Each amino acid has at least one amine and one acid functional
group as the name implies. The different properties result from
variations in the structures of different R groups. The R group
is often referred to as the amino acid side chain. Amino acids
have special common names, however, a three letter abbreviation
for the name is used most of the time. A second abbreviation
, single letter, is used in long protein structures.Consult the
table on the left for structure, names, and abbreviations of
20 amino acids.
There are basically four different classes of amino acids
determined by different side chains: (1) non-polar and neutral,
(2) polar and neutral, (3) acidic and polar, (4) basic and polar.
Principles of Polarity:
The greater the electronegativity difference between
atoms in a bond, the more polar the bond. Partial negative charges
are found on the most electronegative atoms, the others are partially
positive. Review the polarity of functional
Non-Polar Side Chains:
Side chains which have pure hydrocarbon alkyl groups
(alkane branches) or aromatic (benzene rings) are non-polar.
Examples include valine, alanine, leucine, isoleucine, phenylalanine.
The number of alkyl groups also influences the polarity.
The more alkyl groups present, the more non-polar the amino acid
will be. This effect makes valine more non-polar than alanine;
leucine is more non-polar than valine.
QUES. List all amino acids with non-polar side chains.
|Rank the following according to increasing
non-polarity i.e. 1 = least non-polar, 4 = most non-polar.
leu; phe; val; ala
Polar Side Chains:
Side chains which have various functional groups such as acids,
amides, alcohols, and amines will impart a more polar
character to the amino acid. The ranking of polarity will depend
on the relative ranking of polarity for various functional groups
as determined in functional groups.
In addition, the number of carbon-hydrogens in the alkane or
aromatic portion of the side chain should be considered along
with the functional group.
Example: Aspartic acid is more polar than serine because an
acid functional group is more polar than an alcohol group.
Example: Serine is more polar than threonine since threonine
has one more methyl group than serine. The methyl group gives
a little more non-polar character to threonine.
Example: Serine is more polar than tyrosine, since tyrosine
has the hydrocarbon benzene ring.
QUES. List all amino acids by abbreviation which are considered
|Rank the following amino acids by
increasing polarity. i.e.
1 = more non-polar.
ser ; glu ; asp ; lys ; ala ; gln
|Which amino acid is most insoluble
in water: isoleucine or
|Which amino acid is most soluble in water: lys