Impact of Post-translational Modifications on Protein Charge

Consider the process of protein synthesis, during which amino acids are linked together to form polypeptide chains. Each amino acid consists of a central carbon atom, an amino group, a carboxyl group, a hydrogen atom, and a variable R group that determines the properties of the amino acid. Notably, certain amino acids can undergo various post-translational modifications once incorporated into a protein. These modifications can significantly influence the protein's function, stability, and interactions.

Based on this information, which of the following post-translational modifications is most likely to affect the charge of the amino acid side chains in a protein?