from Fisher Scientific, Sigma-Aldrich, or Acros Organics and used
without further purification. FMOC-protected amino acids and Wang
resins were purchased from Anaspec, Inc. or Chem-Impex International.
β-endorphin (YGGFMTSEKS QTPLVTLFKN AIIKNAYKKG E) was purchased
from AnaSpec Inc. (Cat # 24319), and RRLIEDNEYTARG was purchased from
Enzo (Cat # BML-P307-0001). AKAKTDHGAEIVYK was synthesized according
to a modified solid-phase peptide synthesis protocol.21 (link)