Properties of amino acids: physical and chemical

September 18, 2020 Gaurab Karki Biochemistry 0




Properties of amino acids: physical and chemical
Properties of amino acids: physical and chemical
  • Study about properties of protein within single cell is known as Proteomics.

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:

Fig. Chemical reaction of amino acids due to carboxyl (-COOH) group

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

fig. Chemical reaction of amino acids due to both -COOH and -NH2 group

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

fig. Chemical reaction of amino acids due to amino (-NH2) group

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

fig. 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