Module 5. Amino acids and peptides

Lesson 15

15.1 Introduction
  • Molecular formula - free carboxylic and amino groups
  • Several behaviors (properties) - not in accordance with the structure - abnormal behavior/properties - e.g.
  1. Relatively non-volatile, crystalline solids - melt with decomposition at fairly high temperature (~ 200°C) (if amine and carboxylic acid)
  2. Insoluble in non-polar solvents (benzene) but soluble in polar solvents (water)
  3. Neutral substances have Ka 1.6 x 10-12 and Kb 2.5 x 10-12 -aliphatic amines have kb 10-14 and carboxylic acids have ka 10-5
  4. Aqueous solutions - have a high dielectric constant - dipole movement -reflection occurrence of both positive and negative charges on the same molecule
  • These abnormal properties can be explained by - existence of dipolar (salt like) structure of amino acids

Fig. 15.1

The inner salt structure - results due to salt forming character of acids and amines
  • Inner salt - Zwitter ion or ampholyte or dipolar ion.

Fig. 15.2

Electrostatic attraction (-------) between oppositely charged groups - stabilizes crystalline state - therefore high melting point.


Fig. 15.3

Weak electrostatic interaction (-------) between amino acid molecules and water molecules - increases solubility (decrease in potential energy=P.E.).
  • Zwitter (dipolar) form of amino acid remains in equilibrium with non-zwitter (apolar) form of amino acid- therefore amino acid can undergo the typical reactions of amines and carboxylic acid

Fig. 15.4

15.2 Amphoteric Nature of Amino Acids
  • Compound - reacts both with acid and base - amphoteric compound
  • This characteristic - amphoteric nature
  • Zwitter ion of amino acids in water - acts as an acid as well as base - proton donner as well as proton acceptor- i.e. have amphoteric nature (Fig. 15.5)
  • Acidic group of amino acid:- NH+3 - donates proton
  • Basic group of amino acid:- COO- - accept proton
15.3 Isoelectric Point of Amino Acids
  • Amino acid in solutions - action of electric field - moves towards anode or cathode - depending on pH of the solution
  • In alkaline solution - amino group predominates - therefore migrate towards anode
  • In acidic solution - cation predominates - therefore migrate towards cathode
  • At particular pH - Zwitter ion may be present (or cation and anion present in equal amount) - no net migration of amino acid - isoelectric point pH - pI
  • “A particular pH at which a particular amino acid does not have net charge and does not migrate under the influence of an electric field is called isoelectric point of that amino acid”.
  • Each amino acid has a characteristic isoelectric point - e.g. 6.1 for gly
  • In general,
  • Monoamino monocarboxylic acid - pI - 6.0
  • Diamino monocarboxylic acid - pI = > 6.0
  • Monoamino dicarboxylic acid - pI = < 6.0
  • At pI amino acid has minimum solubility - used for separation of amino acids in mix.
Table 15.1 Solubility of different amino acids in water at 25°C (g/l)

t 15.1
  • When Zwitterions is titrated with acid, the –COO- group becomes protonated (-COOH)
  • pH at which concentration of –COO- and –COOH are equal- known as pKa1 (--logKa1)
  • When Zwitterions is titrated with base, the –NH3+ group becomes deprotonated (-NH2)
  • pH at which concentration of –NH3+ and –NH2 are equal - known as pKa2 (--logKa2)
  • In addition to α-NH2 and α-COOH groups, side chains of some amino acids also contain ionizable groups.
Arg., Asp., Cys., Glu., His., Lys, and Tyr
  • pH at which concentration of their respective ionized and unionized form are equal - known as pKa3 (-logpKa3)
  • pI of amino acids can be estimated from their pKa1, pKa2 and pKa3, using the following expressions
  • For neutral amino acids pI - (pKa1 + pKa2)/2 (no charged side chain)
  • For acidic amino acids pI - (pKa1 + pKa3)/2
  • For basic amino acids pI - (pKa2 + pKa3)/2
  • In proteins, the α-COOH of one amino acid is coupled to α-NH2 group of the next amino acid through peptide (amide) bond - these groups are not free to ionize
  • The only ionisable groups in protein are
  • N-terminal –NH2
  • C-terminal –COOH group and
  • Ionizable groups present in side chain of amino acids
  • The pKa of ionisable groups in side chain of amino acids are different from those of free amino acids
  • In protein, the pKa3 values of
  • Acidic amino acids (Glu and Aps) are larger and
  • Basic amino aicds (Arg and Lys) are smaller than those of the corresponding free amino acids
  • Degree of ionization of a group at a given pH of solution can be calculated using Henderson-Hasselbach equation.
pH - pKa + log [Conjugated base]/ [Conjugated acid]

15.4 Hydrophobicity
  • One of the major factors affecting physicochemical properties of peptides and protein is Hydrophobicity of the constituent amino acid residues.
  • Structure, solubility, taste, interaction with other elements and/or constituents
  • Defined as the excess free energy of a solute dissolved in water compared to that in an organic solvent (e.g. ethanol) under similar conditions
Table 15.2 Hydrophobicity of amino acid side chain at 25 °C (kJ/mol)

t 15.2

Last modified: Tuesday, 25 September 2012, 10:50 AM