Organic Chemistry

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 K_a 1.6 x 10^-12 and K_b 2.5 x 10^-12 -aliphatic amines have k_b 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)

• When Zwitterions is titrated with acid, the –COO- group becomes protonated (-COOH)

• pH at which concentration of –COO- and –COOH are equal- known as pK_a1 (--logK_a1)

• When Zwitterions is titrated with base, the –NH₃⁺ group becomes deprotonated (-NH₂)

• pH at which concentration of –NH₃⁺ and –NH₂ are equal - known as pK_a2 (--logK_a2)

• 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 pK_a3 (-logpK_a3)

• pI of amino acids can be estimated from their pK_a1, pK_a2 and pK_a3, using the following expressions

• For neutral amino acids pI - (pK_a1 + pK_a2)/2 (no charged side chain)
• For acidic amino acids pI - (pK_a1 + pK_a3)/2
• For basic amino acids pI - (pK_a2 + pK_a3)/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 pK_a3 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)