The influence of the binding of low molecular weight surfactants on the thermal stability and secondary structure of IgG

The effect of low molecular weight surfactants on the thermal stability of immunoglobulin G is studied by differential scanning calorimetry. The corresponding change in the secondary structure is investigated using circular dichroism spectroscopy and the rate of aggregate formation, both in the presence and absence of surfactant, is monitored by dynamic light scattering. At low surfactant concentrations (SDS/Tween 20 mixture) the thermal stability of the protein was not affected. With increasing surfactant concentration the protein structure is perturbed, most probably due to hydrophobic interaction with the surfactant, leading to a lower thermal stability. At even higher concentrations the surfactant molecules encapsulate the protein molecules, so that the unfolded state is strongly suppressed due to restricted conformational freedom in a confined volume. Interaction with the surfactant mixture at intermediate concentration influences the secondary structure of IgG strongly, i.e. α-helix and random coil conformations are promoted and the amounts of β-sheets and β-turns are reduced.