1. PROPERTIES OF AMINO ACID
SIDE CHAINS:
CLASSESOF α-AMINOACIDS

2. PROPERTIES OF AMINO ACID SIDE CHAINS: CLASSES OF α-AMINO ACIDS
The 20 amino acids contain, in their different side
chains, a remarkable collection of chemical
groups. It is this diversity of the monomers that
allows proteins to exhibit such a great variety of
structures and properties.
Many ways have been proposed to group the
amino acids into classes, but none is wholly
satisfactory.

3. AMINO ACIDS WITH ALIPHATIC SIDE
CHAINS
Glycine, alanine, valine, leucine, and isoleunine
have aliphatic, or alkaline, side chains.
Isoleunine for example, has a much greater
tendency to transfer from water to a
hydrocarbon solvent than does alanine.The
more hydrophobic amino acids such as
isoleunine are usually found within a protein
molecule.

4. Proline which is difficult to fit in any category,
shares many properties with the aliphatic amino
acids. Although it is a cyclic amino acid, its side
chain has a primarily aliphatic character.
However, the rigidity of the ring, compared with
the flexibility of most amino acid side chains,
often makes the folding of proline residues into
protein structures difficult.

5. AMINO ACIDS WITH HYROXYL- OR
SULFUR- CONTAINING SIDE CHAINS
Serine, cystein, threonine, and
methionine can be placed in this
category.
These amino acids, because of their
weakly polar chains, are generally
more hydrophilic than their aliphatic
analog, although methionine is fairly
hydrophobic.

6. Among the group, cystein is noteworthy in two
respects.
First, the side chain can ionize at moderately high
pH
Second, oxidation can occur between pairs of
cystein side chains to form a disulfide bond.

7. AROMATIC AMINO ACIDS
Phenylalanine, tyrosine, and tryptophan, carry
aromatic side chains. Phenylalanine, together
with the aliphatic amino acids.Tyrosine and
Tryptophan have some hydrophobic
character as well, but it is tempered by the
polar groups in there side chains. In addition,
tyrosine can ionize at high pH.

8. The aromatic amino acids, like most compounds
carrying conjugated rings, strongly absorbs light
in the near-ultraviolet region of the spectrum.
This characteristic is frequently used for the
analytical detention of proteins, by measuring
absorption at 280 nm.

9. BASIC AMINO ACIDS
Histidine, lysine, and arginine carry basic
groups in their side chains.
The basic amino acids are strongly polar, and as
consequences they are usually found on the
exterior surfaces of proteins, where they can
be hydrated by the surrounding aqueous
environment.

10. ACIDIC AMINO ACIDS AND THEIR
AMIDES
Aspartic acid and glutamic acids are the only
amino acids that carry negative charges at pH
7.
The pKa is values of the acidic amino acids are
so low that even when the amino acids are
incorporated into proteins, the negative
charge on the side chain is retained under
physiological conditions. hence, these amino
acid residues are often referred to as
aspartate and glutamate

11. Companions to aspartic and glutamic acids are
their amides, asparagine and glutamate. Unlike
their acidic analogs, asparagine and glutamine
have uncharged side chains, although they are
decidedly polar. Like the basic and acidic amino
acids, they are definitely hydrophilic and tend to
be on the surface of a protein molecule, in
conatct with the surrounding water.