FLUORESCENCE QUENCHING OF ALLOPHYCOCYANIN AND PHYTOCHROME BY ESTRIOL*

Abstract— At 293 K the long-wavelength absorption and emission band of 1.4 μM allophycocyanin is decreased by estriol (Δ¹-³-⁵⁽¹⁰⁾-estratriene-3,16α, 17β-trio!) in the range 0.8-6.6 μM in the presence of 11% alcohol (vol/vol). The binding of estriol is shown to be of high affinity, 1:1 with allophycocyanin. The free energy of this binding process (ΔG^°) is -33.6 kJ mol' and single binding site dissociation constant (K_D) 1.0 ×10–6M. Estriol at 21 μM effectively quenches the fluorescence of 1.4 M large molecular weight phytochrome in its red absorbing form at 77 K while having little or no effect on the phototransformation difference spectrum at 293 K.