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

4.5.2 · Chemistry › Biomolecules

1. Amino acid kya hota hai? (WHAT)

α-carbon (glycine ko chhodkar, jisme R = H hota hai) par 4 alag-alag groups hote hain → yeh ek chiral centre hai, isliye amino acids optically active hote hain aur naturally L-form mein paaye jaate hain.

2. Zwitterion (WHY yeh banta hai)

Internal salt hone ke consequences:

• High melting points (ionic lattice) — yeh saaf melt hone ki bajay char ho jaate hain.
• Water mein soluble, non-polar solvents mein poorly soluble.
• Yeh amphoteric hote hain (acids aur bases dono se react karte hain).

Toh jaise-jaise pH badhta hai, charge cation → zwitterion → anion hota jaata hai: +1 se 0 se –1.

3. Isoelectric point pI (HOW formula milta hai)

pI ko first principles se derive karo

Ek simple amino acid mein do acidic ionisations hote hain. Inhe lost proton ke hisaab se number karo:

$\underset{\text{net }+1}{\underbrace{{\text{H}}_3\stackrel{+}{N}-\text{CHR}-\text{COOH}}}\stackrel{p{K}_{a1}(\approx 2)}{\to }\underset{\text{zwitterion, net }0}{\underbrace{{\text{H}}_3\stackrel{+}{N}-\text{CHR}-{\text{COO}}^{-}}}\stackrel{p{K}_{a2}(\approx 9)}{\to }\underset{\text{net }-1}{\underbrace{{\text{H}}_2\text{N}-\text{CHR}-{\text{COO}}^{-}}}$

Hum woh pH chahte hain jahan net charge = 0 ho, yani jahan zwitterion bilkul cation aur anion ki equal amounts ke beech mein ho.

Har equilibrium ke liye Henderson–Hasselbalch use karo: $\text{pH}=p{K}_a+\log \frac{[\text{base form}]}{[\text{acid form}]}$

pI par cation (+1) aur anion (–1) equal concentration mein hote hain (woh cancel hote hain), aur zwitterion maximum hota hai. Mathematically, set up karo:

$\text{pH}=p{K}_{a1}+\log \frac{[\text{zwit}]}{[\text{cation}]}\text{aur}\text{pH}=p{K}_{a2}+\log \frac{[\text{anion}]}{[\text{zwit}]}$

Dono equations add karo: $2\text{pH}=p{K}_{a1}+p{K}_{a2}+\log \frac{[\text{zwit}]}{[\text{cation}]}\cdot \frac{[\text{anion}]}{[\text{zwit}]}$ $2\text{pH}=p{K}_{a1}+p{K}_{a2}+\log \frac{[\text{anion}]}{[\text{cation}]}$

pI par, $[\text{anion}]=[\text{cation}]$ toh log term $=\log 1=0$ ho jaata hai:

Acidic aur basic side chains (3 pKa's)

Agar R khud ionise karta ho (jaise Asp/Glu mein extra –COOH hota hai; Lys/Arg mein extra basic N hota hai), toh teen pKa values hote hain. Rule:

Electrophoresis link (Forecast-then-verify): Forecast — pH 7 par, kya glycine, aspartate, lysine + ya – electrode ki taraf move karenge? Verify: pH 7 > pI(Gly 5.97) → net –, anode (+) ki taraf jaayega. pH 7 > pI(Asp 2.95) → strongly –, anode. pH 7 < pI(Lys 9.75) → net +, cathode (–) ki taraf jaayega. ✔

4. Classification

Flashcards

Connections

• Proteins — peptide bond and structure
• Henderson–Hasselbalch equation
• Acids and bases — pKa and pH
• Optical isomerism and chirality
• Electrophoresis and separation techniques
• Buffers — why pI matters for solubility

Concept Map