Disclosed herein is pyrroline-carboxy-lysine (PCL), a pyrrolysine analog, which is a natural, biosynthetically generated amino acid, and methods for biosynthetically generating PCL. Also disclosed herein are proteins, polypeptides and peptides that have PCL incorporated therein and methods for incorporating PCL into such proteins, polypeptides and peptides. Also disclosed herein is the site-specific derivatization of proteins, polypeptides and peptides having PCL or pyrrolysine incorporated therein. Also disclosed herein is the crosslinking of proteins, polypeptides and peptides having PCL or pyrrolysine incorporated therein.