Hydrogen Bonds
Hydrogen Bonds
Hydrogen Bonds
PANEL 22: Water and Its Influence on the Behavior of Biological Molecules
WATER
WATER STRUCTURE
Two atoms, connected by a covalent bond, may exert different attractions for
the electrons of the bond. In such cases the bond is polar, with one end
_
slightly negatively charged (d ) and the other slightly positively charged (d+).
d+
electropositive
region
O
d
d+
electronegative
region
Although a water molecule has an overall neutral charge (having the same
number of electrons and protons), the electrons are asymmetrically distributed,
which makes the molecule polar. The oxygen nucleus draws electrons away
from the hydrogen nuclei, leaving these nuclei with a small net positive charge.
The excess of electron density on the oxygen atom creates weakly negative
regions at the other two corners of an imaginary tetrahedron.
HYDROGEN BONDS
Because they are polarized, two
adjacent H2O molecules can form
a linkage known as a hydrogen
bond. Hydrogen bonds have
only about 1/20 the strength
of a covalent bond.
Hydrogen bonds are strongest when
the three atoms lie in a straight line.
bond lengths
d+
d+
H
2d
hydrogen bond
0.27 nm
2d
hydrogen bond
0.10 nm
covalent bond
d+
HYDROPHILIC MOLECULES
HYDROPHOBIC MOLECULES
Substances that dissolve readily in water are termed hydrophilic. They are
composed of ions or polar molecules that attract water molecules through
electrical charge effects. Water molecules surround each ion or polar molecule
on the surface of a solid substance and carry it into solution.
H
H
H
O
H H
H
O
H
O
H
O
O _
O_
H
H
H+
H
H d+
d
d
H
_ d
H
N H
O
H
+ Cl
_ Na+
H
d
Od
_
H
O C
H
d O
_
O H d+ d+
d
O
H
H
H
H
N H
O
H
H O
H
O
H
O
H
H
H
H
Ionic substances such as sodium chloride
dissolve because water molecules are
attracted to the positive (Na+) or negative
_
(Cl ) charge of each ion.
C
C
O
H
O
H
O
H
C
H
O
H
H
O
H
CHAPTER 2 PANELS
109
WATER AS A SOLVENT
Many substances, such as household sugar, dissolve in water. That is, their
molecules separate from each other, each becoming surrounded by water molecules.
sugar
dissolves
water
molecule
sugar crystal
sugar molecule
ACIDS
HCl
H+
Cl
hydrochloric acid
(strong acid)
hydrogen ion
chloride ion
H
hydronium ion
hydroxyl ion
(water acting as (water acting as
a weak base)
a weak acid)
O
H+
H2O
C
O
H+
OH
hydrogen
ion
hydroxyl
ion
pH
BASES
OH
(weak acid)
H+
conc.
moles/liter
10
_1
_2
ACIDIC
The acidity of a
solution is defined
by the concentration
of H+ ions it possesses.
For convenience we
use the pH scale, where
10
_3
10
_4
10
_5
10
_6
10
pH = _log10[H+]
10 7
_
10 8
_
[H+] = 10 7 moles/liter
ALKALINE
_9
10
_10
10
_11
10
_12
10
_13
10
_14
10
pH
1
2
3
4
5
6
7
8
9
H+
NH4+
ammonia
hydrogen ion
ammonium ion
NaOH
sodium hydroxide
(strong base)
sodium
ion
OH
hydroxyl
ion
10
11
12
13
14
Many bases found in cells are partially dissociated and are termed
weak bases. This is true of compounds that contain an amino
group (NH2), which has a weak tendency to reversibly accept
an H+ ion from water, increasing the quantity of free OH ions.
NH2
H+
NH3+