Conformation and aggregation of melittin: dependence of pH and concentration

Melittin, a 26-residue peptide from bee venom, is transformed from a largely random to a largely alpha-helical conformation at elevated pH. At 3 x 10(-5) M melittin, circular dichroism spectra show a transition with a pK near 9.6. At 8 x 10(-5) M, two approximately equal transitions occur with pKs at 7.2 and 9.6. At 6 x 10(-4) M, a single transition is seen with a pK of 6.8, followed by a more gradual increase to at least pH 11. The transitions near pH 7 presumably arise from deprotonation of the alpha-amino group. When the amino groups are acetylated or succinylated, a 60% alpha-helical conformation is adopted at neutral or low pH. The acylated melittins form more stable oligomers than does native melittin.