Chlorophyll rate constants

TABLE VI Spectroscopic Data, Quantum Yields, and Rate Constants Chlorophyll a in Methanol at 20°C (from ref. 86) ... 

A series of chlorophyll-like donor (a chlorin) linked having C60 (chlorin-C60) or porphyrin-C60 dyads with the same short spacer have been synthesized as shown in Schemes 13.1 and 13.2 [39, 40]. The photoinduced electron-transfer dynamics have been reported [39, 40]. A deoxygenated PhCN solution containing ZnCh-C60 gives rise upon a 388-nm laser pulse to a transient absorption maximum at 460 nm due to the singlet excited state of ZnCh [39]. The decay rate constant was determined as 1.0 X 10u s-1, which agrees with the value determined from fluorescence lifetime measurements [39]. This indicates that electron transfer from 1ZnCh to C60 occurs rapidly to form the CS state, ZnCh +-C60 . The CS state has absorption maxima at 790 and 1000 nm due ZnCh+ and C60, ... 

Figure 2.9. Dependence of maximum rate constant of ET on the edge-to-edge distance in photosynthetic RCs of bacteria and plant PSI 1, Ao - A, 2, IT - QA 3, A,-Fx H - P+ 5, C559 -P+ 6, Fx - FA 7, QA - P 8, P - Bel 9, P700 - Ao 10, Qa - QB P700 is the chlorophyll dimer, Ao is chlorophyll, A, is phylloquinone and Fx and FA are the 4 Fe-4S clusters. The straight line is related to the dependence of the attenuation parameter for spin exchange (Yse) in homogeneous non-conducting media. Filled circles correspond to a regular dependence, open circles to a deviation (Likhtenshtein, 1996). Reproduced with permission.

Table I. Rate Constants For Chlorophyll A Hydrolysis in Media Of Different Composition.

The primary processes of photochemistry involve the light absorption event, which we have already discussed, together with the subsequent deexcitation reactions. We can portray such transitions on an energy level diagram, as in Figure 4-9 for chlorophyll. In this section we discuss the various deexcitation processes, including a consideration of their rate constants and lifetimes. 

The rate constants for reactions of all-trans retinal and retinol with have been measured in various solvents. Rate constants increase with dielectric constant, thereby suggesting that charge transfer is involved. Electronically excited triplet species generated during peroxide-catalysed aerobic oxidation of substrates gives rise to fluorescence from acceptors in micelles. Micelle-solubilized chlorophyll is an excellent detector of enzyme-generated triplet carbonyl species, and micelles make photochemistry without light possible for... 

Based upon a detailed analysis of reaction transients, a mechanism was proposed for chlorophyll a-photosensitized transmembrane oxidation-reduction of aqueous phase donors and acceptors that included electron transfer between juxtaposed Chi a+ r-cations and Chi a molecules as the transmembrane charge-transfer step [112]. The maximum apparent first-order rate constant for this step was 10 s , which seems large for thermal electron transfer between chlorophyll molecules located at the opposite membrane interfaces, even considering that nuclear activation barriers may be relatively small for this reaction. Transverse flip-flop diffusion of Chi b across the membrane is 10 -fold slower than transmembrane redox under these conditions, so this alternative mechanism is almost certainly unimportant. Kinetic mapping studies have shown that some of the Chi a becomes localized within the membrane at sites that are inaccessible to aqueous phase electron acceptors, presumably within the membrane interior [114]. This suggests the possibility of a transverse hopping mechanism involving electron transfer over relatively short distances from buried Chi a to interfacial Chi a+, followed by electron transfer from Chi a at the opposite interface to the buried Chi a" ". 

Fig. 2. (A) Flash-induced AA in the SDS-fractionated PS-1 core complex (CPI) at 5 K [ with and o without DCIP] (B) Flash-induced AA in TSF-I particles containing dithionite and neutral red at pH 10 and frozen while being illuminated (C) left AA induced by 300-ns, dye laser flashes [710 nm for the blue and green region 590 nm for the red region] insets show individual AA transients at 696 and 480 nm (C) right The difference between the difference spectrum in the left panel and that of P700. (D) Plot of the rate constant vs. reciprocal temperature. Figure source (A) Mathis, Sauer and Remy (1978) Rapidly reversible flash-induced electron transfer on a P-700 chlorophyll-protein complex isolated with SDS. FEBS Lett 88 277 (8) Sauer, Mathis, Acker and van Best (1979) Absorption changes of P-700 reversible in milliseconds at low temperature in Triion-solubilized photosystem I particles. Biochim Biophys Acta 545 469 (C and D) Shuvalov, Dolan and Ke (1979) Spectral and kinetic evidence for two eariy electron acceptors in phoiosystem I. Proc Nat Acad Sci, USA 76 771,773.

