Peptides are used to simulate proteins. In order to mimic the performance of proteins, we need to synthesize polypeptides with similar structures and charges to proteins. When a peptide is "cut out" from a protein, the number of charges at both ends will be different from that of the gene body protein. We need to change the compositing strategy to make them consistent. In general: If the sequence is from the C-terminus of the protein, shield the N-terminus by acetylation; if it is a sequence from the N-terminus of the protein, shield the C-terminus by amidation; if it is from the middle part of the protein, use acetylation and amidation to shield the Both ends are shielded.