Of course, amino acids, as individual molecules,
do not necessarily exist on their own. They’re
joined by peptide bonds to form a polypeptide
chain, which then goes on to form a protein.
An amino acid unit, such as that which I showed
you in the previous slide, which is found
within a polypeptide polymer, is called a
Here we have an example of how a dipeptide
can be formed. We have, in this case, two
amino acids coming together and they’re
linking in in the order from the nitrogen
to the carboxylic acid which is attached to
a nitrogen to form a peptide or amide bond.
So, you should recognise that as an amide
bond from our discussion of the reactions
of amines with acid chlorides and acid anhydrides.
And a polypeptide chain has a direction and
this is when it comes to the biological synthesis
of polypeptides for protein synthesis because
its building blocks have a different end:
the alpha amino and the alpha carboxylic acid
And by convention, from a biological perspective,
the amino end is called the N-terminus and
is taken as the beginning of a polypeptide
chain and the C end is known as the C-terminus,
which is taken as the end. However, when we
actually synthesise these, those roles tend
to be switched around a bit by virtue of some
of the restrictions on group protection and
so forth, which fall beyond the scope. But,
if you are interested, I’m sure you can
find out more. So, N-terminus here on the
left hand side and C-terminus on the right
hand side, shown in red.
Right. So, here we have an example of aspartame,
a dipeptide which is synthesised from aspartic
acid and phenylalanine methyl ester. So, note
the identification of the alpha amino acid
component. If we look at our aspartic acid,
the alpha amino acid component is where we
have that stereogenic center, which is indicated
by the dashed lines going up to the NH2 unit.
And what we’re showing here is the formation
of a carboxylic… of an amide from the carboxylic
acid and the amine of the phenylalanine methyl
ester. And this is an amide bond which is
formed between the two. Now, if you recall,
I said that it was rather difficult for us
to be able to synthesise this just using heat
and a bit of acid. And the reality is that
if you’re making dipeptides, you would usually
use a coupling agent such as DCC, EDCI, a
combo or a number of others.
So, we’ve seen how to create a dipeptide
from two amino acids and polypeptides consist
of a large number of amino acids joined together
in the same way. The boundary for the definition
of a protein in terms of the number of amino
acids is actually rather blurred.
In some of the cases of the larger proteins,
thousands of amino acids, or residues, are
the result. If, on the other hand, we look
at some of the smaller proteins, then hundreds
of amino acids can be used to form them. Proteins
are very important and the orientation of
amino acids and their hydrogen bonding and
also their potential for ionisation are absolutely
essential for their biological activity.
And this is what we will see in Module IV.
At the moment, though, we… this brings an
end to Module III and we’ll be focusing
more along the biological applications of
the chemistry that you’ve learned so far in
the next module.