Mimicking the photosynthetic triplet energy-transfer relay

Journal Article · Wed Jun 30 00:00:00 EDT 1993 · Journal of the American Chemical Society; (United States)

DOI:https://doi.org/10.1021/ja00066a038· OSTI ID:5747563

Gust, D; Moore, T A; Moore, A L; Krasnovsky, Jr, A A; Liddell, P A; Nicodem, D; DeGraziano, J M; Kerrigan, P; Makings, L R; Pessiki, P J ^[1]

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1. Arizona State Univ., Tempe (United States)

In the reaction centers of photosynthetic organisms, chlorophyll triplet states are sometimes formed by recombination of charge-separated intermediates. These triplets are excellent sensitizers for singlet oxygen formation. Carotenoid polyenes can provide photoprotection from singlet oxygen generation by rapidly quenching chlorophyll triplet states via triplet-triplet energy transfer. Because in bacteria the reaction center carotenoid is not located adjacent to the bacteriochlorophyll special pair, which is the origin of the charge separation, it has been postulated that quenching may occur via a triplet relay involving an intermediate chlorophyll monomer. We now report the synthesis and spectroscopic study of a covalently linked carotenoid (C)-porphyrin (P)-pyropheophorbide (Ppd) triad molecule which mimics this triplet relay. The pyropheophorbide singlet-state C-P-[sup 1]Ppd (generated by direct excitation or energy transfer from the attached porphyrin) undergoes intersystem crossing to the triplet C-P-[sup 3]Ppd. In oxygen-free solutions, this triplet decays to [sup 3]C-p-Ppd through a triplet-transfer relay involving an intermediate C-[sup 3]P-Ppd species. In aerated solutions, quenching of C-P-[sup 3]Ppd by the attached carotenoid competes with singlet oxygen sensitization and thus provides a degree of photoprotection. In a similar traid containing a zinc porphyrin moiety, triplet transfer is slow due to the higher energy of the C-[sup 3]P[sub Zn]-Ppd intermediate, and photoprotection via the relay is nonexistent. The triplet relay ceases to function at low temperatures in both the natural and biomimetic cases due to the endergonicity of the first step. 37 refs., 6 figs., 1 tab.

• DOE Contract Number: FG05-87ER75361; FG02-88ER13969
• OSTI ID: 5747563
• Journal Information: Journal of the American Chemical Society; (United States), Vol. 115:13; ISSN 0002-7863
• Country of Publication: United States
• Language: English

Related Subjects

14 SOLAR ENERGY
37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ORGANIC COMPOUNDS
ELECTRONIC STRUCTURE
PHOTOCHEMISTRY
SPECTROSCOPY
CAROTENOIDS
CHLOROPHYLL
MASS SPECTROSCOPY
NUCLEAR MAGNETIC RESONANCE
OXYGEN
PHOTOSYNTHESIS
ZINC COMPLEXES
CARBOXYLIC ACIDS
CHEMICAL REACTIONS
CHEMISTRY
COMPLEXES
ELEMENTS
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
HYDROCARBONS
MAGNETIC RESONANCE
NONMETALS
ORGANIC ACIDS
ORGANIC NITROGEN COMPOUNDS
PHOTOCHEMICAL REACTIONS
PHYTOCHROMES
PIGMENTS
PORPHYRINS
PROTEINS
RESONANCE
SYNTHESIS
TERPENES
140505* - Solar Energy Conversion- Photochemical
Photobiological
& Thermochemical Conversion- (1980-)
400102 - Chemical & Spectral Procedures
400500 - Photochemistry