To determine thermodynamic parameters for the SIM/SUMO interaction, we studied the temperature dependence of the NMR-derived exchange parameters for the parent peptide, along with those for a strong and a weak binding N-Me variant -Me-Ile2712 and Me-Lys2716, respectively (Figs. S7, S8, S13). Van't Hoff analysis of the calculated binding affinities revealed SIM binding to be an entropically-driven process (Table and Fig. ). In comparison to the parent SIM peptide, the entropy of Me-Ile2712 binding at 25 °C (TΔSbind) increased by ≈15 kJ/mol, which was only partially compensated by ≈12 kJ/mol increase in enthalpy (ΔHbind) (Fig. , Table ), leading to a more favourable free energy of binding (ΔGbind). Thus, the increased affinity for Me-Ile2712, relative to the parent, is entropy driven. For Me-Lys2716, both ΔHbind and TΔSbind were diminished, resulting in a smaller ΔGbind and a weak affinity.