Physical properties of amino acids

• Amino acids are colorless, crystalline substance.
• Most amino acids are tasteless but some are sweet. (E.g. Glycine, Alanine) and some are bitter (Eg. Arginine)
• Amino acids have high melting point (200-300)^oC due to ionic property.
• Solubility:
  • Solubility of amino acids depends upon polarity, iso-electric point, nature of solvent (pH) and temperature.
  • Amino acids are soluble in water and ethanol (i.e. polar solvent) and insoluble in non-polar solvent like benzene, ether etc.
  • Amino acids are insoluble at iso-electric point.
  • Solubility depends upon pH of solvent and temperature.
  • Eg. Tyrosine is soluble in hot water.

• Amphoteric property:
  • Amino acids can act as acid and base due to their dipolar i.e. zwitter ion nature.

• Titration curve of amino acids:
  • Most of the amino acids are monoamino-monocarboxylic acid.
  • When they are in fully protonated form they can be titrated twice.
  • Titration curve is the graph made between pH of amino acids and volume of acid or base added. It is always sigmoidal.
  • pKa= tendency of an acid (-COOH) to loose proton.
  • This tendency decreases 10 folds as pKa value increase by 1 unit.
  • At pKa, there is equimolar concentration of +ve ion and zwitter ion.

Important information from titration curve:

• pKa, pKb and pI value can be calculated.
• All the monoamino-monocarboxylic aminoacids (diproteic) have nearly about pKa and pkb values.
• Buffering zone (pKa+/-1) or (pKb+/-1).
• Glycine: pKa=2.3, pKb=9.6
• Alanine: pKa= 2.26, pKb=9.4
• Aspartic acid: pKa+2.09, pKb=9.82, pKR=3.86
• Histidine: pKa= 1.82, pKb=8.95, pKR=10.53

• Note:
  • The α-carbonyl group of mono-amino-mono carboxylic amino acids is stronger acid than carboxyl group of the aliphatic acid.
  • It is because of amino group and its +ve charge which increase tendency of carboxyl group to dissociate the -ve charge.
  • Similarly, the α amino group of diproteic amino acid is stronger base than amino group of comparable aliphatic amines. It is because of presence of carboxyl group.

• Absorption spectrum:
  • Amino acids absorbs the light at 280nm so the concentration of amino acids can be measured.

• Isomerism in amino acids:
  • Except glycine, all amino acids exists in super impossible mirror image i.e. D and L form.
  • This configuration is given by Emil Fischer.
  • This configuration indicate absolute concentration but not indicate the optical activity of amino acids.
  • – CHO group can be converted into -COOH group so they are compared.
  • -OH group compared with -NH₂ group.
  • -R- group compared with -CH₂OH group.

Chemical properties of amino acids:

• Why chemical properties of amino acid is importance?
• Chemical reactions of amino acids are important:
  • For identification and analysis of amino acids in protein.
  • For identification of amino acid sequences in protein.
  • For identification of specific amino acid residue of native protein that are required for biological functioning e.g. haemoglobin (Histidine has role in it).
  • For chemical modification of amino acids residue in protein molecules to produce change in biological activity.
  • For chemical synthesis of the polypeptides (for medical purpose).

Types of chemical reactions given by amino acids:

• Reaction due to -COOH group.
• Reaction due to -NH₂ group
• Reaction due to both
• Reaction due to R-group

1. Chemical reactions of amino acids due to -COOH group:

2. Chemical reactions of amino acids due to both -COOH and -NH2 group:

3. Chemical reaction of amino acids due to -NH2 group

4. Chemical reaction of amino acids due to functional group:

Functions of amino acids:

• Precursor for synthesis of proteins and polypeptides.
• Used for glucogenic and ketogenic degradation.
• For synthesis of porphyrin.
• For synthesis of melanin: Melanin is complex polymeric structure made up of tyrosine and also may contain tryptophan.
• For synthesis of creatine and creatine phosphate.
• For synthesis of plant hormones (auxin).
• For synthesis of neurotransmitter.
• For synthesis of animal hormones (thyroid- thyroxine)
• For synthesis of lignin, tannin, papaverine (alkaloids).
• For synthesis of vitamins (Ascorbic acid), niacin.
• Synthesis of antibacterial agents (penicillin G).

Properties of amino acids: physical and chemical

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