Design, synthesis and photophysical properties of C₆₀-modified proteins

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 Fe^2+/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 E_1/2.1 = -372, E_1/2.2 = -555, and E_1/2.3 = -1028 mV which are attributable to Fe^2+/3+, C₆₀^0/1-, 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.