Photophysical, rotational and translational properties of Radachlorin photosensitizer upon binding to serum albumins.

Introduction: Although photophysical properties of Radachlorin photosensitizer (PS) were extensively studied in solutions and cells, no data is available on variations of its characteristics upon binding to serum albumins, which are major transporters in blood and nutrients in cell culture media.

Objectives: The primary objective of this study was to analyze changes in photophysical properties of Radachlorin molecules upon their binding to human and bovine serum albumins at different microenvironment properties.

Methods: Experiments were performed using time-resolved fluorescence spectroscopy and fluorescence recovery after photobleaching. Variations in fluorescence spectra and lifetime, fluorescence anisotropy, rotational and translational diffusion of PS molecules upon binding to albumins were studied in normal, basic and acidic conditions and at different concentrations of albumin and PS molecules.

Results: Radachlorin molecules effectively bind to both types of serum albumins, which causes changes in photophysical properties of the PS. A minor red shift of the fluorescence spectrum, an increase in fluorescence lifetime and anisotropy and substantial decrease of translational and rotational mobility of PS molecules were observed upon their binding to albumins. The analysis of rotational diffusion time provided robust evaluation of the bound fraction of PS molecules. Both the highly acidic microenvironment and increase in alcohol concentration above 40% resulted in detachment of PS molecules from albumins. Photophysical properties of Radachlorin in complexes with BSA and HSA were found to be slightly different.

Conclusions: Binding of Radachlorin photosensitizer to either BSA or HSA affects significantly its photophysical properties, which may also vary with microenvironment acidity and alcohol concentration.