Download Amino acid theory sheet

Amino acids are amongst the most important chemicals that exist on this planet,
without them life Earth would not exist. Amino acids are used to make the individual
proteins and enzymes that make up all living organisms. There are 21 amino acids,
10 of these are essential amino acids and must be eaten by humans as the body
cannot make them
General Formula: COOHCHRNH2
The backbone of amino acids only
contains carbon, oxygen, hydrogen
and nitrogen. What makes each
amino acid different is the R group
attached to this. Each amino acid has
a different R group which leads its
own individual properties. The R
group can be simple as a hydrogen
atom (glycine) or much more
complicated such as alcohols, amines
and aromatics.
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Remember the general
formula of an amino acid is:
COOHCHRNH2
Glycine (GLYCIN) is the simplest amino
acid. It contains the general amino acid
backbone but its R group is H. This
means that there are two hydrogen
atoms on the central carbon.
You
may have noticed that the crystal structure of glycine above was slightly different from
the general formula of amino acids; the hydrogen atom has gone onto the nitrogen to make
COO- and NH3+ species. This is called a zwitterion. A zwitterion is a species with no
overall net charge but has two parts of the molecule that are charged. The crystal structure
above shows glycine in its solid state as a zwitterion, all other amino acids form zwitterions
in their solid states. They crystalize in a lattice like most salts with strong ionic
intermolecular interactions between the charges. Zwitterions can be controlled by changing
pH, an acidic pH will produce a neutral COOH and a protonated NH3+ and basic conditions
will form COO- and a neutral NH2. The pH which COOH and NH2 exist is called the
isoelectric point.
Produced by Steven Carman at Newcastle University as part of a MChem project.
Most of amino acids have four different groups attached to the central carbon atom, (apart
from glycine) this means that they have chiral centres. This causes each acid to have two
isomers whose mirror images cannot be superimposed onto one another, which is called
stereoisomerism. The optical isomers of amino acids behave in the same way as all
optical isomers and rotate polarised light which is often why they are noted with either (+)
or (-) for the way they rotate the light. If you do not fully understand this refer to the
basic optical isomerism theory sheet. They also can be assigned as R or S as well
using the same rules used in the optical extension theory sheet. However it is
convention to assign R as L and S as D in amino acids. All natural amino acids are in the L
conformation.
L-Alanine
D-Alanine
L-Alanine
D-Alanine
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At UK A Level you will not be required to assign R and S or L and
D. All you need to know is that amino acids have chiral centres
and therefore have two optical isomers that are nonsuperimposable mirror images of each other. You will be required
to explain and draw this.
Amino acids form peptide bonds with one another.
A peptide bond is a condensation reaction between
the amine of one amino acid and the carboxylic
acid forming an amide. Amino acids form chains all
bonding in the same way leading to larger proteins.
Once this chain has formed the R groups interact
between one another both intra and intermolecular,
to form the structure (primary, secondary tertiary
and quaternary) and activity of the protein.
Produced by Steven Carman at Newcastle University as part of a MChem project.