Deprotonated 2,3-dihydrobilindiones - Models for the chromophore of the far-red-absorbing form of phytochrome

Chromophore anions of synthetic 2,3-dihydrobilindiones can serve as models for the chromophore of the far-red-absorbing form of phytochrome because of the strong resemblance of their UV/Vis spectra, Significant red shifts of their long-wavelength maxima result from the elongation of the chromophores by deprotonation and the donor-acceptor reversal upon the conversion of the ring A lactam (N-->C=O) into the lactim anion (N=C <-- O-). The best resemblance was observed in the case of a 15-E configured chromophore anion. C-13 NMR spectroscopic investigations demonstrated that changes in chromophore structure are caused not only by regioselective deprotonation of the 2,3-dihydrolactam moiety of ring A, but also by the subsequent dipyrrin tautomerization within rings B and C. Generally, the chromophore structure can be interpreted in terms of an oxygen-donor/oxygen-acceptor-substituted polyene. The remarkable red shift of the long wavelength maximum results from the strong donor power of the oxygen atom on ring A, where the negative charge is localized, A simple model can be proposed for the interconversion of the structurally different chromophores of the red-absorbing form (Pr) and the far-red-absorbing form (Pfr) of phytochrome, based on oppositely charged chromophore ions of different geometry and stability: a stable 15Z-configured cation for physiologically inactive Pr and a labile 15E-configured anion for physiologically active Pfr, Interconversion requires only two types of reactions, namely Z/E photoisomerization and proton transfer.