Protein-encapsulated chlorophyll a molecules for biological solar cells

This paper reports a novel design of an artificial pigment-protein complex, created by supramolecular binding of chlorophyll a (Chl a ) in the hydrophobic cavity of s ingle- r ing mutant of bacterial GroEL (SR). The results show that Chl a molecules assembled in SR nano-ring retain their native structure, and an increase of loading amount (up to ~26 pigment molecules per protein nano-ring) with the equilibrium concentration of Chl a upon dialysis has been observed. It is further seen that the nanoscale organization of Chl a in SR leads to a significant quenching of Chl a fluorescence, especially at high loadings. Moreover, the photostability of Chl a encapsulated in SR is significantly enhanced in comparison with free Chl a , as evidenced by an up to 6-fold increase of its half-life time. Both the interaction of Chl a with SR and the interaction between Chl a molecules contribute to photostability. Finally, it is shown that the Chl a -SR complex has good photoreduction activity as well as photoelectrochemical activity under visible light illumination. The maximum photoelectric conversion efficiency of the Chl a -SR-sensitized solar cells is around 1.09%. The strategy presented herein is believed to be instrumental in the design of effective photoenergy conversion materials.