Micelles of Nonionic Surfactants Resist the Cold Denaturation of Apo-Myoglobin

Proteins undergo denaturation upon cooling, which is favored by the hydration of hydrophobic residues at low temperatures. Nonionic surfactants, forming hydrophobic interactions with proteins, might alter their cold denaturation process. The present work investigates the effect of three nonionic surfactants, tween20, tween40, and tween80, on the cold-induced denaturation of apo-myoglobin (apoMb). The surfactants increase the helicity of the protein at monomeric concentrations. The helicity is lost with the addition of micellar concentrations of the surfactants at room and higher temperatures, whereas the micelles do not affect the secondary structure at subzero temperature (268 K). Thermal denaturation studies reveal that the surfactants at their monomeric concentrations do not alter the stability of apoMb during heat denaturation, but marginally destabilize the protein during cold denaturation. The micellar concentrations of the surfactants increase the stability of the protein against heat denaturation. During cooling, the protein is initially partially unfolded; however, the micelles later resist further denaturation. The temperature at which protein unfolding is inhibited is referred to as the resistance temperature (T_res), a distinct behavior so far observed with the surfactants alone. The inhibition of unfolding could be observed even in the presence of a mild denaturant, 1 M urea, though the resistance temperature is shifted to lower temperatures. Thus, the study suggests that nonionic surfactants could be used to prevent the cold-induced denaturation of proteins.