THE SPECTROSCOPIC AND PHOTOCHEMICAL PROPERTIES OF LOCKED-15,15'-CIS-SPHEROIDENE IN SOLUTION AND INCORPORATED INTO THE REACTION-CENTER OF RHODOBACTER-SPHAEROIDES R-26.1

The spectroscopic and photochemical properties of the synthetic carote noid, locked-15,15'-cis-spheroidene, were studied by absorption, fluor escence, circular dichroism, fast transient absorption and electron sp in resonance spectroscopies in solution and after incorporation into t he reaction center of Rhodobacter (Rb.) sphaeroides R-26.1. HPLC purif ication of the synthetic molecule reveals the presence of several di-c is geometric isomers in addition to0 the mono-cia isomer of locked-15, 15'-cis-spheroidene. In solution, the absorption spectrum of the purif ied mono-cis sample was red-shifted and showed a large cis-peak at 351 nm compared to unlocked all-trans spheroidene. Molecular modeling and semi-empirical calculations reveal how geometric isomerization and st ructural factors affect the room temperature spectra. The spectroscopi c studies of the purified locked-15,15'-mono-cis molecule in solution reveal a more stable manifold of excited states compared to the unlock ed spheroidene. Reaction centers of Rb. sphaeroides R-26.1 in which th e locked-15,15'-cis-spheroidene was incorporated show no difference in either the spectroscopic properties or photochemistry compared to rea ction centers in which unlocked spheroidene was incorporated or to Rb. sphaeroides wild type strain 2.4.1 reaction centers which naturally c ontain spheroidene. The data suggest that the natural selection of a c is-isomer of spheroidene for incorporation into native reaction center s of Rb. sphaeroides wild type strain 2.4.1 is more determined by the structure or assembly of the reaction center protein than by any speci al quality of the cis-isomer of the carotenoid that would affect its a bility to participate in triplet energy transfer or carry out photopro tection.