Unfolding of acrylodan-labeled human serum albumin probed by steady- state and time-resolved fluorescence methods

Steady-state and time-resolved fluorescence spectroscopy was used to follow the local and global changes in structure and dynamics during chemical and thermal denaturation of unlabeled human serum albumin (HSA) and HSA with Cys³⁴. Acrylodan fluorescence was monitored to obtain information about unfolding processes an acrylodan moiety bound to 34 in domain I, and the emission of the Trp residue at position 214 was used to examine domain II. In addition, Trp-to-acrylodan resonance energy transfer was examined to probe interdomain spatial relationships during unfolding. Increasing the temperature to less than 50°C or adding less than 1.0 M GdHCl resulted in an initial, reversible separation of domains I and II. Denaturation by heating to 70°C or by adding 2.0 M GdHCl resulted in irreversible unfolding of domain II. Further denaturation of HSA by either method resulted in irreversible unfolding of domain I. These results clearly demonstrate that HSA unfolds by a pathway involving at least three distinct steps. The low detection limits and high information content of dual probe fluorescence should allow this technique to be used to study the unfolding behavior of entrapped or immobilized HSA.