title: The photophysics of monomeric bacteriochlorophylls c and d and their derivatives:  properties of the triplet state and singlet oxygen photogeneration and quenching.
abstract: Measurements of pigment triplet-triplet absorption, pigment phosphorescence and photosensitized singlet oxygen luminescence were carried out on solutions containing monomeric bacteriochlorophylls (Bchl) c and d, isolated from green photosynthetic bacteria, and their magnesium-free and farnesyl-free analogs.  The energies of the pigment triplet states fell in the range 1.29-1.34 eV.  The triplet lifetimes in aerobic solutions were 200-250 ns; they increased to 280 +/- 70 microseconds after nitrogen purging in liquid solutions and to 0.7-2.1 ms in a solid matrix at ambient or liquid nitrogen temperatures.  Rate constants for quenching of the pigment triplet state by oxygen were (2.0-2.5) x 10(9) M-1 s-1, which is close to 1/9 of the rate constant for diffusion-controlled reactions.  This quenching was accompanied by singlet oxygen formation.  The quantum yields for the triplet state formation and singlet oxygen production were 55-75% in air-saturated solutions.  Singlet oxygen quenching by ground-state pigment molecules was observed.  Quenching was the most efficient for magnesium-containing pigments, kq = (0.31-1.2) x 10(9) M-1 s-1.  It is caused mainly by a physical process of singlet oxygen (1O2) deactivation.  Thus, Bchl c and d and their derivatives, as well as chlorophyll and Bchl a, combine a high efficiency of singlet oxygen production with the ability to protect photochemical and photobiological systems against damage by singlet oxygen.
authors.affiliation: Department of Chemistry and Biochemistry, Arizona State University, Tempe 85287-1604.
authors.forename: A A
authors.initials: AA
authors.lastname: Krasnovsky
authors.forename: P
authors.initials: P
authors.lastname: Cheng
authors.forename: R E
authors.initials: RE
authors.lastname: Blankenship
authors.forename: T A
authors.initials: TA
authors.lastname: Moore
authors.forename: D
authors.initials: D
authors.lastname: Gust
mesh_terms: D001419:Bacteria; D001429:Bacteriochlorophylls / Q000737:chemistry / Q000378:metabolism* / Q000528:radiation effects; D055592:Biophysical Phenomena; D001703:Biophysics; D008027:Light; D008163:Luminescent Measurements; D010100:Oxygen / Q000032:analysis / Q000378:metabolism*; D018462:Photobiology; D013054:Spectrophotometry, Atomic
chemical_list: D001429:Bacteriochlorophylls; D010100:Oxygen