein or protein-lipid complex. At 30°C, both 1 M HCL and 0.5 to 1 M NaOH rapidly inactivated 70 per cent of the sites, but over a range of Ph 4.4 to 11.3, there was no effect. The sites are inactivated by strong chelating agents at 0.05 to 0.2 M and by reagents which reduce trivalent iron, and 4 to 10 atoms of iron per site are removed from the cells. Prolonged incubation in iron solutions, but not in solutions of Cu, Mn, or Mg, reversed to a considerable extent inactivation by EDTA. It is suggested that the sites probably bind trivalent iron tightly as chelation bridges which are essential to their structure. These structural bridges are broken when iron is removed by chelating agents or reduction, and are reformed in the presence of iron. Other experimental evidence indicates that amine, sulfhydryl, and carbonyl groups are not structural components of the sites.

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