S. Al-Ttaee, Muthanna, et al. “Synthesis of New Opioid Analgesic Peptide Analogues to Enkephalin (Leucine- and Methionine-Enkephalin)”. Iraqi Journal of Pharmaceutical Sciences, vol. 17, no. 1, Mar. 2017, pp. 39-46, https://doi.org/10.31351/vol17iss1pp39-46.