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A B S T R A C THydrolysis of ferric solutions leads initially to mono-and dinuclear species which interact to produce further species of higher nuclearity. These polynuclear species age eventually to either crystalline compounds or to an amorphous precipitate (amorphous iron(l1 I ) hydroxide hydrate).Amorphous iron(lI1) hydroxide hydrate is thermodynamically unstable and gradually transforms to a-FeO(0H) and a-Fe203. These crystalline products form by competing mechanisms and the proportion of each in the final product depends on the relative rates of formation. The master variable governing the rates at which these compounds form is pH. Other important factors are temperature and the presence of additives.Most additives retard the transformation and by suppressing formation of a-FeO(0H) lead to an increase in the amount of a-Fe,03 in the product; some additives also directly promote formation of the latter compound. Metal ions can o f e n replace a proportion of Fe in the a-FeO(0H) and a-Fe,O, lattices. At high enough concentrations they can induce formation of additional phases. Additives may also modify the morphology of the crystalline products.

The reductive mobilisation of iron from ferritin, the principal protein of iron storage, was studied. The kinetic characteristics of iron release by dithionite, thioglycollate, and dihydroriboflavin 5'-phosphate (FMNH,) were found to differ widely. The dependence on pH is most pronounced for the dithionite reduction which proceeds 100 times faster at pH 4 than at pH 7. The experimental data can be consistently explained in terms of specific interactions of products or educts with interfacial iron(II1) hydroxide of the ferritin core. Surface complexes with the product sulfite are postulated in the dithionite reaction, and with the educt in the thioglycollate reaction. Iron(I1) complexes with the radical anion FMN' are suggested to be involved in the iron release by FMNH?. The mobilisation of iron by a series of thiols of different size and coordinative properties confirmed the importance of surface complex formation. N o evidence was found for predominant effects of hindered shell penetration.Ferritin, as an iron storage protein, is a key molecule in iron metabolism. It maintains iron in a form from which it can easily be mobilised as and where it is required. Although the amino acid sequences of many ferritins are known [ I -51 and the three-dimensional structure of horse spleen ferritin has bccn solved at 0.28-nm resolution [6], the mechanisms of ferritin iron deposition and mobilisation are not yet really understood.

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