Dr. Kalpeshkumar C Nakarani

Tutor cum Second year Resident

Amphoteric nature of Aminoacids

Zwitterion At a specific pH the amino acid carries both the charges in equal number and exists as dipolar ion or ‘zwitterion’ At this point net charge on it is zero.

Isoelctric pH (pI) The pH at which a molecule exists as a ‘zwitterion’ or ‘dipolar ion’ and carries no net charge is known as isoelectric pH (pI).

pKa = pK of an acid = the pH at which the protonated and unprotonated species are present at equal concentration.

Amino acids bears at least two ionisable weak acid groups• - COOH (Stronger acid)• - NH3

+ (Weaker acid)

R-COOH R-COO- + H+

R-NH3+ R-NH2 + H+

Acidic (Protonated

form)

Conjugate base (Unprotonated form)

Titration of R-COOH(e.g. Acetic acid) pKa = 4.76

Titra

tion

of R

-NH 3

+

(e.g

. Met

hyla

min

e) p

Ka =

10.

6

• The perturbed pKa of glycine is caused by repulsion between the departing proton and the nearby positively charged amino group on the -carbon atom, as described in Figure.

• Similarly, the pKa of the amino group in glycine is perturbed downward relative to the average pKa of an amino group. This effect is due partly to the electronegative oxygen atoms in the carboxyl groups, which tend to pull electrons toward them, increasing the tendency of the amino group to give up a proton. Hence, the -amino group has a pKa that is lower than that of an aliphatic amine such as methylamine

The pI value can be calculated by taking the average pKa values corresponding to the ionisable group

𝑝𝐼=𝑝𝐾 𝐶𝑂𝑂𝐻 +𝑝𝐾𝑁𝐻 32

The aminoacids with an ionisable R group have more complex titration curve

Aspartate Glutamate Lysine Arginine Histidine Tyrosine Cysteine

Titration curve of GLUTAMATE

pK(COOH)=2.19pK(R) = 4.25pK (NH3) = 9.67

Structure at physiological pH

Titration curve of HISTIDINE

pK(COOH)=1.82pK(R) = 6.0pK (NH3) = 9.17

Simply, How to find pI of the AA containing ionisable R-group?

Write out all possible ionic structures, proceeding from strongly acidic to basic solution.

Identify the isoionic/ Zwitterionic representation(The species which bears no net charge)

The pI is the pH at the midpoint between the pK values on the either side of the isoionic species

Thank You