Binding of metals

Mammalian metallothioneins typically bind seven metal ions in cluster structures, with bridging sulfur groups, as seen in the x-ray structure of the Cd5Zn2MT complex (86). It is therefore difficult to develop a simple formation-constant description for the binding of metal ions to MT (87), considering that protonation-deprotonation equilibria of the free protein itself should also be taken into account. However, the usefulness of Table VIII as a guide to the affinity of metal ions for mercapto donor ligands is seen in that the ability of metal ions to... 

The primary binding site of adenine is N(9) but when this is blocked as in adenosine it becomes N(l). This makes the binding of metals to adenosine weaker than the binding to N(7) of guanosine. The N(7) of guanosine is a known major site of alkylation too. 

Meares, C.F. (1986) Chelating agents for the binding of metal ions to antibodies. Nucl. Med. Biol. 13, 311-318. 

The most dramatic rate retardations of proton transfers have been observed when the acidic or basic site is contained within a molecular cavity. The first kinetic and equilibrium studies of the protonation of such a basic site were made with large ring bicyclic diamines [72] (Simmons and Park, 1968 Park and Simmons, 1968a). It was also observed (Park and Simmons, 1968b) that chloride ion could be trapped inside the diprotonated amines. The binding of metal ions and small molecules by macrocyclic compounds is now a well-known phenomenon (Pedersen, 1967, 1978 Lehn, 1978). In the first studies of proton encapsulation, equilibrium and kinetic measurements were made with several macrobicyclic diamines [72] using an nmr technique. 

Even if the solubility limit of relevant solid phases is not exceeded, a large fraction of trace metals in natural waters often occurs bound to particles of various size ranges (nanometres to millimetres). An important process for binding of metals to particles is adsorption to mineral surfaces (often oxides or hydroxides) [29]. 

In the following section, the role of the various types of complexes mentioned above will be discussed with regard to various mechanisms of interactions at biological interphases. It is clear that metal ions and hydrophilic complexes cannot distribute into the membrane lipid bilayer or cross it. The role of hydrophilic ligands has thus to be discussed in relation to binding of metals by biological ligands. In contrast, hydrophobic complexes may partition into the lipid bilayer of membranes (see below, Section 6). 

Binding of metal ions to carrier ligands occurs in competition to the ligands in the medium (Figure 9a, adapted from [192]). The metal ions react with biological carrier ligands. 

Table 3. Representative affinity constants for the binding of metal to transport sites or whole cells/organisms. Ionic strengths and pH values are given for the conditional constants. In the column Comments , information on the method of determination (Km = Michaelis-Menten constant WC = whole-cell titrations) the type of constant (CC = conditional constant IC = intrinsic constant) and special conditions (Cl = competitive inhibitors NICA = nonideal competitive adsorption) are given...

Stability constants (ethylendiamine, glycinate, oxalate), surface complex formation constants and solubility products (sulfides) of transition ions. The surface complex formation constant is for the binding of metal ions to hydrous ferric oxide =Fe-OH + Me2+ =FeOMe++ H+ K. ... 

Example 2.3 Stoichiometry of H+ Release by Binding of Metal Ions ... 

Binding of metal ions by amorphous silica. 300 mg Si02 dm-3, [Mej = 10 5 MJ. The fraction of metal ion bound was calculated from peak currents measured with differential pulse polarography. 

In a simplified form Eq. (ii) was used, decades ago, to assess metal ion adsorption to surfaces, by plotting log ([Meads] / [Me2+]) vs pH. (Kurbatov et al., 1951). The slope of this curve gives an idea on n. The model for this "Kurbatov-plot" assumes that the adsorbent =S is present in large excess and that the adsorption at constant pH is not affected by surface charge. Fig. 2.11 gives an example for the binding of metal ions to amorphous Si02. 

Discuss the binding of metal ions and of ligands in terms of the Lewis-acid-base theory. 

The Binding of Metals to the Settling Particles. The partition of metals between particles and the water depends... 

Simple steady-state models may be used in order to relate quantitatively the mean concentration in the lake water column and the residence time of metal ions to the removal rate by sedimentation (for a detailed treatment of lake models see Imboden and Schwarzenbach, 1985). In a simple steady-state model, the inputs to the lake equal the removal by sedimentation and by outflow the water column is considered as fully mixed mean concentrations and residence times in the water column can be derived from the measured sedimentation fluxes. The binding of metals to the particles is fast in comparison to the settling. 

A recently developed method to prepare particles of a narrow size distribution is the dendrimer-assisted method. In this method (Fig. 1.1), metal ions in a solution are complexed to a dendrimer, mostly with the amine groups in the outer shell of an OH-terminated poly(amidoamine), for example. The complexed metal ions are subsequently reduced to metal atoms which agglomerate into a metal particle. Because dendrimers can be prepared with high purity, and there is a strong binding of metal ions... 

---