Molecular conformations of met-enkephalin: Comparison of the zwitterionic and cationic forms

Apparent discrepancies in published assignments of the pmr spectrum of Met-enkephalin are due to the existence of two forms, cationic and zwitterionic. The pmr parameters of these two forms indicate different secondary conformations. The pmr parameters of the zwitterionic form are indicative of a 2–5 β-I turn but no secondary conformation can be suggested for the cationic form with the present data.