The effects of temperature on streptavidin-biotin binding using affinity isothermal titration calorimetry

An entropically-driven binding interaction at a certain temperature may change to an enthalpically-driven process at another temperature, depending on the polarization state of the groups that are involved in binding. The streptavidin-biotin complex has been extensively studied across biological, medical, chemical and material science fields using various techniques, however, not much has been reported on this interaction across a broad temperature range, between 2 °C and 40 °C using biophysical techniques. In this study, we determined how the forces involved in the streptavidin-biotin complex formation are affected by the reaction temperature using the Affinity ITC (TA Instruments). We observed that this complex formation is a spontaneous binding process, indicated by a negative Gibbs energy (ΔG) at all temperatures tested. The observed negative heat capacity (ΔCp) ~ −459.9 cal/mol K highlights the polar solvation of the interaction that corresponds to a decreasing enthalpy (more negative) (ΔH) with increasing reaction temperature. The stoichiometry (n) of 0.98 was estimated at 25 °C. An increase in reaction temperature resulted in an almost two-fold increase or more in n, notably from 1.59 to 3.41 between 30 °C and 40 °C. Whereas, at lower reaction temperatures, 2 °C to 10 °C, higher molar binding ratios were recorded, i.e. 2.74 to 5.76. We report an enthalpically-driven interaction between 30 °C and 40 °C whereas, an entropically-favourable interaction is observed at lower temperatures, suggestive of an interaction dominated by nonpolar interactions at lower temperatures and polar interactions at higher temperatures. Consequently, alterations in the polarisation state of streptavidin result in moderate binding affinity of biotin to streptavidin at higher reaction temperatures, K D 10-4 ≤ 10-5 M.