The N-terminal amino group of a polypeptide can be modified covalently, e.g., acetylation − C ( = O ) − C H 3 {\displaystyle \mathrm {-C(=O)-CH_{3}} } The positive charge on the N-terminal amino group may be eliminated by changing it to an acetyl group (N-terminal blocking). (wikipedia.org)
A phosphate group can be attached to the sidechain hydroxyl group of serine, threonine and tyrosine residues, adding a negative charge at that site and producing an unnatural amino acid. (wikipedia.org)
The recent elaboration and documentation of this concept has depended on ad- vances in our understanding of the mechanism of protein synthesis and the concomitant appreciation of the variety of roles played by RNA in that process. (nih.gov)