The manner in which atoms are bounded together is of critical importance in all biochemical reactions. Three kinds of bonds may be briefly considered here: electrovalent, covalent and hydrogen.
1. Electrovalent bond:
An electrovalent bond is formed by the transfer (not sharing) of one or more electrons from the outer shell of one element, say sodium, to another, say chlorine. Sodium has a single (“extra”) outer electron; it lacks seven of the eight electrons needed for stability of his outer shell. Chlorine has seven outermost electrons; its outermost shell is one electron short of complete stability.
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Each atom would gain greater stability by combining with the other. The sodium tends to give up its single electron; the chlorine tends to saturate its outer electron shell by accepting the electron. Each becomes ionized thereby.
The two ions therefore combine, the sodium transferring its electron to the chlorine, to form sodium chloride (NaCl), common table salt. The bond holding the two ions together is electrostatic and is therefore called as electrovalent or ionic bond.
Such bonds are not very strong. Electrovalent bonds are typical of inorganic compounds. Since electrovalence results in aggregations of ions, the compounds thus formed are actually ion aggregates, not true molecules.
However, the term molecule is commonly used for them. Unlike “true” molecules ions held together by electrovalence are readily separated; i.e., such compounds readily ionize or dissociate in aqueous solutions.
2. Covalent Bond:
By covalence is meant the merging or linking together by sharing unpaired outermost electrons of two atoms os that the shared electrons are attracted by each nucleus. Paired inner electrons such as the two in the k (inner) shell of carbon are very strongly held by the nucleus and are not shared.
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It is the unpaired outer (valence) electrons that tend to be shred; unpaired electrons seek “mates.” In the carbon atoms there are four unpaired outer (valence) electrons; in hydrogen, one; in nitrogen, three; in oxygen two.
When electron pairs are shared (not transferred), the shared electrons are strongly held by each nucleus. Each pair of shared electrons is called a covalent bond or electron- pair bond: a strong and stable type of bond. Covalent bonds are most common in organic compounds.
If two or three pairs of electrons are shared between two atoms, double and triple covalent bonds, respectively, are formed. Instead of being stronger, however, double and triple bonds are progressively less stable.
Anhydro bonds:
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Among the most important covalent bonds are those formed by the removal of the elements of water from two adjacent molecules: a hydrogen ion from one, a hydroxyl ion from the other? The two ions combine to form water.
The residues of the two altered molecules combine by what is sometimes called an Anhydro bond, a type of covalent bond. The process is sometimes called anhydrosynthesis.
Anhydro bonds are extremely important in the process of polymerization, the mechanism by which complex macromolecules (proteins, polysaccharides, nucleic acids) containing thousands of smaller subunits (e.g., glucose, amino acids, nucleotides), are put together by living cells.
Anhydro bonds are important also because they can be readily made or disconnected by enzyme action. Anhydrosynthesis, polymers and their functions in living cells are more fully discussed farther on.
Some common and important Anhydro bonds that will be mentioned again are here illustrated:
Ester bonds:
Between the carboxyl group of an organic acid and the hydroxyl group (R—OH) of an alcohol, forming an ester (organic salt). These bonds occur in fats, oils and waxes and related compounds:
Between phosphoric acid and a hydroxyl group of a sugar, forming a phosphorylated sugar: these bonds are important in polymerization and the storage and transfer of energy hi the cell:
Thioester bonds:
Between the carboxyl group of an organic acid and a sulfhydryl (—SH) group in sulfhydryl-containing compounds; important in the storage and transfer of energy in the cell:
Peptide bonds:
Between the carboxyl group of one amino acid and the amino (R—NHj) group of another, forming peptides, polypeptides and proteins; basic to the synthesis of living matter:
Glycoside bonds:
Between hydroxyl or related groups in simple sugar molecules (e.g., glucose) to form various disaccharides, and polysaccharides (polymers) like starch and cellulose. The glycoside bond is basic to the synthesis of many structural materials and energy-storing compounds in the cell:
Note that all five reactions are indicated as being reversible. This is because most enzymic reactions are reversible unless too much energy is released by the reaction in one direction.
Hydrolysis:
When the anhydrosynthetic process is reversed, the elements of water are reinstated and the constituent residues or subunits are separated and released in their original form.
This process, called hydrolysis, is exceedingly important in biology because it is the basic reaction of virtually all processes of digestion of proteins, fats, polysaccharides (e.g., starches) and many other compounds with Anhydro bonds.
3. Hydrogen Bonds:
As previously noted, hydrogen is unique in having only one electron, one proton and no neutron. The single proton exerts a strong, electrostatic, attractive force that extends beyond its single electron.
Hydrogen therefore tends to exert an unbalanced electropositive effect in certain molecules or groups containing it. Such molecules or groups tend to be polarized. Strongly polarized groups or molecules (dipoles), like magnets, attract other polarized groups or molecules.
When sufficiently strong the attraction results in the formation of a hydrogen bond through the sharing of a hydrogen atom (not an electron) between adjacent polarized groups or molecules; especially those containing oxygen or nitrogen in polarized groupings For example, water molecules as well as ammonia molecules, all polarized groupings, become associated through hydrogen bondings:
(In the above diagrams hydrogen bonds are indicated by……) In many combinations the proton of hydrogen is actually transferred to the other atom and the bonds are called proton bonds.
As will appear, hydrogen bonds, although relatively weak, are essential in the phenomenon of inheritance and the reproduction of living cells. This is because they help to support the structure of genes (DNA).