Amino acids — zwitterion, isoelectric point pI, classification (essential, non-essential): Foundations

This page is the toolbox. We name every symbol the parent note leans on, draw the picture behind it, and say why the topic can't work without it. Nothing here assumes you've met chemistry notation before. Read top to bottom — each rung of the ladder needs the one below it.

1. Atoms, bonds, and a "group" — the alphabet

Before any charges, we need to read a molecular sketch.

Why the topic needs this: the entire story is these two groups arguing over one proton. If you can't spot them in a sketch, none of section 2 onward makes sense.

Figure 1 — what to look for: the sketch below draws the whole amino-acid skeleton. In the centre is the carbon that holds everything; on its left (violet) is the base hand $- NH_{2}$ , on its right (magenta) is the acid hand $- COOH$ , with an $H$ above and the swappable side chain $R$ (orange) below. Fix this four-armed picture in mind — every later idea is about the two coloured hands passing a proton.

Figure — Amino acids — zwitterion, isoelectric point pI, classification (essential, non-essential)

2. The proton $H^{+}$ and electric charge — the currency

Everything that follows is one particle moving around.

Figure 2 — what to look for: the top row shows the acid hand $- COOH$ losing a proton to become $- COO^{-}$ (reads $-$ because it kept the electron); the bottom row shows the base hand $- NH_{2}$ grabbing a proton to become $- NH_{3}^{+}$ (reads $+$ ). The line at the bottom adds the two signs: $(+ 1) + (- 1) = 0$ — that's the zwitterion being balanced. Read the figure as "the charge sign follows the proton".

3. Acid, base, and "donates a proton" — the verbs

Why the topic needs this: the phrase "the acidic proton hops onto the amine" is exactly an acid handing a proton to a base inside one molecule. That hop is the zwitterion being born.

4. The logarithm and square brackets — two bits of notation first

The pH formula in the next section uses two pieces of shorthand, so we define them before we use them.

5. pH — how crowded the water is with protons

Figure 3 — what to look for: this is the "dial" picture. Slide left (low pH, many protons) and the molecule fills up with protons → net $+ 1$ (cation). Slide to the middle and it's the balanced zwitterion → net $0$ . Slide right (high pH, protons scarce) and both hands empty → net $- 1$ (anion). Watch the coloured circle's charge change as you move along the pH arrow.

6. $p K_{a}$ , Henderson–Hasselbalch, and pI — the balance pH

This is where the symbols let us calculate instead of just describe.

Now bring in the equation that connects a $p K_{a}$ to a pH.

Worked example WHY the pI is the

average of the two flanking $p K_{a}$ 's At the pI the leftover cation ( $+ 1$ ) and leftover anion ( $- 1$ ) are present in equal amounts so they cancel. Write Henderson–Hasselbalch for each of the two ionisations that flank the zwitterion: $pH = p K_{a 1} + lo g_{10} \frac{[ zwit ]}{[ cation ]} pH = p K_{a 2} + lo g_{10} \frac{[ anion ]}{[ zwit ]}$ Add the two equations (this makes the middle $[zwit]$ cancel inside the logs): $2 pH = p K_{a 1} + p K_{a 2} + lo g_{10} \frac{[ zwit ]}{[ cation ]} \cdot \frac{[ anion ]}{[ zwit ]} = p K_{a 1} + p K_{a 2} + lo g_{10} \frac{[ anion ]}{[ cation ]}$ At the pI, $[anion] = [cation]$ , so the ratio is $1$ and $lo g_{10} 1 = 0$ : $2 pH = p K_{a 1} + p K_{a 2} \Rightarrow pI = \frac{p K _{a 1} + p K _{a 2}}{2}$ That is why you average: sitting at the midpoint pH of the two reactions makes exactly as much cation as anion, so they cancel to net zero. See Acids and bases — pKa and pH.

7. Electrodes and moving in a field — what "doesn't migrate" means

The pI is defined by behaviour in an electric field, so we must say what that field does.

8. How the pieces feed the topic

The diagram below is a prerequisite map. How to read it: each box is one idea from this page, and every arrow means "you must understand the box at the tail before the box at the head makes sense." Follow the arrows and you are literally walking the safe learning order — nothing points backwards. It's useful because it shows why we built the symbols in this sequence: the zwitterion sits in the middle because charge, functional groups, and the proton verbs all have to arrive first.

9. The α-carbon and chirality — the geometry

Why the topic needs this: "optically active, occurs as the L-form" is a direct consequence of the α-carbon carrying four different bricks.

10. Where these tools reappear

The zwitterion's balanced charge → why it sits still at its pI → matters for Electrophoresis and separation techniques.
$p K_{a}$ , pH and net charge → why an amino acid is least soluble at its pI → Buffers — why pI matters for solubility.
The α-carbon and the peptide bond that joins amino acids → Proteins — peptide bond and structure.
The full Hinglish walkthrough of this topic → 4.5.02 Amino acids — zwitterion, isoelectric point pI, classification (essential, non-essential) (Hinglish).
Parent topic to return to → Amino acids — parent note.

Equipment checklist

Can you answer each before reading the parent note? Reveal to check.

What does the raised

+

H^{+}

physically mean?

The particle is missing one electron, so it carries one unit of positive charge — a bare proton.

Why does

- COO^{-}

carry a minus?

It let go of its proton but kept the electron the proton left behind, so it has one spare negative unit.

What is a zwitterion?

A molecule with both a

+

and a

-

on different groups at once, giving net charge zero — an internal salt.

What is the exact mathematical definition of pH?

pH = - lo g_{10} [H^{+}]

, the negative base-10 log of the molar proton concentration.

At low pH, is the amino acid a cation or anion?

Cation (

+

) — lots of protons flood the groups and they fill up.

What does a group's

p K_{a}

tell you?

The pH at which that group is exactly half holding / half releasing its proton — its personal tipping point.

Which group does

p K_{a 1}

belong to, and which does

p K_{a 2}

p K_{a 1}

(≈2) is the carboxyl

- COOH

;

p K_{a 2}

(≈9) is the ammonium

- NH_{3}^{+}

Why is the pI the average of the two flanking

p K_{a}

's?

Adding the two Henderson–Hasselbalch equations and setting [cation]=[anion] gives

2 pH = p K_{a 1} + p K_{a 2}

, so pI is their midpoint.

What is the pI (isoelectric point)?

The pH at which the molecule's net charge is zero, so it doesn't migrate in an electric field.

Toward which electrode does a net-positive molecule move, and why?

The cathode (negative electrode) — the field pushes positive charges toward the negative terminal.

Can a side chain R change the pI?

Yes — if R has its own ionisable group it adds a third

p K_{a}

and shifts the pI up (basic R) or down (acidic R).

What do the square brackets in

[H^{+}]

denote?

The molar concentration — moles of that species per litre of solution.

Which base does "

lo g

" use on the pH scale, and what is

lo g 1

Base 10 (

lo g_{10}

), and

lo g_{10} 1 = 0

Why is the α-carbon usually chiral?

It carries four different groups (acid hand, base hand, H, side chain R), so its mirror image can't be superimposed.