All molecules are held together by a number of forces. With regards to
your question the most relevant are:
1) van der Waals interactions
2) ionic bonds
3) hydrogen bonding
The van der Waals interactions are weaker, and are caused by the
electrons surrounding the atoms in the molecule temporarily moving so
that there are more at one end of the molecule than the other. This
results in a temporary dipole, similar to the dipole found in a
magnet. The "positive" end of this molecule (the one with less
electron density) then attracts more electrons towards it from the
surrounding molecules, inducing the same sort of dipoles in them.
These temporary dipoles help to attract the molecules towards each
other and these interactions are present in all molecules.
With some compounds, however, there are permanent charge separations
present, ie one part of the molecule is positively charged and another
is negatively charged. The simplest example of this would be something
such as sodium chloride (salt) where the sodium ions have a positive
charge and the chloride ions have a negative charge. These permanent
charge separations result in the ionic bonding, and as the effect is
always present and not fluctuating, the bonds are far stronger
(melting point of sodium chloride: 1074 K (801 °C)).
Ionic compounds therefore are held together more strongly than
non-ionic compounds. So where does that leave us with amino acids?
Well, amino acids contain both an amine (NH2) group and a carboxylic
acid (COOH). When drawn as I have drawn them in the brackets they do
not have any charge. However, when both of the groups are present a
proton can be exchanged between the acid and the amine to give a
positively charged (NH3+) and negatively charged (COO-) residue. When
this happens there is a permanent charge separation within the
molecule. When a molecules such as this contains both a positive and a
negative charge this is called a zwitterion as has already been noted.
The permanent charge separation in these zwitterions results in ionic
bonding, and hence the individual molecules are held more tightly
together.
Then there is hydrogen bonding. This is where a proton can be bound to
more than one other atom (usually oxygen, nitrogen, sulfur, etc).
Hydrogen bonds are also stronger than van der Waals interactions.
Potentially there are protons which can be bound to both the oxygen of
the carboxylic acid and the nitrogen of the amine as a form half-way
between the completely charge separated states and the non-ionic
states.
However, the zwitterionic form is probably the most important
contribution. Indeed, this structure helps to explain other properties
of amino acids, such as their solubility in water.
Another point to note is that each of the amino acids has a different
side-chain which affects its properties.
Hope that helps. |
