PubMed · 14374044
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H NIELSEN. 1955-02-10. Chlorophyll.. https://pubmed.ncbi.nlm.nih.gov/14374044/
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The effects of dibromothymoquinone (DBMIB) and methylviologen (MV) on the Chl a fluorescence induction transient (OJIP) were studied in vivo. Simultaneously measured 820-nm transmission kinetics were used to monitor electron flow through photosystem I (PSI). DBMIB inhibits the reoxidation of plastoquinol by binding to the cytochrome b(6)/f complex. MV accepts electrons from the FeS clusters of PSI and it allows electrons to bypass the block that is transiently imposed by ferredoxin-NADP(+)-reductase (FNR) (inactive in dark-adapted leaves). We show that the IP phase of the OJIP transient disappears in the presence of DBMIB without affecting F(m). MV suppresses the IP phase by lowering the P level compared to untreated leaves. These observations indicate that PSI activity plays an important role in the kinetics of the OJIP transient. Two requirements for the IP phase are electron transfer beyond the cytochrome b(6)/f complex (blocked by DBMIB) and a transient block at the acceptor side of PSI (bypassed by MV). It is also observed that in leaves, just like in thylakoid membranes, DBMIB can bypass its own block at the cytochrome b(6)/f complex and donate electrons directly to PC(+) and P700(+) with a donation time tau of 4.3 s. Further, alternative explanations of the IP phase that have been proposed in the literature are discussed.
Electron transfer (ET) processes in reaction centers (RC) of photosystem II (PSII) are prerequisites of oxygen generation. They are promoted by energy transfer from antenna to RC. Here, we calculated the redox potentials of chlorophylla/beta-carotene (Chla/Car) in PSII CP43/CP47 antenna complexes, solving the linearized Poisson-Boltzmann (LPB) equation based on the PSII crystal structure. The majority of antenna Chla redox potentials for reduction/oxidation were lower than those of RC Chla. Hence, ET events with excess electrons remain localized in the RC. Simultaneously antenna Chla can serve as an efficient cation sink to rereduce RC Chla if normal PSII function is inhibited. Especially three antenna Chla (Chl-47, Chl-18, and Chl-12) and two Car bridging the space between Chl(Z(D1)) and cytochrome (cyt) b559 have the same level of oxidation redox potential. Together with Chl(Z(D2)) they form an electron hole transfer pathway and temporary storage device guiding from the oxidized P680(+.) Chla to the cyt b559. This path may play a photoprotective role as efficient electron hole quencher.
Porphyrins and their metal derivatives are strong protein binders. Some of these compounds have been used for radiation sensitization therapy of cancer and are targeted to interact with cellular DNA and protein. The presence of several high-affinity binding sites on human serum albumin (HSA) makes it possible target for many organic and inorganic molecules. Chlorophyll a and chlorophyllin (a food-grade derivative of chlorophyll), the ubiquitous green plant pigment widely consumed by humans, are potent inhibitors of experimental carcinogenesis and interact with protein and DNA in many ways. This study was designed to examine the interaction of HSA with chlorophyll (Chl) and chlorophyllin (Chln) in aqueous solution at physiological conditions. Fourier transform infrared, UV-visible, and CD spectroscopic methods were used to determine the pigment binding mode, the binding constant, and the effects of porphyrin complexation on protein secondary structure. Spectroscopic results showed that chlorophyll and chlorophyllin are located along the polypeptide chains with no specific interaction. Stronger protein association was observed for Chl than for Chln, with overall binding constants of K(Chl) = 2.9 x 10(4)M(-1) and K(Chln) = 7.0 x 10(3)M(-1). The protein conformation was altered (infrared data) with reduction of alpha-helix from 55% (free HSA) to 41-40% and increase of beta-structure from 22% (free HSA) to 29-35% in the pigment-protein complexes. Using the CDSSTR program (CD data) also showed major reduction of alpha-helix from 66% (free HSA) to 58 and 55% upon complexation with Chl and Chln, respectively.