Download Amino Acids - Newcastle University

Amino acids are often referred to as ‘the building blocks of life’. This is because they combine in
different sequences to form proteins, which are fundamental to all living organisms. There are 21 amino
acids, 9 of which are called ‘essential’ because they cannot be naturally found in the body. This
worksheet will explain what makes an amino acid, along with some of their interesting features.
Structure
General formula:
All amino acids have the same backbone, which contains exclusively
COOHCHRNH2
carbon, oxygen, hydrogen and nitrogen. The R you can see in the
general formula represents the different R group attached to each
amino acid. These different R groups determine the chemical
personality of an amino acid; they each have a distinct shape, size and charge distribution
and can be large and bulky or small, and polar or non-polar. The R group is important
because the properties of amino acids dictate how
they interact with each other and the environment
they’re in.
Glycine (GLY) is the simplest amino acid as it’s R
group is simply a hydrogen atom. There are
aliphatic and aromatic R groups, as well as
nitrogen, sulfur or oxygen containing groups.
Zwitterions
As you can see from the general formula and the glycine molecule above, an amino acid
has a basic amine group and an acidic carboxylic acid group. However, the hydrogen
from the carboxylic acid is able to migrate to the amine, leaving a zwitterion. Overall, a
zwitterion has no electric charge, but separate parts of the ion are positively and
negatively charged. Amino acids actually exist as zwitterions even in the solid state. They
crystallise in a lattice like most salts with strong intermolecular ionic interactions. The
nature of the ions can be controlled by adjusting the pH of a solution of amino acids, as
demonstrated below with glycine.
In alkali solution (increased
pH), H+ is removed from
the NH3+ group leaving
NH2CHRCOO-.
The amino acid in
it’s zwitterionic
form, NH3+CHRCOO-.
In acidic solution (decreased
pH), COO- picks up H+, giving
NH3+CHRCOOH.
At the isoelectric point, there is no overall charge on the molecule.
Optical Isomers
Most amino acids have four different groups attached to the central carbon atom (excluding
glycine, which as you have seen has two hydrogen atoms). This means that they are chiral;
the central carbon atom is a chiral centre. Because of this, each amino acid has two
enantiomers whose mirror images cannot be superimposed onto one another – this is a
type of stereoisomerism called optical isomerism.
D-enantiomers rotate polarised light clockwise. Lenantiomers rotate polarised light anticlockwise. All
natural amino acids have L-configuration.
You don’t need to memorise all of the amino acids. Just
remember that the R groups are all different, and it’s these
that determine the chemical personality of an amino acid.
R-groups
Glycine (H)
Gly, G
Alanine (CH3)
Ala, A
Valine (CHCH3CH3)
Val, V
Isoleucine (CHCH3CH2CH3)
Ile, I
Serine (CH2OH)
Ser, S
Threonine (CHCH3OH)
Thr, T
Cysteine (CH2SH)
Cys, C
Methionine (CH2CH2SCH3)
Met, M
Phenylalanine (CH2Ph)
Phe, F
Tyrosine (CH2PhOH)
Tyr, Y
Tryptophan
Trp, W
Aspartic acid (CH2COOH)
Asp, D
Glutamic acid (CH2CH2COOH)
Glu, E
Asparagine (CH2CONH2)
Asn, N
Glutamine
(CH2CH2CONH2)
Gln, Q
Histidine
His, H
Lysine
(CH2CH2CH2CH2NH3+)
Lys, K
Arginine
(CH2CH2CH2NHCNH2NH2+)
Arg, R
Produced by Lucy Jakubecz at Newcastle University as part of an MChem project.
Leucine (CH2CHCH3CH3)
Leu, L
Proline
Pro, P