Energetics of cyclodextrin-induced dissociation of insulin

The energetics of dissociation of bovine insulin oligomers in aqueous solution under various conditions have been investigated by dilution microcalorimetry. Addition of cyclodextrins increases dissociation of insulin oligomers in solution in a manner consistent with interaction of these cyclic polysaccharides with protein side chains. For example, assuming monomer-dimer equilibrium, in the absence of cyclodextrins dilution data (25 degrees C, pH 2.5) are consistent with a dimer dissociation constant (K-diss) of about 12 mu M and a dimer 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 approximate to 1.7 mM). Qualitatively similar results are observed with alpha-cyclodextrin and other beta-cyclodextrin derivatives.