Conformational studies of aqueous melittin: thermodynamic parameters of the monomer-tetramer self-association reaction

Abstract

The self-association reaction in which 4 melittin molecules associated to form an aqueously soluble tetramer was studied fluorescent spectroscopy. At 23.degree. C , pH 7.15, .GAMMA./2 0.50, Kd was 3.20 .times. 10-16 M3. At 23.degree. C, .GAMMA./2 0.60, melittin had an amino acyl group with a proton Ki at .apprx. 10-6 M, which was un-ionized for tetramer formation to occur. The change in Kd with temperature indicated the forward reaction (tetramer formation) proceeded primarily by entropic changes, with .DELTA.H.degree. = -20.3 kJ/mol of monomer and .DELTA.S.degree. = 211 J/(K .cntdot. mol monomer). The observed enthalpic and entropic values for the tetramerization reaction were consistent with the expected contributions of nascent H bonds and hydrophobic stabilization to the reaction. The ionic strength dependence of the tetramerization reaction was consistent with an Edsall-Wyman treatment of activity coefficients. The calculated charge of melittin varied from 2.5 (pH 10.53, .GAMMA./2 < 0.08)-.apprx. 6 (pH 7.15, .GAMMA./2 > 0.3) and showed a strong dependence on .GAMMA./2.