Energetics of Cyclodextrin-Induced Dissociation of Insulin

Abstract

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 °C, pH 2.5) are consistent with a dimer dissociation constant (Kdiss of about 12 μM and a dimer dissociation enthalpy (ΔHdiss) of +41 kJ mol−1. Addition of methyl-β-cyclodextrin (up to 200 mM) makes dissociation significantly more endothermic (ΔHdiss = 79 kJ mol−1) and reduces the apparent dimer dissociation constant by more than two orders of magnitude (Kdiss≈1.7 mM). Qualitatively similar results are observed with α-cyclodextrin and other β-cyclodextrin derivatives.