The contributions of individual elements
المؤلف:
Peter Atkins, Tina Overton, Jonathan Rourke, Mark Weller, and Fraser Armstrong
المصدر:
Shriver and Atkins Inorganic Chemistry ,5th E
الجزء والصفحة:
777-778
2025-10-28
64
The contributions of individual elements
Key point: Elements are selected by Nature for their inherent useful properties and their availability. In this section we summarize the major roles of each element and correlate what we have discussed with emphasis on the element rather than the type of reaction that is involved. Na, K, and Li The ions of these elements are characterized by weak binding to hard ligands and their specificity is based on size and hydrophobicity that arises from a lower charge density. Compared to Na+, K+ is more likely to be found coordinated within a protein and is more easily dehydrated. Both Na and K are important agents in control ling cell structure through osmotic pressure, but whereas Na is ejected from cells, K is accumulated, contributing to a sizeable potential difference across the cell membrane. This differential is maintained by ion pumps, in particular the Na+ ,K-ATPase, also known as the Na-pump. The electrical energy is released by specific gated ion channels, of which the K channel (Section 27.3) has been studied the most. An important related issue is the widespread use of simple Li compounds (particularly Li2CO3) as psychotherapeutic agents in the treatment of mental disorders, notably bipolar disorder (manic depression). One possibility is that hydrated Li binds tightly in the Na or K channels in place of the dehydrated ions that are normally transported selectively in the filter region of these proteins. As Li ions are highly labile, we can be confident that the aqua ion itself, or a complex with an abundant ligand, is the active species. As a simple aqua ion, Li is strongly solvated, in fact the solvated radius is greater than that of Na. Mg Magnesium ions are the dominant 2+ ion in cytoplasm and the only ones to occur above millimolar levels in the free, uncomplexed state. The energy currency for enzyme catalysis, ATP, is always present as its Mg2 complex. Magnesium has a special role in the light-harvesting molecule chlorophyll because it is a small 2+ cation that is able to adopt octahedral geometry and can stabilize a structure without promoting energy loss by fluorescence. The Mg2 ion is a weak acid catalyst and is the active metal ion in rubisco, the highly abundant enzyme responsible for removing from the atmosphere some 100 Gt of CO2 per year. Rubisco is activated by weak binding of Mg+2 to two carboxylates and a special carbamate ligand, leaving three exchangeable water molecules. Ca Calciumions are important only in eukaryotes. The bulk of biological Ca is used for structural support and devices such as teeth. The selection of Ca for this function is due to the insolubility of Ca carbonate and phosphate salts. However, a tiny amount of Ca is used as the basis of a sophisticated intracellular signalling system. The principle of this process is that Ca is suited for rapid coordination to hard acid ligands, especially carboxylates from protein side chains, and has no preference for any particular coordination geometry. Mn Manganese has several oxidation states, most of which are very oxidizing. It is well suited as a redox catalyst for reactions involving positive reduction potentials. One reaction in particular, in which H2O is used as the electron donor in photosynthesis, is re sponsible for producing almost all the O2 in the Earth’s atmosphere. This reaction involves a special Mn4Ca cluster. Manganese (II) is also used as a weak acid base catalyst in some enzymes. Spectroscopic detectability varies depending on oxidation state: EPR has been useful for Mn (II) and for particular states of the Mn cluster that constitutes the catalyst for the evolution of O2. Fe Versatile Fe is probably essential to all organisms and was certainly a very early element in biology. Three oxidation states are important, namely Fe(II), Fe(III), and Fe(IV). Active sites based on Fe catalyse a great variety of redox reactions ranging from electron transfer to oxygenation, as well as acid base reactions that include reversible O2 binding, dehydration/hydration, and ester hydrolysis. Iron-containing active sites feature ligands ranging from soft donors such as sulfide (as in FeS clusters) to hard donors such as carboxylate. The porphyrin macrocycle is particularly important as a ligand. Iron (II) in various coordination environments is used to bind O2 , either reversibly or as a prerequisite for activation. Iron (III) is a good Lewis acid, whereas the Fe (IV) O (ferryl) group may be considered as Nature’s way of managing a reactive O atom for insertion into C H bonds. Cells contain