NMR studies on the monomer-tetramer transition of melittin in an aqueous solution at high and low temperatures

Melittin, a peptide of 26 amino acid residues, has been used as a model peptide for protein folding and unfolding, and extensive research has been done into its structure and conformational stability. Circular dichroism (CD) studies have demonstrated that melittin in an aqueous solution undergoes a transition from a helical tetramer to a random coil monomer not only by heating but also by cooling from room temperature (i.e., heat- and cold-denaturation, respectively). The heat-denaturation has been also examined by nuclear magnetic resonance (NMR) experiments, however, no NMR data have been presented on the cold-denaturation. In this paper, using proton ((1)H) NMR spectroscopy, we show that melittin undergoes conformational transitions from the monomer to the tetramer to the monomer by elevating temperature from 2 to 70 °C. Only melittin including a trans proline peptide bond participates in the transitions, whereas melittin including a cis proline one does not. The tetramer has maximum conformation stability at around 20 °C, and cooperativity of the heat-denaturation is extremely low.