Phytochrome Models. 11. Photophysics and Photochemistry of Phycocyanobilin Dimethyl Ester

Silvia E. Braslavsky, Dagmar Schneider, Klaus Heihoff, Santiago Nonell, Pedro F. Aramendia, Kurt Schaffner

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Phycocyanobilin dimethyl ester (3) undergoes a self-sensitized oxidation by a singlet oxygen [O2(1g)] 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(1g)]-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, Φisc < 10−4, estimated by the energy-transfer method in flash photolysis. A total rate constant of kq = (3.4 ± 0.3) × 108 M−1 s−1 was determined for the quenching of O2(1g) 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.

Idioma originalAnglès
Pàgines (de-a)7322-7334
Nombre de pàgines13
RevistaJournal of the American Chemical Society
Estat de la publicacióPublicada - 1 de set. 1991
Publicat externament


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