Biological systems alkaline earth metals

In spite of the overwhelming importance of the channel mechanism for the transport of alkali and alkaline earth metal ions in biological systems, only carrier transport has been studied extensively by chemists. Studies on ion channel mimics of simple structures have long been limited to antibiotic families of gramicidin, amphotericin B, and others. Several pioneers have reported successful preparation of non-peptide artificial channels. However, their claims have been based on kinetic characteristics observed for the release of metal ions through liposomal membrane and lacked the very critical proofs of channel formation. Such a situation was... 

Not all complexes are purely electrostatic. In fact, many metal complexes in biological systems have covalent interactions as well. In these cases, the ligand donates a pair of electrons (acting as a Lewis base) to the metal, which functions as a Lewis acid. Therefore, metals can be evaluated based on their abilities to accept electron pairs. Alkali metal ions (Na+, K+) and alkaline earth metals (Mg2+, Ca2+) tend to not form stable complexes with Lewis base ligands. Transition metal ions, particularly those with vacant ri-orbitals, will form more stable complexes with Lewis base-acting ligands. 

The alkali- and alkaline-earth metals are widespread on earth (four of the eight most common elements in the earth s crust are 5-block elements) and their compounds are ubiquitous in daily life. Considering that an estimated one-third of all proteins require a metal ion for their structure or function," and that the most common metals in biological systems are from these two families... 

Potentiometric probes are the oldest forms of electrochemical sensors. They can conveniently be used for studying many interesting chemical systems not accessible to voltammetric techniques. In particular, alkali and alkaline earth metal ion concentrations, of importance in biological systems,... 

The problem of demonstrating the involvement of a metal ion in a biological reaction has always been a very difficult one, since the most important metals (such as the alkali metals, alkaline-earth metals, and zinc) possess very few convenient handles . The replacement of the native metal by a less retiring one has become a well-established technique, and the n.m.r. work of Cohn and her school on systems in which the paramagnetic Mn + ion has been substituted for the diamagnetic Mg + ion needs no introduction. The interaction of transfer-RNA and related compounds with Mn + has recently been demonstrated by the enhancement of the proton relaxation rate (p.r.r.) technique. 

Neither the alkali and alkaline earth metal cations nor their associated anions can permeate lipid bilayers. However, some non-biological ions can do so by virtue of their lipid solubility. Whilst these ions are of no importance in living systems, they may well provide data for understanding the mechanisms by which carrier molecules can trandocate ions across cell and other biological membranes. 

From a biological point of view, potassium is an essential element in all forms of life. Each organism has a closely maintained potassium level and a relatively fixed potassium-sodium ratio. For instance, in the human body, the ratio of potassium between the cell and plasma is ca. 27/1. Potassium is the primary inorganic cation within living cells, and sodium is the most abundant cation in extracellular fluids. Moreover, potassium is a chief element of fertilizers. Potassium forms alloys with all the alkali metals. Complete miscibility exists in the K-Rb and K-Cs binary systems. The latter system forms an alloy eutectic melting at ca. -38°C. Modification of the system by the addition of sodium results in a ternary eutectic melting at ca. -78°C. Potassium is essentially immiscible with all of the alkaline-earth metals (when they are liquid), as well as with molten zinc, aluminum, and cadmium. 

In biological systems, ionophores selectively transport alkali- and alkaline-earth metal ions for example, vali-nomycin is a selective carrier for K+ and monensin for Na+, bnt they also can bind other cations for which they have not been designed by natnre and this is an area of active cnrrent research with natnral ionophores. 

The development of probes for the toxic heavy metals mercury, cadmium and lead in biological systems is of special interest In this context, selectivity is particularly important since alkali and alkaline earth metals (e.g. Na", K", ... 

Following the original paper, reports of the synthesis of new crowns and crown-like molecules proliferated. A typical property of these systems is their ability to form stable complexes with the alkali metal and alkaline earth ions. Prior to the synthesis of the crowns, the coordination chemistry of the above ions with organic ligands had received very little attention. A further impetus to the study of such complexes was the recognition of the important role of Na+, K+, Mg2+ and Ca2+ ions in biological systems. 

---