Destabilization of human serum albumin by polyethylene glycols studied by thermodynamical equilibrium and kinetic approaches

Both polyethylene glycols (PEG) of M w 8,000 and that of 10 000 stabilize the native compact state of human albumin showing negative preferential interaction with the protein. Interaction between these polymers and the protein is thermodynamically unfavorable, and becomes even more unfavorable for denatured protein whose surface areas are larger than those of native ones. PEG of low M w 1000 and 4000 did not show steric exclusion, interacting favorably with hydrophobic side chains made available when the protein was unfolded and leading to a stabilization of the unfolded state, which is manifested as a lowering of the thermal transition temperature. Perturbation of the absorption spectrum of albumin by PEGs confirms that at high temperature the polymers preferentially interact with the denatured state of albumin, but is excluded from the native state at low temperature. This observation is consistent with the fact that PEG is hydrophobic in nature and may interact favorably with the hydrophobic side chain exposed upon unfolding. The lower activation energy for thermal unfolding in the presence of PEG 1000 is in favour of preferential interaction of this polymer with human albumin. PEG of low M w favours the ionization of the tyrosine residues of albumin. It is apparent that pKa decreased with the increase in M w of synthetic polymer. Such variation might be a consequence of the change in dielectric constant at the domain of the protein by PEG binding.