PMID-sentid Pub_year Sent_text comp_official_name comp_offsetprotein_name organism prot_offset 2173185-5 1990 Substitution of the natural glycine residue by D-arginine residue in position 2 of the enkephalin molecule truncated at the C-terminus increased the mu-receptor binding potency of the tetrapeptide, whereas its delta receptor binding potency declined by more than one order of magnitude. D-Arginine 47-57 proenkephalin Rattus norvegicus 87-97