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∆_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 stereoisomers peroxides (8–10) formed from two units of 3. Near-IR emission of O₂(¹∆_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, Φ_isc < 10⁻⁴, estimated by the energy-transfer method in flash photolysis. A total rate constant of k_q = (3.4 ± 0.3) × 10⁸ M⁻¹ s⁻¹ was determined for the quenching of O₂(¹∆_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.