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Elements (metals), chemical characteristics explaining biological functions

Matter is made up of elements which bind together to form molecules, one of the simplest being the water molecule.

Living organisms are primarily made up of organic molecules formed by elements of low atomic mass, hydrogen, carbon, nitrogen and oxygen, bound together by covalent bonds. These four elements represent more than 96% of the body mass, water alone accounts for approximately 60%.

Among the other elements, calcium is the most abundant because of its high concentration in bone, close to 25%. For a body mass of 70 kg, calcium represents more than1 kg, sodium, potassium, chlorine and sulfur, approximately100 g each one, magnesium 30 g . The other elements, including iron, are present in the body in low or very low quantities.

The biological properties of the elements result from their physicochemical characteristics: ionization, polarizing capacity, polarizability, oxidoreduction potential.

Ionization, polarizing capacity

In the body, the elements sodium, potassium, calcium, magnesium, copper, zinc, iron, are ionized by loss of electrons, they are cations. Chlorine and bromine are ionized by gaining an electron and become chloride and bromide anions.

The cations ionized by loss of electrons have an ion radius much smaller than the anions ionized by gain of electrons. They interact more strongly with their polar environment. The value of the ion radius is an approximation because one cannot say where an ion or an atom finishes.

The ions react between them and with organic molecules with nucleophilic or electrophilic domains by electrostatic forces of attraction or repulsion.

The interactions are all the more strong when the charge of the ion is high and its radius is little. The ratio charge versus radius indicates the polarizing capacity of an ion.

An ion is surrounded by a certain number of other ions with charges of the opposite sign (or by extension of molecular sites carrying a charge of opposite sign). The preferential number of interactions between an ion and the other ions of opposite sign is called the coordination number. This number depends mainly on the relative size of the ions involved, on the electrostatic forces of attraction between ions of opposite sign and repulsion between ions of the same sign. The coordination number generally lies between 2 and 8 and very frequently from 4 to 6. The ion radius increases when the coordination number increases.

The interactions between an ion and water molecules play a very important part in biology. The hydration of an ion results from its interactions with the dipoles of water molecules. An ion present in the body in hydrated form is however regarded as being in a free state , in opposition to the ions bound to proteins or to other organic molecules. Because of the polarity of the water molecule, the cations interact with the oxygen atom of the water molecule and the anions with the hydrogen atoms.

Enthalpy of hydration, i.e. the energy necessary to detach water molecules from an ion, is expressed negatively in Kj/mol-1. The more the ion is strongly charged, the greater is the enthalpy.

Lithium has a more polarizing effect than sodium because it has a smaller ion radius and similarly calcium, because it has a double charge.

The trivalent cations are strongly bound to water and exert their influence on several concentric layers of water molecules.

In the biological environment, cations, except the alkalis sodium and potassium, interact preferentially with endogenous molecules containing sulfur or nitrogen atoms rather than with water. They are bound to amino acids, proteins; zinc, for example, having a coordination number of 4 in the catalytic site of the carbonic anhydrase enzyme, is bound to the nitrogen atoms of three histidine residues and to an OH group of a water molecule. It thus induces the transfer of this OH group to carbon dioxide.

Elements

Li+

Na+

K+

Mg+

Ca+

Fe3+

F-

Cl-

Charge

1

1

1

2

2

3

- 1

- 1

Ion radius A°

0.68

0.97

1.33

0.66

0.99

0.64

1.33

1.81

R = charge/radius

1.47

1

0.75

3

2

4.6

0.75

0.55

Enthalpy of Kj hydration/mol

-519

-406

-322

-1921

-1577

-4430

-515

-381

Ion radius and charge of some elements

Polarizability

Mobility of the electronic cloud, i.e. its polarizability, increases with the number of electrons. The not very polarizable ions are called hard, the polarizable ones soft ions.

