Antibodies against beta-amyloid peptides (A beta s) are considered an important therapeutic opportunity in Alzheimer's disease. Despite the vast interest in A beta no thermodynamic data on the interaction of antibodies with A beta are available as yet. In the present study we use isothermal titration calorimetry (ITC) and surface plasmon resonance to provide a quantitative thermodynamic analysis of the interaction between soluble monomeric A beta(1-40) and mouse monoclonal antibodies (mAb). Using four different antibodies directed against the N-terminal, middle, and C-terminal A beta epitopes, we measured the thermodynamic parameters for the binding to A beta. Each antibody species was found to have two independent and equal binding sites for A beta with binding constants in the range of 10(7) to 10(8) M-1. The binding reaction was essentially enthalpy driven with a reaction enthalpy of Delta H-A beta(0) approximate to -19 to -8 kcal/mol, indicating the formation of tight complexes. The loss in conformational freedom was supported by negative values for the reaction entropy Delta S-A beta(0). We also measured the heat capacity change of the 1mAb:2A beta reaction. Delta C-p,A beta(0) 0 was large and negative but could not be explained exclusively by the hydrophobic effect. The free energy of binding was found to be linearly correlated with the size of the epitope.