very little uncomplexed Fe (II) and extremely low levels of Fe(III). These ions are toxic, particularly in terms of their reaction with peroxides, which generates the hydroxyl radical. Primary uptake into organisms from minerals poses problems because Fe is found predominantly as Fe (III), salts of which are insoluble at neutral pH (see Fig. 5.12). Iron uptake, delivery, and storage are controlled by sophisticated transport systems, including a special storage protein known as ferritin. Iron porphyrins (as found in cytochromes) show intense UV–visible absorption bands and most active sites with unpaired electrons give rise to characteristic EPR spectra. Co Cobalt and nickel are among the most ancient biocatalysts. Cobalt is processed only by microorganisms and higher organisms must ingest it as vitamin B12 in which Co is complexed by a special macrocycle called corrin. Complexes in which the fifth ligand is a benzimidazole that is covalently linked to the corrin ring are known as cobalamins. Cobalamins are cofactors in enzymes that catalyse alkyl transfer reactions and many radical-based rearrangements. Alkyl transfer reactions exploit the high nucleophilicity of Co(I). In the special cofactor known as coenzyme B12, the sixth ligand to Co(III) is a carb anion donor atom from deoxyadenosine. Radical-based rearrangements involve the ability of coenzyme B12 to undergo facile homolytic cleavage of the Co C bond, producing stable low-spin Co (II) and a carbon radical that can abstract a hydrogen atom from substrates. Cobalamin-containing enzymes show strong UV–visible absorption bands; EPR spectra are observed for Co (II). Ni Nickel is important in bacterial enzymes, notably hydrogenases, where it also uses the+3 and +1 oxidation states, which are rare in conventional chemistry. A particularly remarkable enzyme, coenzyme A synthase, uses Ni to produce CO and then react it with CH3-(provided by a cobalamin enzyme) to produce a C C bond in the form of an acetyl ester. Nickel is also found in plants as the active site of urease. Urease was the first enzyme to be crystallized (in 1926), yet it was not until 1976 that it was discovered to contain Ni. Cu Unlike Fe, copper probably became important only after O2 had become established in the Earth’s atmosphere and it became available as soluble Cu (II) salts rather than in soluble sulfides (Cu2 S). The main role of Cu is in electron transfer reactions at the higher end of the potential scale and catalysis of redox reactions involving O2. It is also used for reversible O2 binding. Both Cu (II) and Cu(I) are strongly bound to biological ligands, par ticularly soft bases. Free Cu ions are highly toxic and almost absent from cells. Zn Zinc is an excellent Lewis acid, forming stable complexes with ligands such as N and S donors, and catalysing reactions such as ester and peptide hydrolysis. The biological importance of Zn stems largely from its lack of redox chemistry, although its common adoption of di-, tri-, and tetrathiolate ligation provides a link to the redox chemistry of cysteine/cystine interconversions. Zinc is used as a structure former in enzymes and proteins that bind to DNA. A major problem has been the lack of good spectroscopic methods for studying this d10 ion. In some cases, Zn enzymes have been studied by EPR, after substituting the Zn by Co (II). Mo and W Molybdenumis an abundant element that is probably used by all organisms as a redox catalyst for the transfer of O atoms derived from H2O. In these oxo-transfer enzymes the Mo is always part of a larger pterin-containing cofactor in which it is coordinated by a special dithiolene ligand. Interconversion between Mo (IV) and Mo(VI) usually results in a change in the number of terminal oxo ligands, and recovery of the starting material occurs by single-electron transfer reactions with Mo(V) as an intermediate. Aside from oxo-transfer and related reactions, Mo has another intriguing role, that of nitrogen fixation, in which it is part of a special FeS cluster. Use of W is confined to prokaryotes, where it is also used as a redox catalyst, but in reactions where a stronger reducing agent is required. Si Silicon is often neglected among biological elements, yet its turnover in some organ isms is comparable to that of carbon. Silica is an important material for the fabrication of the exoskeleton and of prickly defensive armour in plants. Pt, Au, Bi, and Ru These elements have no known deliberate biological functions and are foreign agents to biological systems, acting under normal conditions as poisons. However, used in controlled procedures, and dressed up by complexation to target a particular site, they are potent drugs, active against a range of diseases and disorders.
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