Amino Acids

Topics: Amino acid, Acid, Protein Pages: 29 (570 words) Published: March 30, 2015
Lesson Learning
Outcomes
Upon completion of this lecture,
students should be able to:
understand the structures and

properties of amino acids
know the groups of amino acid

What are amino acids?
Amino acids are molecules that when combined with

each other  proteins.
Amino acids contain a central tetrahedral carbon atom
(α-carbon)
amine group, carboxyl group, R-side chain
The R-side chain determines the different amino acids

There are 20 common amino acids
Amino acids can join via peptide bonds
Several amino acids occur only rarely in

proteins
Some amino acids are not found in proteins

3D structure of amino acids
Why is the 3D structure (stereochemistry) of

amino acids important?
Amino acids appear in chiral (mirror image)

except for glycine where the R = Hydrogen
Chirality of amino acids can only be seen in 3D
structure

The position of amino group on the left or

right side of the α-carbon determines the L
or D designation.
L-amino acids are found in proteins
D-amino acids are often found in bacterial

cell walls and in some antibiotics.

There are 20 common amino acids
Amino acids can join via peptide

bonds
Several amino acids occur only rarely
in proteins
Some amino acids are not found in
proteins

Amino Acids Can Join Via Peptide Bonds

20 Common Amino Acids
Non-polar amino acids
Polar, uncharged amino acids
Acidic amino acids
Basic amino acids

(a) nonpolar (hydrophobic)

(b) polar, neutral

c) acidic

(d) basic

Several Amino Acids Occur
Rarely in Proteins
Hydroxylysine, hydroxyproline - collagen
Carboxyglutamate - blood-clotting proteins
Pyroglutamate – in bacteriorhodopsin
Phosphorylated amino acids – a signaling device

Isoelectric point
Isoelectric point is the point along the pH

scale where the molecule (amino acid) exist in
a neutral form with a zero net charge
Iso = equal

Why is it important?
So we can predict the charge of the amino acid
at a certain pH
The general structure of amino acid:

The general structure of amino acid:
• Proton
acceptor
• Makes it
basic

• Proton
donor
• Makes it
acidic

Amino acids can act as both
acid and bases
Carboxyl group (-vely charged) and amino

group (+vely charged) of the general
structure – charged at neutral pH
Amino acids that exist with a net charge of
zero is called - zwitterion

What happens when an amino acid is placed

in acidic solution (lots of H+)?
The amino group will accept the excess H+ and

form NH3 – positively charged
The carboxyl group will also gain proton and
become uncharged
Thus the amino acid will be positively charged

What happens when an amino acid is placed

in basic solution (lots of oH-)?
The amino group will be deprotonated the

excess oH- and form NH2 – no charged
The carboxyl group will also be deprotonated
and become negatively charged
Thus the amino acid will be negatively charged

The point on the pH scale that we
can find the zwitterion is what we
call the isoelectric point.

Spectroscopic Properties
All amino acids absorb infrared

wavelengths
Only Phe, Tyr, and Trp absorb UV
Absorbance at 280 nm is a good diagnostic
device for amino acids – useful for detection
of proteins contamination in dna extraction

Separation of Amino
Acids
Mikhail Tswett, a Russian botanist,
first separated colorful plant pigments
by ‘chromatography’
Many chromatographic methods exist
for separation of amino acid mixtures
Ion exchange chromatography
High-performance liquid
chromatography

Chromatographic fractionation of a
synthetic mixture of amino acids on
ion exchange columns

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