STABILIZING INTERACTIONS

Covalent Bond

Molecules are formed by sharing of electrons between atoms. The term covalent bond was proposed by lewis in 1966. This bond is a very common inorganic compound. Ionization does not occur. The atoms are also neither lost nor gain electrons. these compounds are called molecular compounds. The covalent bonds are also called homopolar bonds. They are strong bonds compared to ionic bonds. Covalent bonds are endergonic. Both the formation and breakdown of covalent bonds occur by enzyme action in cells. eg . Methane.

COVALENT BOND

2   Ionic Bonds or Electrostatic Bonds

The term electrovalent bond was proposed by Kossel in 1961. The bond is formed by ionization. Hence the bond is called an ionic bond and the compound is called an ionic compound. Ionic bonds result from the electrostatic attraction between two ionized groups of opposite charges. They are formed by the transfer of one or more electrons from the outermost orbit of an electropositive atom to the outermost orbit of an electronegative atom. This transfer results in the formation of a ‘cation’ and an ‘anion’, which get consequently bound by an ionic bond. Common examples of such compounds include NaCl, KBr, and NaF. Concerning protein chemistry, positive charges are produced by the epsilon amino group of lysine, guanidium group of arginine, and imidazolium group of histidine. Negative charges are provided by beta and gamma carboxyl groups of aspartic acid and glutamic acid.

IONIC BOND

Hydrogen Bonds

The term hydrogen bonds were suggested by Latimer and Rodenbach. These are formed by sharing hydrogen between two electron donors. Hydrogen bonds result from electrostatic attraction between an electronegative atom and a hydrogen atom that is bonded covalently to a second electronegative atom. Normally, a hydrogen atom forms a covalent bond with only one other atom. However, a hydrogen atom covalently bonded to a donor atom may form an additional weak association, the hydrogen bond with an acceptor atom. In biological systems, both donors and acceptors are usually nitrogen or oxygen atoms, especially those atoms in amino (NH2) and hydroxyl (OH) groups. About protein chemistry, hydrogen releasing groups are -NH (imidazole, indole, peptide); -OH (serine, threonine), and -NH2 (arginine lysine). Hydrogen accepting groups are COO–, (aspartic, glutamic) C=O (peptide); and S–S (disulfide). The DNA structure is maintained by hydrogen bonding between the purine and pyrimidine residues

 

HYDROGEN BOND

Hydrophobic Interactions

Non-polar groups have a tendency to associate with each other in an aqueous environment; this is referred to as hydrophobic interaction. These are formed by interactions between nonpolar hydrophobic side chains by eliminating water molecules. The force that causes hydrophobic molecules or nonpolar portions of molecules to aggregate together rather than dissolve

HYDROPHOBIC INTERACTIONS

in water is called the ‘hydrophobic bond’. This serves to hold lipophilic side chains of amino acids together. Thus, nonpolar molecules will have minimum exposure to water molecules.

 Van Der Waals Forces

These are very weak forces of attraction between all atoms, due to oscillating dipoles, described by the Dutch physicist Johannes van der Waals (1837-1923). He was awarded the Nobel prize in 1910. These are short-range attractive forces between chemical groups in contact. Van der Waals interactions occur in all types of molecules, both polar and nonpolar. The energy of the van der Waals interaction is about 1 kcal/mol and is unaffected by changes in pH. This force will drastically reduce when the distance between atoms is increased. Although very weak, van der Waals forces collectively contribute maximum towards the stability of protein structure, especially in preserving the nonpolar interior structure of proteins.