The hard ions, like calcium and magnesium, do not interact with the sulfur atoms of molecules, whereas the soft ions like lead, cadmium, mercury, interact preferentially with the sulfur atoms. These soft ions establish covalent bonds of low energy with carbon and form organometallic complexes like Pb(CH2-CH3)4, Hg (CH3)2, As (CH3)3.

There are cations having the same charge, a similar ion radius but a different number of electrons. They have resemblances but not the same properties, and can be biological antagonists. It is the case for potassium, rubidium and thallium on the one hand and calcium and cadmium or lead on the other hand.

Element, ion

K+

Rb+

Tl+

Ca2+

Cd2+

Ion radius in A°

1.33

1.47

1.47

0,99

0.97

Number of electrons

18

36

80

18

46

Ion radius of some essential and toxic elements

Beside the elementary ions that we have seen, there are also molecular ions such as sulfate SO42- phosphate PO43- etc. Moreover, any molecule with acid-basic function can, by exchange of a proton H+, take a positive or a negative charge.

Redox potential

Certain elements, particularly iron and copper, have the capacity to yield or acquire electrons reversibly. As the electrons do not exist in a free state in solution, the oxidant binds the electrons yielded by the reductant. The compound which loses electrons is called reductant, that which gains them is called oxidant.

The redox potential of a reaction is expressed in volts, compared to that of hydrogen. In biology, iron particularly plays an essential role in electron transfer during cellular respiration.

Biological consequences

Essential elements 

Many essential elements are present in the body in the form of ions.

The ions do not cross the biological membranes across the lipid bilayer but through protein structures, channels, pumps etc

The passage of an ion through a channel of a biological membrane needs the ion to be attracted by electrostatic charges to approach  the entry of the  channel ,  and to be at least  partially dehydrated .

  • The alkaline monovalent cations, sodium, potassium and rubidium, have a rather important ion radius, as well as chloride and bromide anions, and are nearly 100% in a free state . They do not bind to proteins in the biological environment but are only hydrated. Moreover, as their hydration energy is low (see enthalpy of hydration), they are easily removed from the water molecules which surround them and pass through protein structures of the biological membranes.
    They are involved in cell polarization and depolarization.
    The alkaline ions do not form complexes with the organic molecules but can be introduced into cavities of organic molecules called cryptands. The combination element-cryptand is called cryptate or clathrate.
  • The bivalent ions, calcium and magnesium, are present in tissues both in unbound but hydrated form and bound to organic molecules. Biologically, calcium, in addition to its role in the constitution of bones, has two functions: it participates in cellular polarization while crossing the plasma membrane and it modulates the activity of certain proteins, in particular enzymes, and thus plays a role of intracellular messenger.
  • The other bivalent ions like zinc, copper, iron, selenium, present in the biological milieu, are not bound to water molecules but more or less specifically to various molecules, amino acids and proteins. They are not involved in the polarization function.
  • The very polar ions - either because they carry three positive charges, like ferric iron  and aluminium, or because they have an ion radius of very low dimension, such as beryllium - are not present in the free state, except  in an extremely fugacious way during their transfer from  one molecule to another. These ions, when they are present in water, create around themselves concentric layers of water molecules which are directed, the oxygen atoms turned towards the cation. They do not cross the biological membranes and are practically not absorbed after oral intake.
  • Iron and copper present in metallo-enzymes ensure electron transfers, through their redox potential. They are present in the cytochromes of mitochondria and the P-450 cytochromes of microsomes.
  • Zinc does not participate in electron transfer, but is involved in the stabilization of various protein structures in particular conformations (for example enzymes, zinc fingers).

The deficiency or excess of an essential element causes pathological disorders.

Toxic elements

Certain elements, when they are present in the body, elicit toxic effects which have two main explanations:

  • Resemblance to essential elements, in particular in the ion state. Thus the monovalent ion thallium enters into competition with potassium ion, the divalent ions lead and cadmium with calcium, arsenate with phosphate.
  • Different reactivity: the toxic elements have a great affinity for certain endogenous molecules, in particular those which have one or more HS groups , of which they inhibit the activity.
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  Last update : August 2007  
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