Leah here from leah4sci.com/mcat and in this video we're going to look at zwitterions and
finding the charge of an amino acid.
Amino Acid are often drawn in the neutral form such as glycine represented here.
We have the alpha carbon with the hydrogen and side-chain, the alpha carboxy group and
the alpha amine.
That's how we get amino acid and amine and then acid.
But the problem is that with the amine being basic and the carboxylic acid being acidic,
this is not the correct form for drawing an amino acid since acids and bases will react
with each other.
If I put this two molecules together, nitrogen would use its lone pair of basic electrons
to grab the acidic hydrogen from the carboxy group giving it back the electrons and when
enough amino acids react with each other, we would get something that looks like this.
The carboxy group is now deprotonated to give us a negative carboxylate, the amine group
now has three hydrogens because it's protonated, no longer has a lone pair but gets a positive
charge which is the correct form in nature.
Depending on the pH, you're more likely to see this form in nature, the neutral form
cannot exist because of the acid base interactions.
In nature they're not going to react with each other, they're going to react with the
solution but the idea is the same, we're going to get a zwitterion or dipolar ion.
Zwitterion comes from the german word 'zwitter' which means double and ion 'charged'.
The zwitterion has at least two charged groups on the molecule, acidic and basic side-chains
can have a third charge in their variable group.
Let's break this down and see how you find the charger in amino acid at any given pH.
The key here is to recognize that acids will donate a proton and bases will accept or attack
a proton.
If you're not solid with this concept make sure you reviewed the acid base tutorial link
below.
Once again, we have the neutral form of glycine, in order to figure out the charge, you want
to start with a neutral form and then look toward the PKA values.
The PKA values one for each acid or base is going to tell you at which pH it's going to
gain or lose a proton.
To find the charge, ask yourself 'what is the pH of any ionizable group' and will that
group be protonated or deprotonated base on the PKA.
Before we look at finding amino acid charge at a specific pH, we have to understand the
relationship of pH and PKA.
The acid base tutorial series link below, explains the relationship of ka to acid concentration.
ka is equal to H plus times A minus over HA and that tells us that KA is proportional
to the H plus concentration where H plus concentration tells us the pH.
From KA, we can find the PKA given that PKA is equal to negative log of the KA and finally
the relationship of pH and PKA can be found with the Henderson Hasselbalch Equation where
pH is equal to Pka plus the log of conjugate base over acid.
If you're not comfortable with this, see the tutorial link below.
So what do we make of this expression?
Pka tells us how likely the molecule is to take a proton from solution or donate a proton
to the solution.
And what the pH tell us?
Well if we have a low pH, that means we have an acidic solution.
An acidic solution means that we have lots of H plus floating around, so many H pluses
that we just start pumping them on to the molecules dissolved in solution.
A high pH tells us that we have a basic solution.
Basic solutions have lots of OH minus and OH minus as a negative ion is looking to take
protons away from our molecule dissolved in solution.
Now you have to compare the two, how does the acidity or basicity of the solution compare
to the molecules desire to hold on to that H plus.
When you're comparing the pH and the Pka, just think of it as a battle for the proton.
The higher value has the stronger desire for the proton and takes it away from the other,
if the pH is higher, the solution wins and if the Pka is higher, the molecule wins.
When the pH is less than the Pka, we're looking at a low pH and therefore an acidic solution,
which one wins?
If the Pka is higher, the molecule is stronger and the molecule takes the proton away from
solution, you can also think of it as a low pH has a lot of protons and therefore some
of it gets donated to the molecule.
Therefore if the pH is less than the Pka, the molecule will be protonated.
If the pH is greater than the Pka, we're looking at a higher pH or basic solution.
The pH means that the solution wins and takes the proton away from the molecule, that means
the molecule is going to be deprotonated.
And finally, when the pH is near the Pka, we have a stalemate.
The pH representing the solution once the proton just as strongly as the Pka which represents
the molecule and this puts us into the buffer zone.
