Properties of the Metal and Anion Sites

The rise in the 465-nm absorbance as Fe is added to the apoprotein (generally as a ferric nitrilotriacetate or ferric citrate complex) can be used to monitor iron binding and forms the basis of iron titrations that demonstrate the presence of two specific sites per molecule (Fig. 17). 

The ionization of tyrosine on metal binding (PhOH PhO -P H ) also causes a dramatic increase in absorbance in the UV region, both at 245 nm and -290 nm, due to perturbations of the tt-tt transitions of the aromatic ring. This effect has been widely exploited for metal titration studies, using UV difference spectroscopy (122-125). Typical metal titration difference spectra are shown in Fig. 18. These can also be used to determine the number of metal ions that are bound (Fig. 18) and to determine metal binding constants, at least for the weaker-binding metal ions (126,127). 

Transition elements Group 13 elements f-block elements  

Most often it is the third criterion that has not been demonstrated to be met. The metal ions that have definitely been shown to bind specifically, by these criteria, are Cr , VO, Mn, Co, Cu, and Ga , as well as Fe 152), but it is highly probable that most of the species in Table VI do bind in the two specific sites. 

Binding constants have been determined for some metal ions, but their determination is complicated by the presence of two very similar sites and by the pH and (bi)carbonate-dependence of the values. Readers are referred to an excellent discussion by Aisen and Harris (17). For Fe , at ambient HCOa concentrations, the log K values are estimated as 19.4 and 20.7 for the two sites (107) and it is generally the case for most metal ions analyzed that the two sites differ by approximately one unit in their log K values (Table VII). 

Selected Binding Constants for Metals Bound to Transferrins0 

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D. Similarities with Bacterial Binding Proteins Properties of the Metal and Anion Sites... 

The computer simulation of clay minerals or other complex soil phases is often limited by the large unit cells and supercells required to properly represent the bulk or surface structure of the sorbing material. Furthermore, multicomponent substitutions in the structure, especially for clay minerals, lead to complex disordered systems that are not amenable to molecular simulation based on a simple unit cell to represent the system. Recently, Cygan and Kirkpatrick (2002) determined the sorp-tion sites of several complex clay phases based on the forcefield method of Pat-(abiraman et al. (1985). This approach allows for the evaluation of the fundamental atomistic properties that control the sorption of molecules, metals, and anions onto the internal or external surface of clays. It is possible to evaluate the influence of crystallography, cleavage surface, composition, isomorphic substitution, and sur-l ace protonation state on the relative sorption energy. 

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