Human Serum Albumin Dimerization Enhances the S₂ Emission of Bound Cyanine IR806

Abstract

Cyanine molecules are important phototheranostic compounds given their high fluorescence yield in the near-infrared region of the spectrum. We report on the frequency and time-resolved spectroscopy of the S 2 state of IR806, which demonstrates enhanced emission upon binding to the hydrophobic pocket of human serum albumin (HSA). From excitation−emission matrix spectra and electronic structure calculations, we identify the emission as one associated with a state having the polymethine chain twisted out of plane by 103°. In addition, we find that this configuration is significantly stabilized as the concentration of HSA increases. Spectroscopic changes associated with the S 1 and S 2 states of IR806 as a function of HSA concentration, as well as anisotropy measurements, confirm the formation of HSA dimers at concentrations greater than 10 μM. These findings imply that the longer-lived S 2 state configuration can lead to more efficient phototherapy agents, and cyanine S 2 spectroscopy may be a useful tool to determine the oligomerization state of HSA.

Explanation of inconsistencies in the determination of human serum albumin thermal stability

“…This fact indicates the presence of intermolecular interactions, probably due to the formation of dimers with a stabilizing effect on the interacting domains. This is consistent with the tendency of HSA to form reversible dimers via hydrophobic interactions [54]. The formation of such dimers can be induced by various factors, including a high temperature [55][56][57].…”