The above general formula describes 19 of the 20 amino acids except proline, whose side chain is irregular, in that it is covalently bound to both the α-carbon and the backbone nitrogen. The covalent bonds among different amino acids in a protein are called peptide bonds. The 20 amino acids can be divided into three categories based on R-group differences: non-polar, polar and charged polar.
The functional properties of food proteins are directly attributable to the amino acid R-group properties: structural (size, shape and flexibility), ionic (charge and acid–base character) and polarity (hydrophobicity/ hydrophilicity). At neutral pH, most free amino acids are zwitterionic, i.e. they are dipolar ions, carrying both a positive and negative charge, as shown in the general formula above.
Since the primary amino and carboxyl groups of amino acids are involved in peptide bonds within a protein, it is only the R groups (and the ends of the peptide chains) that contribute to charge; the charge of a protein being determined by the charge states of the ionisable amino acid R groups that make up the polypeptide, namely aspartic acid (Asp), glutamic acid (Glu), histidine (His), lysine (Lys), arginine (Arg), cysteine (Cys) and tyrosine (Tyr).
The acidic amino acids are Tyr, Cys, Asp and Glu (note: Tyr and Cys require pH above physiologic pH to act as acids, and therefore, are less important charge contributors in living systems). Lys, Arg and His are basic amino acids. The amino acid sequence and properties determine overall protein structure. Some examples are as follows: Two residues of opposite charges can form a salt bridge. For example, Lys and Asp typically have opposite charges under the same conditions, and if the side chains are proximate, then the negatively charged carboxylate of Asp can salt-bridge to the positively charged ammonium of Lys. Another important inter-residue interaction is covalent bonding between Cys side chains.
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Under oxidising conditions, the sulfhydryl groups of Cys side chains (–S–H) can form a thiol covalent bond (–S–S–) also known as a disulphide bond. Lastly, hydrophobic (non-polar) amino acids are generally sequestered away from the solvent in aqueous solutions (which is not always the case for food products), since interaction with polar molecules is not energetically stable