R ECAP : C OVALENT / P OLAR C OVALENT / I ONIC B ONDS
Covalent bonds are bonds where the electrons are shared between atoms. In polar
covalent bonds, the electron cloud is distorted, meaning that the electrons are not
shared equally. This may cause a permanent dipole, which is the cause of the hydrogen
bond.
Ionic bonds are bonds where an electronegative entirely seizes an electron from
another atom. Here, the electron is “stolen” and not shared.
P EPTIDE B OND
Peptide bonds are formed between the amino group (R-NH2) of one amino acid and the
carboxyl (R-COOH) group of another amino acid. Water is released and the nitrogen in
the amino group becomes covalently bonded with the carbon in the carboxyl group.
Peptide bonds are a form of covalent bonds.
G LYCOSIDIC B OND
A glycosidic bond occurs between the hemiacetal group (C – and the hydroxyl group (OH)
of another molecule. Like a peptide bond, the formation of a glycosidic bond is a
condensation reaction which releases water.
Moreover, glycosidic bonds allow monosaccharides to bond together to form
polysaccharides. Sucrose, for example, consists of fructose and glucose bonded together.
D ISULFIDE B OND
This is a type of bond that occurs between two thiol groups (R-SH). This is significantly
weaker than the C-H and C-C covalent bond.
INTERMOLECULAR BONDS ARE AN ESSENTIAL PART OF BIOCHEMISTRY. IN GENERAL, THE
STRENGTH OF THE BOND IS MUCH WEAKER THAN THAT OF A COVALENT OR AN IONIC BOND.
H YDROGEN B ONDS
Note that a hydrogen bond is an intermolecular bond – it is different from
intramolecular bonds such as a covalent or an ionic bond. Hydrogen bonds occur
between polar covalent molecules (i.e. the molecule is not entirely covalent) where
hydrogen is covalently bonded to an electronegative atom such as oxygen, nitrogen or
fluorine. This causes the hydrogen to be slightly positively charged (i.e. a permanent
dipole has formed) – which can be denoted by a δ+ (the symbol is the Greek alphabet
delta). Ultimately this can be demonstrated in the diagrams below:
The hydrogen bond donor is the hydrogen atom which is bound to another
electronegative atom (e.g. fluorine, nitrogen, oxygen). The bond acceptor is the
electronegative atom itself.
Hydrogen bonds are extremely important as they allow the nitrogenous bases
(cytosine, guanine, etc.) to bond with each other.
L ONDON D ISPERSION ( VAN DER W AALS ) F ORCE
The London dispersion forces also act between molecules. However, they are caused
by momentarily induced dipoles – meaning that the dipole is caused by a random
alignment of electrons in the electron cloud, instead of the covalent bonding.
Macromolecules such as proteins and DNA contain numerous sites of potential van der
Waals interactions that the cumulative effect of these small binding forces can be
enormous.