Solution structure of human beta-endorphin in helicogenic solvents: an NMR study.

beta-Endorphin is the largest natural opioid peptide. The knowledge of its bioactive conformation might be very important for the indirect mapping of the active site of opioid receptors. We have studied beta-endorphin in a variety of solution conditions with the goal of testing the intrinsic tendency of its sequence to assume a regular fold. We ran NMR experiments in water, dimethylsulfoxide and aqueous mixtures of methanol, ethylene glycol, trifluoroethanol, hexafluoracetone trihydrate and dimethylsulfoxide. The solvent in which the peptide is more ordered is the hexafluoracetone trihydrate/water mixture. The helical structure detected for beta-endorphin in this mixture at 300 K extends for the greater part of its address domain, hinting at a possible mechanism of interaction with opioid receptors: a two-point attachment involving an interaction of the helical part of the address domain (PLVTLFKNAIIKNAY) with one of the transmembrane helices and a classical interaction of the message domain (YGGF) with the receptor subsite common to all opioid receptors.