The binding of four epitope-related peptides and three library-derived, epitope-unrelated peptides of different lengths (10-14 amino acids) and sequence by anti-p24 (HIV-1) monoclonal antibody CB4-1 and its Fab fragment was studied by isothermal titration calorimetry. The binding constants K(A) at 25 degrees C vary between 5.1 x 10(7) M (-1) for the strongest and 1.4 x 10(5) M (-1) for the weakest binder. For each of the peptides complex formation is enthalpically driven and connected with unfavorable entropic contributions; however, the ratio of enthalpy and entropy contributions to deltaG(0) differs markedly for the individual peptides. A plot of -deltaH(0) vs -TdeltaS(0) shows a linear correlation of the data for a wide variety of experimental conditions as expected for a process with deltaC(p) much larger than deltaS(0). The dissimilarity of deltaC(p) and deltaS(0) also explains why deltaH(0) and TdeltaS(0) show similar temperature dependences resulting in relatively small changes of deltaG(0) with temperature. The heat capacity changes deltaC(p) upon antibody-peptide complex formation determined for three selected peptides vary only in a small range, indicating basic thermodynamic similarity despite different key residues interacting in the complexes. Furthermore, the comparison of van't Hoff and calorimetric enthalpies point to a non-two-state binding mechanism. Protonation effects were excluded by measurements in buffers of different ionization enthalpies. Differences in the solution conformation of the peptides as demonstrated by circular dichroic measurements do not explain different binding affinities of the peptides; specifically a high helix content in solution is not essential for high binding affinity despite the helical epitope conformation in the crystal structure of p24.