Chemistry, Third edition

Hydrogen bonding in biology

1. Proteins

Proteins are the basic ‘building blocks’ of living tissue. Protein molecules consist of

long chains of atoms, containing polar C=O and H–Nbonds. Hydrogen bonding

can occur between these two groups:

Many protein molecules have the chains of atoms twisted or coiled, and these

coils are held together by many hydrogen bonds. These hydrogen bonds help to give

the protein its shape (Fig. 5.19).

Because the hydrogen bonds holding the protein molecule together are weak, rela-

tive to the covalent bonds in the long chain of the molecule, they can be broken by

heating.Clearegg white contains proteins suspended in water. When the egg white is

heated, the increased vibrations of the protein molecules break the hydrogen bonds

giving them their shape and they unravel. The protein turns the familiar colour of

cooked egg white. When a protein loses its shape (and function) in this way it is said

to be denatured.

2. DNA

Deoxyribonucleic acid (DNA) stores the genetic information present in living cells.

It allows the cell to make proteins according to a definite sequence of atoms. The

DNA molecule consists of two long chains molecules twisted around each other and

held together by hydrogen bonds This structure is called a double helix(Fig. 5.20).

For this discovery, F. Crick, M. Wilkins and J. Watson gained the Nobel Prize for

Medicine in 1962.

FORCES BETWEEN COVALENT MOLECULES 79

BOX 5.6

The boiling points of

hydrides

The graph in Figure 5.18 shows the

boiling points of four sets of

hydrides with similar formulae. Note

that for the set CH 4 , SiH 4 , GeH 4 and

SnH 4 the boiling points increase

with increasing mass of the

molecules. The hydrides NH 3 , H 2 O

and HF, however, have boiling

points much higher than would be

expected from the trends shown by

the other members of their set

because the process of boiling

requires the breaking of hydrogen

bonds. These compounds also have

abnormally high melting points,

latent heats of melting and

vaporization because of hydrogen

bonding between their molecules.

Boiling point /

°C

Molecular mass / u

20 60

–180

–100

–20

100 H 2 O

NH 3

H 2 S

PH 3

AsH 3

HCl

HF

CH 4

SiH 4

GeH 4

SnH 4

HI

SbH 3

H 2 Te

HBr

100 140

–140

–60

20

60

H 2 Se

Fig. 5.18Plot of the boiling points of hydrides.

Fig. 5.19Structure of a

protein.