The global minimum (GM) structure of opiorphin (QRFSR) and its seven derivatives (qRFSR, QrFSR, QRfSR, QRFsR, QRFSr, QRYSR, QRySR) were explored using molecular mechanics method. Based on STRIDE analysis, most of the conformers possessed no secondary structures. Among global minima, only QrFSR epimer contained a beta-turn formed between glutamine (Q(1)) and serine (S-4) residues. The most common hydrogen bonds were formed between the 2nd arginine (R-2) and glutamine (Q(1)) which also appeared in some GMs. According to DFT calculations (omega B97X-D/cc-pVTZI/B3LYP/6-31G(d)), the overall epimerization of the GM structures is a downhill process in the presence of an center dot OH/H2O2 redox system. (C) 2015 Elsevier B.V. All rights reserved.