chlorophyll-a and tetraphenylporphine

Photochemical activity as well as some spectral properties of various porphyrins and their model complexes with proteinoids were studied. Photochemical activity increased from the less advanced biosynthetic chlorophyll precursors to the more advanced compounds. The significant increase was detected as a result of the formation of complexes of phytol-containing porphyrins with proteinoids, which brings about the disaggregation of the pigment clusters formed in the aqueous environment. Hemin is photochemically inactive, either alone or in complex with proteinoid. Although hemoproteinoid was demonstrated to be able to catalyse photochemical electron transfer, its activity is about 20 times lower when compared with the chlorophyll a-proteinoid complex. Experimental results are discussed in context of the early evolution of photosynthesis.

Topics: Chlorophyll; Chlorophyllides; Hemin; Models, Biological; Pheophytins; Photochemistry; Porphyrins; Protoporphyrins; Spectrophotometry, Atomic; Spectrum Analysis; Uroporphyrins

This paper discusses the mechanism of photooxygenation reactions sensitized by dyes, such as: rose bengal, methylene blue, tetraphenylporphin and chlorophyll a. First it is shown more particularly that: the quantum yields of singlet oxygen O2(1 delta g) production gamma delta, and of intersystem crossing singlet S1-triplet T1, gamma is, are not always equal and that the possibilities gamma delta greater than or less than gamma is may occur or are observed; the processes S1 + O2(3 sigma)----T1 + O2(3 sigma) and T1 + O2(3 sigma)----S0 + O2(1 delta g) are mainly if not fully responsible for the quenching of the singlet and triplet excited states of the sensitizer by oxygen. Thereafter, clear indication is given of the considerable complication of the photooxygenation which may arise from particular properties of the investigated substrate A (or of other compounds present in the reaction medium) and which may result in a decrease of the oxygenation quantum yield phi O2. It is shown that this lowering of phi O2 is due to that of gamma delta and/or of phi A, (the probability that O2(1 delta g) yields an oxygenation product AO2) since phi O2 = gamma delta phi A. The latter effect can be induced by the quenching of singlet oxygen by the dye, a process which is quite general and which must be always taken into account in kinetic studies.

Topics: Chlorophyll; Chlorophyll A; Coloring Agents; Eosine Yellowish-(YS); Fluorescence; Kinetics; Light; Methylene Blue; Oxygen; Photochemistry; Porphyrins; Probability; Rose Bengal; Singlet Oxygen; Thermodynamics