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

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 degrees. 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 mu M. 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.

Automated summary

Cyanine molecules have high fluorescence yield in the near-infrared region and show enhanced emission upon binding to human serum albumin (HSA). Spectroscopic analysis and calculations reveal that the enhanced emission is associated with a twisted polymethine chain configuration. Moreover, this configuration is more stable at higher HSA concentrations. The formation of HSA dimers at concentrations above 10 μM is confirmed by spectroscopic changes and anisotropy measurements. These findings suggest that the longer-lived S-2 state configuration can be utilized for more efficient phototherapy agents, and cyanine S-2 spectroscopy can be a useful tool to determine the oligomerization state of HSA.