Basic, Acidic, Neutral Amino Acids Video Lecture - NEET

All right.
So let's go through the
classification of amino acids.
And I've highlighted the word
class within classification
for you, because
I'm going to paint
for you a picture
of a classroom that
is full of 20
different amino acids.
And just picture this as
the most diverse classroom
you've ever seen, because
each amino acid has
their own unique
side chain, and this
makes them distinctly
different from the amino acid
next to them.
And just like a real
classroom full of kids,
even though each amino
acid is unique and special
in their own way,
you can start to see
that some of these
amino acids are
more alike than
they are different.
And we can start to
see these similarities
in the chemical properties
of the side chains,
and this allows us to
group them together
into various categories.
And those chemical
properties include the charge
of the side chain, the
ability of the side chain
to undergo hydrogen bonding,
and also whether or not
we can classify that
side chain as being
either acidic or basic.
So the 20 amino acids
can be split broadly
into kind of two main groups.
The first group includes
the nonpolar amino acids,
and then the second group
includes the polar ones.
And the nonpolar
amino acids can also
be thought of as the
hydrophobic, or water-fearing,
amino acids.
And conversely, you
have the polar ones.
Those can be
considered hydrophilic,
meaning water-loving.
And yet another way that
I like to kind of think
about these two main groups are
the hydrophobic amino acids--
they're kind of like
the water-haters.
They don't really want to
interact with water at all.
They'd rather just
interact with themselves.
Whereas the
hydrophilic amino acids
are very open and welcoming to
interacting with water, and so
they're water-lovers.
And then within the two
groups of nonpolar hydrophobic
and polar hydrophilic
amino acids,
you then have a further
breakdown into subgroups.
And those subgroups
include those amino acids
that have alkyl side chains,
aromatic side chains,
neutral ones, acidic
ones, or basic ones.
So let's take a closer look
at those amino acids that
have alkyl groups
as side chains.
And as you can see here, we have
seven different amino acids,
and I've just drawn out
the side chain for you.
I've left the rest
of the molecule
out just to fit
everything in here.
And we have glycine, alanine,
valine, methionine, leucine,
isoleucine, and proline.
And proline is the exception.
I've drawn out the
entire amino acid there,
because as you can
see, its side chain
forms this interesting ringed
structure with the amino group
in the backbone of the molecule.
So I just included it
there for completeness.
So all these side chains
are made up of alkyl groups,
with the one exception
being glycine,
because its side chain has
only a hydrogen atom in it.
But because it behaves similarly
to an alkyl chain side group,
it gets slumped into this
category of amino acids.
And whenever you
see an amino acid
with an alkyl group
as its side group,
you should be thinking that
this amino acid is nonpolar.
And so they're also
going to be hydrophobic.
Now, let's take a closer look
at those amino acids that
have aromatic groups as part of
their side chain, and remember,
we're still under the umbrella
of nonpolar hydrophobic amino
acids here.
And so I've drawn out for
you here two amino acids,
phenylalanine and tryptophan.
And what should you
be thinking when
you're looking at
these amino acids?
So besides thinking,
oh, those amino acids
must smell really
good, because they're
called aromatic amino acids--
well, that might be true,
but you should also be
thinking the same thing
that you think when
you see amino acids
with alkyl groups as
their side chains.
These amino acids
that you see here
are also nonpolar
and hydrophobic.
And that kind of makes sense,
because aromatic chains
are also just made up of
carbons and hydrogens.
And you weren't wrong if you
thought that aromatic compounds
might smell really good, because
many of our most aromatic herbs
and spices that we're all
familiar with, like basil
or cinnamon and
vanilla, are composed
of the same sorts of ring
structures that we see here.
All right.
So now that we've tackled the
nonpolar hydrophobic amino
acids, let's dive on into the
polar and hydrophilic amino
acids.
The first group that we will
look at is the neutral group.
Here we have serine, threonine,
asparagine, glutamine, cystine,
and tyrosine.
The way that I remember that
these are the polar amino acids
is that these amino acids
have a side chain that
contain an oxygen
or a sulfur atom,
which tends to hog all
the electrons around them
to create a localized
negative charge over that atom
and then a positive charge over
the rest of the side chain.
So you can kind of see
why these amino acids like
to hang out with water
now, since water is also
polar in the same way.
And these amino acids
are considered neutral,
because although they are polar
enough to interact with water,
they're not strongly
polar enough
to qualify as an acid or a base.
So which of the polar
hydrophilic amino acids
do qualify as acidic?
Well, that would be these two
amino acids here, aspartic acid
and glutamic acid.
As you can see,
these amino acids
have a carboxylic acid as
part of their side chain,
which is a very willing,
strong hydrogen donor which
qualifies these amino
acids as acidic.
When these side chains
do donate their hydrogen
and they're left in anion
form, then in that case,
we refer to them as aspartate
and glutamate, respectively.
So you might see them
referred to in that way.
Last but not least, we
have the basic amino acids,
and they're histidine,
lysine, and arginine.
And the way I remember that
these amino acids are basic
is that if you take a closer
look at their side chains,
you see a few nitrogen atoms.
And remember that nitrogen is
a very willing proton accepter,
and this is why they
qualify as basic.