If the pH is equal to the Pka, then we have a fifty-fifty meaning half the molecules are
protonated and half are deprotonated but if the pH is within one unit of the Pka, you're
still going to see the buffer zone but you'll have more protonated or more deprotonated
depending on the direction.
When the pH is within one unit of the PKA, that the pH is lower, you're going to have
more protonated than deprotonated.
If it's higher you'll have more deprotonated than protonated but still within that partially
charged region.
Amino acids have a carboxylic acid in their backbone so let's look at acidic acid, a very
simple carboxylic acid with the Pka of approximately four point eight.
What is this tell us about the molecule?
Acetic acid is happy to donate its proton depending on the solution.
If acetic acid dissolved in an acidic solution, we have too many protons and that means Le
Chatelier's principle says 'we push the reaction towards the reactants to the left' and we
protonate acetic acid.
If acetic acid is dissolved in a basic solution, the base, the OH minus is pulling the protons
out of equilibrium and that means Le Chatelier's principle pushes the reaction to the right,
favoring products and we deprotonate the acid.
Let's look at different pH ranges, we'll start with the pH that is less than the Pka for
example, if pH is equal to two.
A pH have two is significantly lower than four point eight and that means we have so
many protons in solution that we're going to protate every available acetate and get
acetic acid.
The charge is equal to zero, we have a neutral molecule.
If the pH is equal to four point eight, pH is equal to the Pka, we're going to get fifty
percent protonated and fifty percent deprotonated.
Half the molecules grab a proton but the solution takes the proton away from the other half
of the molecules.
The charge in this case, the average charge of all the molecules and solution will be
equal to minus one half.
Why negative half?
We have a charge of zero for the protonated form, a charge of negative one for the deprotonated
form and the average between zero and negative one is negative a half.
This is an estimate, add pH is equal to four point eight, it's negative a half, as the
pH goes up, the charge becomes more negative, as the pH goes down, the charge becomes less
negative with the ideal buffer range within a pH unit above or below the PKA value.
If the pH is equal to three point eight, we have a ten to one ratio of protonated to deprotonated,
following the Henderson Hasselbalch Calculation.
If we raise the pH all the way to eight, it's a very basic solution, we have an overwhelming
amount of OH minus and the OH minus is much more desperate for the proton than the acetic
acid.
That means the OH minus will take away all the protons and we have acetate in solution,
for a charge of negative one.
The numbers will change slightly on an amino acid because the PKA of the carboxy side-chain
will vary depending on what else we have nearby.
But we're gonna use the same principle with a specific Pka value.
Amino acids have amines on their backbone, so let's look at an alkyl amine in this case,
methylamine, but since we're looking at a Pka and methylamine is the base, we have to
look at the conjugate acid protonated form, methylammonium.
Methyl-ammonium will be an equilibrium with it's conjugate base methylamine, giving off
a proton in solution.
Methyl-ammonium has a PKA of ten point six which tells us a pH of ten point six will
give me a buffer between methylamine and methyl-ammonium.
If instead, we tried to apply a PKA value to a base, we're calling this base in acid
and methylamine can actually act as an acid or a base while the acidic form of this molecule
is going to give us CH3, NH with two lone pairs and a negative charge on nitrogen.
This is a very very very strong conjugate base and you're not going to see this in the
side-chain of amino acids.
Amino acid nitrogens will accept a proton to get a positive charge or donate a proton
to become neutral but the neutral amine will never donate a proton to become negative in
physiological conditions.
Let's see what happens at different pH values.
If we start with a pH significantly lower than the Pka, let's put the pH equal to seven,
this means, compare to the methylammonium's desire to hold on to that proton, the pH is
much much much more acidic, that means the pH has so many protons it doesn't want the
proton from methyl-ammonium, the pH is lower than the PKA and so the molecule wins.
The form is going to the methylammonium, CH3, NH3+ and the charge is equal to plus one.