The primary photosynthetic process is carried out by a pigment protein complex the reaction centre (RC) embedded in a lipid bilayer membrane (Figure 6.19) and surrounded by light-harvesting complexes.1477,1481,1482 Thus energy is transferred from LH1 to a bacteriochlorophyll special pair (P) and then through a bacteriochlorophyll molecule (BC monomer) to bacteriopheophytin (BP a chlorophyll molecule lacking the central Mg2 + ion), followed by electron transfer to a quinone Qa in hundreds of ps. The neutral P is then restored by electron transfer from the nearest intermembrane space protein cytochrome c (Cyt c) in hundreds of ns. The rate constants of the... 

Light-driven electron transfers between Chla and hydroquinones [Cho (30), White (200)] as well as between Chla and lumiflavin [Tu (184), 185)] have also been described and interpreted as simple models for some primary steps in photosynthesis [Wang (193)]. Through the use of j8 carotene as a quencher the rate constant for semiquinone radical formation in such a system was found to be 8.8 X 105 1/mol sec at —30° [Cho (30j. A descriptive theory of the relationships between the energy levels in different chlorophylls and the electron-transfer processes between them and other organic electron carriers has also been developed [Nelson (134)]. 

Violaxanthin inhibits in vitro quenching in LHCIIb, CP29 and CP26 (Ruban et al 1994a, 1996 1998 Phillip et al., 1996). As for the effect of endogenous DEPS, the effect is to control the rate constant and amplitude of quenching (Fig. lOB). Maximum effect was found with approx. 1 molecule violaxanthin per chlorophyll but this is the amount added, not bound. 

In ethanolic solution, the yield of fluorescence from Chl is about 32% and that oftriplet formation is 68%. Shipman (1980) has calculated a rate constant for chlorophyll triplet formation from Chi of 1.9 x 10 s , compared to the rate constant for fluorescence emission of 6.5 x 10 s". Measurements on isolated thylakoids also indicate that the yield of chlorophyll a triplet formation is twice that of fluorescence decay (Kramer and Mathis, 1980), with a triplet yield of 0.15 (calculated over all PS I and PS II chlorophylls) when all rc was in the state. More recent studies on purified PS II pigment-protein complexes show that for CP47 at 4 K the yield of fluorescence is 0.11 0.03 and that oftriplets is 0.16 0.03 (Groot et al.,... 

The rate constants for radiationless decay of the triplet state of mesoporphyrin IX dimethyl ester at 77 are 26 s-1 in EtOD and 57 s-1 in EtOH.352 The most probable cause is a decrease in the rate of tautomerism in the porphyrin due to deuteriation of the N—H hydrogen. However, for TPPH2 triplet state in n-octane matrices 363 such tautomerism does not appear to be an important mechanism for radiationless deactivation. Several recent reports deal with the low-temperature e.s.r. spectra of triplet states of porphyrins.364-368 The zero field splittings and depopulation rates of the various spin sub-levels of the triplet state of Zn-chlorophyll-a have been determined by an optically detected magnetic resonance method.359... 

Shetlar has derived non-linear relationships of the Stem-Volmer type suitable for systems in which quenching occurs by more than one mechanism. Rate constants for heavy atom fluorescence quenching of polynuclear aromatic hydrocarbons by 1-iodopropane in benzene have been found to decrease exponentially with the energy difference between the fluorescing state and the nearest lower triplet state (Dreeskamp et al.). Bromocyclopropane has been recommended as a heavy-atom quencher of excited singlet states since it is more photostable than simple alkyl bromides (Flemming, Quina, and Hammond). Laser studies with chlorophyll a have provided evidence for the interesting radiationless intermolecular process Tx + Sx -> T2 + S0 (Menzel). 

Complexes of Ni" and Co", for example, do not undergo demetallation but complexes of Zn" and Mg" do so very readily. To prevent demetallation, either a base (pyridine) was added to neutralize the HCl, or the solvent was changed to CCl4, ° where HCl is not produced. By using dichloroethane/ pyridine solutions rate constants could be determined for the electron transfer from Co"TPP and from chlorophyll a to the radical cations of various metalloporphyrins. 

Pulse radiolysis has been used to study elementary reactions of importance in photosynthesis. Early experiments provided rate constants for electron transfer reactions of carotenoid radical cations and radical anions with chlorophyll pigments.More recent experiments dealt with intramolecular electron transfer in covalently bound carotenoid-porphyrin and carotenoid-porphyrin-quinone compounds. Intramolecular electron transfer reactions within metalloproteins have been studied by various authors much of that work has been reviewed by Buxton, and more recent work has been published. Pulse radiolysis was also used to study charge migration in stacked porphyrins and phthalocyanines. Most of these studies were carried out by pulse radiolysis because this techruque allowed proper initiation of the desired processes and pemtitted determination of very high reaction rate constants. The distinct character of radiolysis to initiate reactions with the medium, in contrast with the case of photolysis, and the recent developments in pulse radiolysis techniques promise continued application of this technique for the study of porphyrins and of more complex chemical systems. 

Plots of pseudo-first-order rate constants, k, versus algal biomass expressed as micrograms chlorophyll a/L were linear (Figure 3). 

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