A peptide chain of 10 residues in length is used in native chemical ligation with a larger recombinantly expressed protein. 
* Residue 1: alanine
* Residue 2: one of glutamine, glycine, arginine, glutamic acid, serine, or methionine
* Residue 3: any one of the 20 amino acids
* Residue 4: acetyllysine
* Residue 5: alanine
* Residue 6: isoleucine
* Residue 7: aspartic acid
* Residue 8: phenylalanine
* Residue 9: acetyllysine
* Residue 10: arginine with the carboxy terminal thioester
With 7 possibilities at Residue 2 and 20 possibilities at Residue 3, the total would be  or 140 different polypeptides in the library.
This peptide library would be useful for analyzing the effect of the post-translational modification acetylation on lysine which neutralizes the positive charge. Having the library of different peptides at residue 2 and 3 would let the investigator see if some change in chemical properties in the N-terminal tail of the ligated protein makes the protein more useful or useful in a different way.