Energetics of cyclodextrin-induced dissociation of insulin.

The energetics of dissociation of bovine insulin in aqueous solution have been investigated by sensitive dilution microcalorimetry. Cyclodextrins increase dissociation of insulin oligomers in a manner consistent with their interaction with protein side chains. For example, assuming monomer-dimer equilibrium, in the absence of cyclodextrins the calorimetric dilution data (25 degrees C, pH 2.5) indicate a dimer dissociation constant (K(diss)) of about 12 microM and an endothermic dissociation enthalpy (delta H(diss) of +41 kJ mol-1. Addition of methyl-beta-cyclodextrin (up to 200 mM) makes dissociation significantly more endothermic (delta H(diss) = 79 kJ mol-1) and reduces the apparent dimer dissociation constant by more than two orders of magnitude (K(diss) approximately 1.7 mM). Qualitatively similar results are observed with alpha-cyclodextrin and other beta-cyclodextrin derivatives. Cyclodextrin-induced insulin dissociation is also observed at pH 7.4.