Design, synthesis and photophysical properties of C60-modified proteins

Abstract

Fullerene–porphyrin conjugates coordinated with Fe and Zn ions (1·Fe and 1·Zn) were designed and synthesized. Compounds 1·Fe and 1·Zn were reconstituted into apomyoglobin successfully to produce C60-modified myoglobins, 1·Fe-Mb and 1·Zn-Mb, respectively. The axial-ligand exchange reaction revealed that 1·Fe-Mb maintains the intrinsic properties of native Mb, except for the autooxidation rate constant, suggesting similar microenvironment of the porphyrin moieties in the proteins. Cyclic voltammogram (CV) of a graphite electrode modified with a film of 1·Fe-Mb–didodecyldimethylammonium bromide (DDAB) showed two reversible redox couples with E0′ = −206 and −1048 mV which are attributable to Fe2+/3+ and the reduction of the porphyrin ring, respectively. Differential pulse voltammogram of an electrode modified with a 1·Fe-Mb–tridodecylmethylammonium bromide (TDAB) film in water containing 0.5 M tetraethylammonium chloride and 10 mM 2,2′,2″-nitrilotriethanol showed three cathodic peaks at E½,1 = −372, E½,2 = −555, and E½,3 = −1028 mV which are attributable to Fe2+/3+, C600/1−, and the reduction of the porphyrin ring, respectively. The electrodes modified with 1·Fe-Mb and 1·Zn-Mb gave anodic photocurrent coupled with on–off light irradiation. The action spectrum of photocurrent for a 1·Zn-Mb–DDAB film was in accord with the UV–vis absorption spectrum of 1·Zn-Mb. Transient absorption spectra of 1·Zn in benzonitrile and 1·Zn-Mb in a 50 mM phosphate buffer at 100 ns after the ns-laser light pulse irradiation at 532 nm showed three absorption maxima at 700, 830, and 1000 nm which are assignable to the triplet excited state of C60 (3C60*), the triplet excited state of the zinc porphyrin (3ZnP*), and C60 radical anion (C60˙−), respectively. Existence of the apparent C60˙− indicates the generation of charge-separation state, ZnP˙+–C60˙−. The rate constants for the generation of the charge separation states in 1·Zn and 1·Zn-Mb calculated from the fluorescence data were 8.2 × 108 s−1 and 6.4 × 108 s−1, respectively, and the corresponding quantum yields were 0.62 and 0.59.