Complexing in Natural Systems

Most natural water systems contain an excess of Ca(II) compared with the available strong ligands. Consequently, although calcium complexes are rather weak, the amount of ligand left over to complex with trace metals is often small, perhaps often negligible. In some situations, however, complex formation is believed to be important in natural waters—for example, when the concentration of dissolved metal ions is higher than expected in the solubiliza-tion of metal ions for transport in soils and plants, and in the sometimes unexpected chemical behavior of trace metals. The enrichment of aqueous trace metals by plants tends to follow the normal stability sequence for metal ion complexes [this is, in part, that the complex stability with a given ligand that follows the order Mn(II) < Fe(II) < Co (II) < Ni(II) < Cu(II) > Zn(II)]. This suggests that trace metal uptake by plants is related to complex formation. There is evidence that unpleasant outbreaks of algal growth (e.g., the Florida "red tide") are linked to the availability of soluble metal complexes.

Most naturally occurring organic ligands are decomposition products or byproducts of living organisms. There are several main types.

Humic acid, fulvic acid, humin, etc. Soils and sediments normally contain organic matter derived from decaying vegetation. Much of this organic matter is made up of fulvic or humic acid materials, or humin. These are members of a related series distinguished primarily on the basis of solubility differences. Fulvic acid is soluble in both acid and base, humic acid in base only, and humin is insoluble. The molecular weights increase in the same sequence, ranging from near 1000 to perhaps 100,000 Da. They are not definite simple compounds, but polymeric materials. Their exact origin is uncertain: One theory suggests that they are oxidized and decomposed derivatives of lignin; another that they are formed by polymerization of simple decomposition products from plant materials like phenols and quinones. The structures contain aromatic rings with oxygen, hydroxyl, and carboxyl groups, and some nitrogen groups, all of which have potential coordination abilities. Typical units are as follows.

COOH

COOH

It is well established that strong interactions between these materials and metal ions take place; humic substances have a high capacity for binding metal ions. They form a potential reservoir of trace metals in soils. The availability of trace metals in soil to plants evidently depends on complex formation. Most soil sources of essential metals, which include V, Mo, Mn, Co, Fe, and Cu, are insoluble, and the small amounts that can be made available through acidic attack of the oxides, carbonates, silicates, and so on in which they are present appear to be inadequate for normal plant needs. Large amounts may be available through a reservoir of coordinated metal held in humic acids in the soil, which may be released on further decomposition, or by an attack from agents such as citric or tannic acids, or other substances generated by plant roots. Humic substances are often associated with color in water, and are sometimes accompanied by a high iron content through complex formation.

Polyhydroxy compounds: carbonates, sugars. The following grouping,

often present in sugars, can chelate, although it does not normally form strong complexes. These substances are widespread in metabolic products and in carbohydrate decomposition products.

Amino acids, peptides, and protein decomposition products. A simple amino acid, such as the following

R-CH-COOH I

FIGURE 9-17 The porphin structure.

is an effective chelating agent. Such molecules can be products of protein hydrolysis, although there is no definite evidence for their importance in natural water systems. Peptides and related substances are more important in this respect; for example, the ferrichromes are very stable, soluble iron complexes. These ligands, which have high specificity for Fe(III), are common in microorganisms and are present as decomposition intermediates. They seem to be useful in transporting iron into cells and making it available to iron-containing enzymes. The iron is bound in an octahedron of oxygen atoms from groups such as the following.

Porphyrins and related compounds. Compounds based on the tetrapyrrole nucleus make up an important class of naturally occurring complexing agents. The parent compound is porphin,11 the structure for which is given in Figure 9-17. Considerable scope for electron delocalization is possible in this ring structure (i.e., resonance forms exist), and partly because of this, these compounds exhibit a high stability. Substituents on the porphin skeleton, and minor changes to the ring structure, are found in the various naturally occurring compounds of this class. The porphyrin structure is planar, with sufficient room in the center of the ring system for binding a metal ion, as shown in Figure 9-18. This is an example of a macrocyclic ligand.