If we raise the pH we'll pass the Pka to a value of thirteen, the pH is higher than the
PKA, that means the pH, the solution has a stronger desire for that proton , methyl-ammonium
will give up its proton to give me the form, CH3, NH2 because we took away the third hydrogen
and the charge on this case is neutral, it's not negative because we started positive so
we drop down to neutral.
Acetic acid started neutral so we dropped down to negative.
And finally, if we take a pH that is approximately the Pka value, for example, a pH of ten, it's
slightly less than the Pka, that means a slightly more acidic than the Pka but we're still in
the buffer zone.
Because the pH is within one pH unit of the Pka but it's towards the acidic side, we're
going to get the acidic CH3, NH3+ in a higher concentration than the CH3 NH2 and that means
the charge will be somewhere between zero and positive one and so we can say is that
zero is less than the charge which is less than positive one.
The exact number can be calculated using the Henderson Hasselbalch Equation, but on the
MCAT you don't have a calculator so you're not expected to know the specifics, instead
you have to be able to pinpoint the range.
If the fifty fifty will be positive one half and it's more acidic that means we're closer
to the plus one, it'll actually be somewhere between positive and a half and positive of
a one.
Let's apply this to the amino acid glycine.
The PKA for the carboxy group is two point three and the Pka for the amine is nine point
six.
Let's see what happens when we have a pH less than, greater than, and equal to every Pka
value.
Let's start at the pH is equal to one.
pH one is less than both Pka values, that means the solution has more protons and the
solution will give protons to the molecule, or you think of it as the Pka is higher than
the pH and so the Pka's desire for the protons win.
The amine group is protonated to NH3+, the carboxy group is protonated to OH and if we
add up both charges, we get positive one for the amine, zero for the carboxy for a net
q of positive one or q is the charge.
If we raise the pH to the first Pka value, we're going to get a buffer at the first Pka.
The carboxy group will be partially protonated and partially deprotonated which means half
of the molecules will have a negative one charge, half of the molecules will be neutral
so we'll say that the charge is equal to negative a half.
The amine has a PKA greater than the pH and will be protonated for a charge of plus one.
The net charge of plus one minus a half is equal to positive one half.
If we raise the pH somewhere between the two Pka values for example, pH is equal to seven,
that's very new physiological pH, that's where we expect to see the zwitterion but let's
prove it.
Though amine PKA is higher than the pH and so it will be protonated, gives me a charge
of plus one, the carobxy Pka is lower than the pH so it will be deprotonated for a charge
of negative one.
If we add a plus one minus one, charge is equal to zero, this is our zwitterion.
I want you to pay attention to a trend that's happening here.
As we raise the pH we went from plus one and were dropping to plus a half, plus zero, so
we kept dropping half, I want you to realize with me that if we raise the pH to the next
PKA value, for this amino acid it should be negative a half and if we raise it above the
PKA value, it should be negative one, let's prove it.
We'll bring the pH to nine point six which is the PKA of the amine, that means half the
amines will be protonated, half will be deprotonated, protonated amines have a charge of plus, deprotonated
amines are neutral, giving me a charge of plus one half.
The pH is much greater than the PKA and so the carboxy group is deprotonated for a charge
of negative one plus one half minus one is equal to negative one half as we predicted
and finally, when we bring the pH while above both PKA values, we expect the pH to win and
steal all possible protons, the amine group will be protonated to a neutral NH2, the carboxy
group will be deprotonated to a charge of negative one, and when we add them up, zero
minus one is equal to negative one once again as predicted.
Another trend to keep in mind here, where the pH is at a PKA, your net charges gonna
be somewhere near half, when the pH is well above or well below the PKA values, you're
gonna get a whole number, what whole number?
This depends on the specific pH and any groups on the molecule including side-chain and backbone,for
even more practice on finding amino acid and peptide charge, be sure to try the quiz along
with my tutorial series and amino acid cheat sheet by visiting my website, leah4sci.com/aminoacids