 Short proteins can also be synthesized chemically by a family of methods known as peptide synthesis , which rely on organic synthesis techniques such as chemical <unk> to produce peptides in high yield . Chemical synthesis allows for the introduction of non @-@ natural amino acids into polypeptide chains , such as attachment of fluorescent probes to amino acid side chains . These methods are useful in laboratory <unk> and cell biology , though generally not for commercial applications . Chemical synthesis is inefficient for polypeptides longer than about 300 amino acids , and the synthesized proteins may not readily assume their native tertiary structure . Most chemical synthesis methods proceed from C @-@ terminus to N @-@ terminus , opposite the biological reaction . 
