Expanded Genetic Code: A List of Non-Natural Amino Acid Systems

The 'expanded genetic code' refers to the addition of artificial or non-natural nucleotides to the standard genetic code, allowing for the incorporation of new amino acids into proteins. Here is a list of some of the expanded genetic codes that have been developed:

1. 21st amino acid system:

   • PylTyr: Incorporates the non-natural amino acid pyrrolysine (Pyl) and the natural amino acid tyrosine (Tyr).
   • PylHis: Incorporates Pyl and histidine (His).
   • PylCys: Incorporates Pyl and cysteine (Cys).
   • PylPhe: Incorporates Pyl and phenylalanine (Phe).
   • PylLys: Incorporates Pyl and lysine (Lys).

2. 22nd amino acid system:

   • Sep: Incorporates the non-natural amino acid O-phosphoserine (Sep).

3. 23rd amino acid system:

   • AzF: Incorporates the non-natural amino acid p-azido-L-phenylalanine (AzF).

4. 24th amino acid system:

   • BocK: Incorporates the non-natural amino acid Nε-(tert-butoxycarbonyl)-L-lysine (BocK).

5. 25th amino acid system:

   • LysK: Incorporates the non-natural amino acid N6-(2-aminoethyl)-L-lysine (LysK).

6. 26th amino acid system:

   • Hpg: Incorporates the non-natural amino acid L-homopropargylglycine (Hpg).

7. 27th amino acid system:

   • LysP: Incorporates the non-natural amino acid N6-propargyl-L-lysine (LysP).

Please note that there are many more variations and combinations of non-natural amino acids that have been developed and used in various studies. This list provides a glimpse of some of the expanded genetic codes that have been explored.