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ABSTRACT: Removal of the central metal from chlorophyll (Chl) molecules is biologically important in terms of production of the primary electron acceptors in photosystem-II photosynthetic reaction centers and the early stage in Chl degradation. The physicochemical properties on demetalation of chlorophyllous pigments are useful in the understanding of such reaction mechanisms in photosynthetic organisms. Here we analyzed the demetalation kinetics of a series of Zn-Chl derivatives with a systematic variation in the electron-withdrawing and -donating substituents at the 3-position of the chlorin macrocycle under acidic conditions to elucidate thoroughly the substitution effects on the demetalation properties of chlorophyllous pigments. Dehydrogenation of the aliphatic group (CH(2)CH(3) → CH═CH(2) → C≡CH) at the 3-position slowed the removal of the central zinc from the chlorin macrocycle. The gradual decrease in the demetalation rate constants of the three zinc chlorins originates from differences in the electron-withdrawing strength of the ethyl, vinyl, and ethynyl groups directly linked to the chlorin π macrocycle. Reduction of the 3(1)-carbonyl groups significantly increased the demetalation rate constants, and the relative ratios of the demetalation rate constants of the zinc chlorins possessing a carbonyl group to those possessing the corresponding hydroxy group were analogous in the cases of 3-formyl- and 3-acetyl-zinc chlorins. The demetalation rate constants of the seven Zn-Chl derivatives possessing various electron-withdrawing and -donating groups exhibited good correlation with the Hammett σ parameters of the 3-position substituents.
Inorganic Chemistry 12/2012; · 4.60 Impact Factor
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ABSTRACT: Demetalation of chlorophyll (Chl) a and its analogs is an important reaction in oxygenic photosynthetic organisms, which produces the primary electron acceptors in photosystem II reaction centers and is crucial in the Chl degradation. From these viewpoints, demetalation reactions of four Chl a analogs, 3,8-divinyl-Chl a (DV-Chl a), 3-devinyl-3-ethyl-Chl a (mesoChl a), 13(2) -demethoxycarbonyl-Chl a (pyroChl a) and protochlorophyll a (PChl a), were kinetically analyzed under weakly acidic conditions, and were compared with that of Chl a. DV-Chl a exhibited slower demetalation kinetics than did Chl a, whereas demetalation of mesoChl a was faster than that of Chl a. The difference in demetalation kinetics of the three chlorophyllous pigments originates from the electron-withdrawing ability of the vinyl group as the peripheral substituent compared with the ethyl group. Removal of the electron-withdrawing and homoconjugating 13(2) -methoxycarbonyl group in Chl a (Chl a → pyroChl a) accelerated demetalation kinetics by two-fold. PChl a possessing the porphyrin-type skeleton exhibited slower demetalation kinetics than Chl a. The structure-dependent demetalation properties of Chl a analogs will be useful for understanding in vivo Chl demetalation reactions in oxygenic photosynthetic organisms.
Photochemistry and Photobiology 07/2012; · 2.41 Impact Factor
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ABSTRACT: Demetalation of three synthetic zinc cyclic tetrapyrroles that possess identical peripheral substituents, zinc methyl bacteriopyropheophorbide a (zinc bacteriochlorin 1), zinc methyl 3-devinyl-3-acetyl-pyropheophorbide a (zinc chlorin 2), and zinc methyl 3-devinyl-3-acetyl-protopyropheophorbide a (zinc porphyrin 3), was kinetically analyzed under acidic conditions to examine the effects of macrocyclic structures on demetalation without peripheral substitution effects. Zinc bacteriochlorin 1 exhibited much slower demetalation kinetics than zinc chlorin 2 and zinc porphyrin 3. These results indicate that the bacteriochlorin skeleton provides significant resistance to the removal of the central metal from the tetrapyrrole ligand. Comparison of demetalation kinetics of 3-acetyl zinc complexes 2 and 3 with that of 3-vinyl zinc complexes under the same reaction condition demonstrated that the relative ratio (5.0 × 10(-2)) of the demetalation rate constant of the 3-acetyl zinc chlorin 2 to that of the corresponding 3-vinyl zinc chlorin 4 resembled the case of the 3-acetyl zinc porphyrin 3 to the 3-vinyl zinc porphyrin 5 (the relative ratio was 6.8 × 10(-2)). These suggest that the electron-withdrawing 3-acetyl group slows down the demetalation from the tetrapyrrole ligands more than the 3-vinyl group and that the 3-acetyl effect is analogous in both chlorin and porphyrin π-systems.
The Journal of Physical Chemistry B 09/2011; 115(40):11757-62. · 3.70 Impact Factor
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ABSTRACT: Reduction of the 7-formyl groups in chlorophyll (Chl) b and its demetalated compound pheophytin (Phe) b was kinetically analyzed by using tert-butylamine-borane complex (t-BuNH(2)·BH(3)), and was compared with that of the 3-formyl groups in Chl d and Phe d. Reduction kinetics of the 7-formyl group in Chl b was similar to that in Phe b in dichloromethane containing 5mM t-BuNH(2)·BH(3). Little difference of the reduction kinetics of the 7-formyl groups between Chl b and Phe b was in sharp contrast to the reduction kinetics of the 3-formyl groups in Chl d and Phe d: the 3-formyl group in Phe d was reduced 5.3-fold faster than that in Chl d. The 7-formyl groups in Chl b and Phe b were reduced more slowly than the 3-formyl groups in Chl d and Phe d, respectively. The difference of the reactivity between the 3- and 7-formyl groups was in line with (13)C NMR measurements of chlorophyllous pigments, in which the chemical shifts of carbon atoms in the 7-formyl groups of Chl b and Phe b were high-field shifted compared with those in the 3-formyl groups of Chl d and Phe d, respectively. These indicate that the 7-formyl groups in chlorophyllous pigments were less reactive for reduction to the corresponding hydroxymethyl groups than the 3-formyl groups due to the difference in electronic states of the formyl groups in the A- and B-rings of the chlorin macrocycle.
Bioorganic & medicinal chemistry 07/2011; 19(13):3901-5. · 2.82 Impact Factor
