ENERGY TRANSFER AMONG THE CHROMOPHORES OF C-PHYCOCYANIN FROM Anabaena variabilis USING STEADY STATE AND TIME-RESOLVED FLUORESCENCE SPECTROSCOPY

Abstract We have investigated the model of energy transfer between sensitizing (s) and fluorescing (f) chromophores for the αβ monomer and for the separated α and β subunits of C-phycocyanin from Anabaena variabilis using fluorescence emission spectroscopy, fluorescence excitation polarization, and picosecond-resolved fluorescence decay kinetics. The fluorescence emission maximum occurs at 640 nm for all samples. The fluorescence excitation polarization is constant ( P = 0.40) across the absorption hand for the α subunit, but it increases across the absorption band towards longer wavelength for the β subunit and the αβ monomer. The fluorescence decay kinetics exhibit two exponential lifetimes of 1.3-1.5 ns and 340-500 ps for the αβ monomer and for the α and β subunit preparations.
We attribute the change in polarization across the absorption band to energy transfer among the three chromophores in the αβ monomer and among the two chromophores in the separated β subunit. The constant, relatively high polarization in the separated a subunit, having only one chromophore, is consistent with the absence of both energy transfer and chromophore rotation. The concentration of the α subunit did not affect the decay kinetics, suggesting that the short lifetime component does not arise from aggregation of the α subunits. The biexponential decay kinetics of the α subunit cannot be explained using the sensitizing-fluorescing model. The possibility of conformational interactions is under investigation.