The Ions

1.1 INTRODUCTION

Only sodium (Na), potassium (K), magnesium (Mg) and calcium (Ca) among the Group 1 and 2 elements are essential in biological systems. Some of the other s-block elements are used in medicine (e.g. lithium, Li and barium, Ba) and/or occur as minor (but useful) contaminants in calcium biominerals (e.g. strontium, Sr).

The metal ions with which we are involved in this book (Na+, K+, Mg2+ and Ca2+) are less striking than many of the other metal ions that have significant biological roles. Metal ions such as iron and copper are coloured in solution, have more than one common oxidation state and are strongly coordinating, all properties lacking in the s-block metals. Nevertheless, the very lack of these properties enable them to be the workhorses in many biological processes. These metals display only one stable oxidation state resulting from the loss of one (M+, Group 1) or two (M2+, Group 2) s-electrons. This enables the metal ions to move around the cell without any danger of being oxidized or reduced. In this way they play many vital, complex and intriguing roles. They aid in neurologic and neuromuscular conduction, help regulate pH, maintain osmolality of body fluids, are involved in muscle contraction and are required for the functioning of many enzymes. They are components, calcium in particular, of many of the solid structures in most organisms.

The Heart Says It All

Animal tissues can be kept alive for experimentation for a short time by immersion in a buffered solution that mimics the ionic composition of animal plasma (Ringer's solution). A graphic illustration of the importance of these ions is shown by the composition of a cardioplegic solution used to preserve a donor heart (for 4-6 hours) prior to a transplant. The solution typically contains NaCl, 144 mM; KCl, 20 mM; MgCl2, 16 mM; CaCl2, 2.4 mM and procaine (a local anaesthetic that blocks nerve sodium channels, see 4.6), 1.0 mM at a pH of 5.5-7.5 at 4 °C.

1.2 OCCURRENCE

1.2.1 Earth's Crust

The s-group metals Na, K, Mg and Ca are, after Al and Fe, the most abundant metals in the earth's crust.

An excess of sodium salts in the soil can cause severe problems in crop production. This is because an influx of sodium ions into plant cells can upset critical biochemical processes by competing with potassium ions in membrane transport and enzyme activation, and can also cause osmotic stress. It has been estimated that about one-third of the world's irrigated land is unsuitable for growing crops because of high sodium ion contamination. Calcium ions, on the other hand, can be beneficial in that they help to maintain or enhance the selective absorption of K ions by plants.

A Possible Solution to Salt Contamination of Soil

Shrubs of the genus Plantago produce seeds that are used as a food for birds and also as a bulk-forming laxative. Plants, even those of the same species, have differing abilities to tolerate salty soil. Plantago mahtima, but not P. media, is salt tolerant. P. mahtima (but not P. media) contains an Na+/H+ antiport protein that permits the interchange of Na+ and H+ions (3.6.1). This allows the plant to sequester Na+ ions in the large intracellular vacuole and away from the cytosol where salt interferes with metabolic processes. This represents one way of defeating a saline environment.

A salt-tolerant Arabidopsis has been engineered by overexpressing a single endogenous gene, AtNHXl, which encodes for an Na+/H+ antiport protein. Similarly, tomato plants have been genetically engineered to produce high levels of an Na+/H+ antiport. These plants flourish in 200 mM salt water, and produce good-looking and tasty fruit that is low in sodium even though the leaves accumulate large amounts of the ion. It could now be possible to produce a whole array of salt-tolerant crop plants, which would enable the use of seawater for irrigation.

1.2.2 The Seas

Most (about two-thirds) of the earth's surface is covered with water, and in the seas the s-group metal ions specified above are the major cations present. On average, there is 3.5 g of salts per 100 mL of seawater. This rises to 4% in the Mediterranean and off the southwest coast of Crete, where near-dry conditions 3-5 million years ago has produced a brine pool with a very high MgCl2 concentration. The great salt lake in Utah is saturated with salts and NaCl crystallizes on the shore. In the Dead Sea, CaSO4 crystallizes out. Nevertheless, certain creatures, tiny crustacean brine shrimp (Artemia) for example, but not fish, can survive in these conditions. Indeed, Artemia cannot live for long in fresh water. Coastal evaporation ponds are a commercial source of Artemia, which is used as fish food.

1.2.3 Biological Materials

The s-group metal ions Na, K, Mg and Ca are found in most cells in mM concentrations, see Table 1.1. It is often very difficult to measure the intracellular concentrations, particularly of the free ions. Inorganic and organic anions, which are lower in concentration outside the cell than inside, maintain cell neutrality.

One can draw one's own conclusions about the significance, if any, of the similarity between the concentrations of these ions in extracellular media and in seawater in which life may have evolved. Although there are significant differences between ion concentrations in vertebrates and invertebrates, the differences between the solute composition of the cells and of the extracellular environment, with the glaring exception of Mg, persist for squid and for mammals. These concentration differences are used to carry out many of the vital tasks that are necessary to maintain life.

1.3 COORDINATION CHEMISTRY

1.3.1 Ion Sizes

There is some disagreement as to the sizes of the ions of the s-block (particularly of Li+). However, Pauling's values have, by and large, stood the test of time and are shown in Table 1.2.

We will find that ion size is particularly relevant when the transfer of ions through channels is being considered. For example, a long-standing vexing question arises as to how a dehydrated Na+ ion, which is substantially smaller than a K+ ion, can be rejected by the very selective K+ channel. Earlier proposals have now been strongly supported and extended by an X-ray crystallographic examination of the K+ channel (KcsA) cloned in mg amounts from the membrane of the bacterium Streptomyces lividans. For more about the channel structure and passage of K+ through the pore see 3.4.2. The constriction (~ 3Å across) at the selectivity filter demands that all coordinated H2Os are stripped from the hydrated ion for passage of the ion through the channel. An ideal fit and coordination by the channel O atoms compensate for the associated energy loss. These criteria apply...