The electron delocalization just referred to may involve orbitals and electrons from the metal as well as those of the organic molecule. Metal ions that tend to form octahedral complexes may have ligands above and below the porphyrin plane; the bonds are also influenced by the electron delocalization in the plane. Examples of compounds of this kind are the iron-containing hemo-proteins, including hemoglobin and myoglobin; cobalt-containing vitamin B12 and related enzymes (based on a slightly modified ring called the corrin structure); and chlorophyll, with magnesium as the metal.

11A substituted porphin is called a porphyrin.

FIGURE 9-18 A porphin ring containing a metal ion.

The structure of the metal ion portions of hemoglobin and myoglobin, used in biological oxygen transport and storage, is shown in Figure 9-19. In addition to the four porphyrin nitrogen atoms in the coordination sphere of the Fe(II) ion, a fifth nitrogen from a histidine group in the amino acid portion of the molecule is attached to one of the remaining coordination sites. The sixth coordination site of the Fe(II), which is the active site, may have a water molecule replaced reversibly by an oxygen molecule (dioxygen). That is, 02 is taken up when its partial pressure is high, and released when it is low. 0ther ligands may be attached to the sixth site in place of water or dioxygen. If such ligands are not replaced readily, as in the case with C0, which bonds strongly to the Fe(II) in hemoglobin, the oxygen transport function is blocked and death can result. This illustrates one process by which the biological function of a species containing a metal ion can be disrupted.

FIGURE 9-19 The iron center in hemoglobin and myoglobin. Groups attached to the porphin ring are not shown.

The exact behavior of the metal ion in this and analogous systems is determined not only by the immediate, coordinate environment that influences the electron density on the metal, but also by the extensive protein structure that is part of the molecule. This protein structure may have indirect effects on electron density and may exhibit important steric effects that control the overall reactions. Thus, iron-containing enzymes with direct coordination structures similar to hemoglobin, but different structures of the protein chains, may not exhibit reversible oxygen uptake. Rather, they may act as electron transfer agents through the conversion of Fe(II) to Fe(III).

Complex formation with organic materials also may influence mineral weathering reactions, largely through solubilizing a weathering product that otherwise would be insoluble, or through the effect of complexing on the stability of different oxidation states of an element. For example, Fe(II) is usually oxidized to Fe(III) only slowly; the reaction may be faster if it can proceed as follows:

Fe(II) + organic ligand ^ Fe(III) 4 organic complex (9-65)

Ligands such as tannic acid, amino acids with SH groups, or phenols are particularly effective. We may also encounter more complicated cycling reactions in which the Fe(III) complex oxidizes the organic ligand, reverting to Fe(II) again. Organic materials in groundwater lower pE and result in solubility and stability of Fe(II) and Mn(II). [Normally, these elements are present as insoluble Mn(IV) and Fe(III) oxide species.] The presence of either of these ions is an indicator of organic contamination of the groundwater source.

Complexation may provide a buffering of free metal ion concentration. To maintain the equilibrium conditions required by the complex stability constant, excess metal may be tied up as a complex, or released from a complex form. A regulation of this sort probably occurs in organisms and may be present in other parts of the environment; for example, organic-rich sediments may have some action of this kind in water bodies.

Complex formation in seawater is significant, but the predominant cations, Ca(II), Mg(II), Na(I), and K(I), are largely present as the aqua species. About 10% of the alkaline earth ions and 1% of the alkali metal ions in seawater are complexed, mostly by the sulfate ion. Relative concentrations are such that this takes roughly half of the sulfate ion concentration. Carbonate and bicarbonate ions are present in much lower concentrations, but the amounts present are extensively complexed with the more abundant cations. This complexing is often compensated for in equilibrium calculations through the use of "stoichiometric" constants valid for seawater conditions (Section 9.1.3).

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