Lesson 15. ZWITTER ION FORM AND ITS PROPERTIES VIZ. MELTING POINT AND VOLATILITY

Module 5. Amino acids and peptides

Lesson 15
ZWITTER ION FORM AND ITS PROPERTIES VIZ. MELTING POINT AND VOLATILITY

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
15.1

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.
15.2

Fig. 15.2

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

15.3

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
15.4

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