Saltman—Spiro ball

A particularly interesting synthetic model for the ferritin core is the iron (III) polymer obtained by Spiro, Saltman, and coworkers from bi-carbonate-hydrolyzed ferric nitrate solutions (6). The polymer has an approximate molecular weight of 150,000 and is about 70 A in diameter. Analytical studies indicate a composition Fc403(0H)4(N03)2 I.5H2O for the polymer. In physical size and shape, the Saltman-Spiro ball is remarkably similar to the ferritin core. The structure that has been sug-... 

Figure 5. Structural model proposed for the Saltman-Spiro ball, adapted from ...

Figure 6. Electronic absorption spectrum of the Saltman-Spiro ball...

Our electronic absorption spectral studies on the ferritin core itself are less definitive (8, 9). Because of strong tail absorption through the visible, we have been able to resolve only a 918-nm LF band. This band is diagnostic of [Fe(III)06]oct coordination, but without better spectra in the 400-700-nm region we cannot distinguish between exclusively [Fe(lII)06]oct and mixed [Fe(III)06]oct-[Fe(III)04]tet coordination structures. There is, however, no positive evidence for the presence of [Fe(III)04]tet units in the core and the analogy with the Saltman-Spiro ball leads us to prefer a model with [Fe(III)06]oct coordination. 

Our structural investigations of hydrolytically polymerized Fe(III) have not been limited to the high molecular weight systems such as the Saltman-Spiro ball. Much more of our research, in fact, has been concentrated in the area of relatively simple binuclear Fe(III) complexes. The polyfunctional ligands EDTA (ethylenediaminetetracetate) and... 

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