Biology Reference
In-Depth Information
Hydrogen
Bonds
O
O
H
H
H
H
O
H
H
Covalent
Bonds
O
O
H
H
H
H
FIGURE 3.3
Hydrogen bonding in water.
Hydrogen bonding occurs when an atom of hydrogen is shared between two electroneg-
ative atoms instead of residing on just one (
Figure 3.3
). In the case of water, the hydrogen is
shared unequally between two oxygens of adjacent waters. Water is unique in that it can form
4 H-bonds, while other H-bonding molecules like HF, ammonia, and methanol can only form
1 H-bond. In fact liquid water contains by far the densest H-bonding of any solvent.
H-bonding accounts for the anomalous physical properties of water that are particularly
important over a narrow temperature range (0
o
C
100
o
C), where water exists as a liquid
and is therefore suitable for biological processes. For example, a similar compound, H
2
S,
has much weaker H-bonding than water and is a gas at biological temperatures.
At any one instant in time a hydrogen is more closely associated with one water's oxygen
(covalent bond length 0.101 nm) than the other water's oxygen to which it is H-bonded (bond
length 0.175 nm). As a result, a covalent bond (~492 KJ/mol) is stronger than an H-bond
(~23.3 KJ/mol). However, even the weaker H-bond is almost 5x stronger than the average
thermal collision fluctuation energy at 25
o
C and both are far stronger than weak van der
Waals interactions. The hydrogen residence is rapidly alternated between the two adjacent
water oxygens with an equilibrium time scale of less than a femtosecond (10
15
sec).
In the solid ice state every water molecule is H-bonded to 4 other water molecules
producing an enormous, highly organized extended water cluster. Upon melting, an initial
assumption would be that the organized water clusters would be completely destroyed,
hence resulting in a fluid. However, as shown in
Table 3.1
, this is not the case. The solid is
transformed into a liquid as a result of breaking a surprisingly small number of H-bonds.
Even near physiological temperature (40
o
C) water exists primarily in extended clusters
where most waters are still H-bonded to 4 other water molecules. It is not until water is in
the gaseous state (100
o
C) that the extended clusters disappear.
e
C. PROPERTIES OF WATER
NO ORDINARY JOE
e
The structure of water and its facility to form H-bonds dictates its extraordinary proper-
ties. For example, water is small, stable, abundant, and is the only pure substance on earth
that exists in all three states of matter: solid; liquid; and gas. Below are listed a few of water's
special properties that are related to its central role in life
[6
8]
.
e
