PHYTOCHROME MODELS .11. PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYCOCYANOBILIN DIMETHYL ESTER

Phycocyanobilin dimethyl ester (3) undergoes a self-sensitized oxidation by a singlet oxygen [O2(1-DELTA-g)] mechanism with a quantum yield of 10(-5)-10(-6), depending on the solvent, excitation wavelength, and concentration. The main photooxidation products are a tripyrrolic aldehyde (7) and three stereoisomeric peroxides (8-10) formed from two units of 3. Near-IR emission of O2(1-DELTA-g)-formed upon sensitization by 3 under both steady-state irradiation and pulsed excitation-constitutes the first direct evidence for the intersystem crossing of a bilatriene. The low photooxidation yield results from a low intersystem crossing yield from 3, PHI-isc < 10(-4), estimated by the energy-transfer method in flash photolysis. A total rate constant of k(q) = (3.4 +/- 0.3) X 10(8) M-1 s-1 was determined for the quenching of O2(1-DELTA-g) by 3. Possible relations between these findings and the photophobic movement of Anabaena variabilis are discussed. In analogy to biliverdin dimethyl ester, two types of conformations (a helical form and a family of stretched forms) for 3 [and for its 3,3'-dihydro-3'-thioethoxy derivatives (3'S*)-5] were detected in methanol at room temperature by stationary fluorescence and excitation spectra. The wavelength dependence of the self-sensitized photooxidation of 3 is due to the selective reaction of the helical conformer, while a selective photoisomerization of the stretched forms of 3 and 5 was observed by time-resolved laser-induced optoacoustic spectroscopy. For 3 the latter process takes place with a quantum yield > 0.5, and the resulting isomer (probably a C-10 stereoisomer of the parent compound) reverts to the ground state with a lifetime of around 200 ns.