Physical and Chemical Properties of Aminoacids
Aminoacids are the organic compound having an amine group and a carboxyl group. Amino acids linked together by peptide bonds to form protein molecules. The 20 amino acids are encoded directly by the genetic codes, based on which several groups of aminoacids are classified. Factors based on which subgroups are divided are charge, hydrophilicity or hydrophobicity, size and functional groups.
The Amino acids are linked together by peptide bonds to form a protein. Peptide bonds are strong, rigid and planar. They have partial double bond characteristics.
Aminoacids linked by peptide bonds
Amino acids are colourless, crystalline solids, water-soluble high melting solids and behave like salts. In aminoacids the interaction of the carboxyl and amino groups results in the transfer of proton from carboxyl group to amino group. Thus the amino acid exists in ionised form known as zwitter ion.
The zwitter ions are dipolar, when the proteins are present in an aqueous solution, the carboxyl group can loose a proton and amino group can accept a proton giving rise to a dipolar ion which is neutral but has both positive and negative charges.
In the ionic form, amino acids show slow amphoteric behavior as they react with acids and bases. In the acidic solution, the carboxyl group (- COO-) accepts a proton and gets converted to carboxyl substituent (- COOH) while in basic solution the ammonium (+NH3) loses a proton and gets converted to amino group (-NH2).
The movement of the aminoacid in an electric field is different in acid and alkaline solution. In acidic solution, an amino acid exists as a positive ion and migrates towards the cathode in an electric field, while in alkaline solution it exists as a negative ion and migrates towards anode. At a certain pH zwitterions exists as a neutral ion and do not migrate to either electrode. This pH is known as the isoelectric point of the amino acid.
Proteins carry out posttranslational modifications, during attachment of additional chemical groups to the amino acids in proteins. Such modifications can produce hydrophobic lipoproteins, or hydrophilic glycoproteins. These modifications will also direct the proteins towards the target membrane. For example to the cystein residues when the fatty acid like palmitic acid is added or removed, it causes the proteins to attach and then detach from cell membranes.