Structures and functions of reaction spaces constructed using metalloprotiens

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Last year, we have specified the amino acid residues responsible for capturing Pd2+ ions on the interior surface of ap-ferritin by X-ray crystal structural studies. In addition, we have examined the residues which ligate to Au3+ ions and found that some of the Au ion binding sites in apo-ferritin are different from those of Pd ions with different coordi-nation structures. On the basis of these observation, we have prepared two type of metal nano-clusters, i.e., alloy and core/shell as shown in Schem 1. In the co-reduction, Au and Pd ions coexist in the apo-ferritin cage. The number of each ion in the cage is regulated by the affinity of each ions to its specific amino acid residues. Consequently, the alloy nano-particle was composed of 133 Au atoms and 165 Pd atoms ((AuPd)・apo-ferritin). On the other hand, we have successfully prepared Au-core/Pd-shell([Au] (Pd)) and Pd-core/Au-shell ([Pd] (Au)) types of nano-particles composed of 201 Au atoms/305 Pd atoms and 169 Au atoms/65Pd atoms, respectively. Among those apo-ferritin composites, [Au] (Pd) apo-ferritin shows the highest catalytic activity for the hydrogenation of olefins.In a separate experiment, we have examined a possible incorporation of organometallic compounds such as [Pd(allyl)C1]2 and [Rh(norbornadiene)C1]2 in apo-ferritin. The crystal structure of Pd(allyl)・apo-ferritin clearly shows that the allyl group ligates to Pd ion in apo-ferritin.