Short chain polyethylene glycols unusually assist thermal unfolding of human serum albumin

• Short chain polyethylene glycols, PEG 200 and PEG 400 facilitate thermal unfolding of human serum albumin.
• The native state of the protein is stabilized by the usual osmolytic effect of PEGs.
• A direct interaction of the small PEGs with the unfolded protein surface makes the overall unfolding process favourable.

In the present study we have investigated the thermal stability of the globular transport protein human serum albumin (HSA), in the presence of two small chain polyethylene glycols (namely PEG 200 and PEG 400). Both near- and far-UV circular dichroism (CD) study reveal that addition of PEG moderately increases the α-helical content of the protein without abruptly changing its tertiary structure. The hydration structure at the protein surface experiences a notable change at 30% PEG (v/v) concentration as evidenced from compressibility and dynamic light scattering (DLS) measurements. Thermal denaturation of HSA in the presence of PEG has been studied by CD and fluorescence spectroscopy using the intrinsic fluorophore tryptophan and it has been found that addition of PEG makes the protein more prone towards unfolding, which is in contrary to what has been observed in case of larger molecular weight polymers. The energetics of the thermal unfolding process has been obtained using differential scanning calorimetry (DSC) measurements. Our study concludes that both the indirect excluded volume principle as well as interaction of the polymer at the protein surface is responsible for the observed change of the unfolding process.

Short chain polyethylene glycols exhibit an unusual phenomenon of facilitating thermal unfolding of human serum albumin. The native state of the protein is stabilized by the usual osmolytic effect of PEGs, however, a direct interaction of the small PEGs with the unfolded protein surface makes the overall unfolding process favourable.Figure optionsDownload high-quality image (134 K)Download as PowerPoint slide