Effect of cadmium sulfide quantum dots on physical properties of R-phycoerythrin as a protein matrix

The synthesis and analysis of nanostructures in the cavities of protein molecules is a promising research field in the industry of nanosystems. In this study, atomic force microscopy (AFM) has been used to evaluate the properties of CdS quantum dots synthesized in the tunnel cavities of R-phycoerythrin, a 290 kDa water-soluble pigment protein responsible for light harvesting in red algae. It has been shown that R-phycoerythrin dissolved in deionized water to a concentration of 50 μg/ml is prone to self-organization into regular spatial structures upon adsorption on the surface of mica, but no such structuring takes place in films prepared from R-phycoerythrin solutions diluted tenfold. In the latter case, protein molecules are deformed, as judged from the analysis of the surface profile. R-phycoerythrin with CdS quantum dots in protein cavities (the concentration of the preparation was (48 μg/ml) loses the self-organization ability and is not deformed upon adsorption on the mica surface. Analysis of AFM images by flicker-noise spectroscopy has shown that incorporation of CdS quantum dots into R-phycoerythrin molecules provides for "smoothing" of the protein surface, with various irregularities being leveled off. Conversely, the irregularity of the protein surface increases when R-phycoerythrin molecules are arranged into three-dimensional branching structures. It is concluded that CdS quantum dots interfere with protein-protein interactions and restrain the conformational mobility of the protein. The anomalously rigid structure of R-phycoerythrin in the presence of CdS is due to its conformational rearrangements during the synthesis of quantum dot.