During senescence, chlorophyll (chl) is metabolized to colorless nonfluorescent chl catabolites (NCCs). A central reaction of the breakdown pathway is the ring cleavage of pheophorbide (pheide) a to a primary fluorescent chl catabolite. Two enzymes catalyze this reaction, pheide a oxygenase (PAO) and red chl catabolite reductase. Five NCCs and three fluorescent chl catabolites (FCCs) accumulated during dark-induced chl breakdown in Arabidopsis (Arabidopsis thaliana). Three of these NCCs and one FCC (primary fluorescent chl catabolite-1) were identical to known catabolites from canola (Brassica napus). The presence in Arabidopsis of two modified FCCs supports the hypothesis that modifications, as present in NCCs, occur at the level of FCC. Chl degradation in Arabidopsis correlated with the accumulation of FCCs and NCCs, as well as with an increase in PAO activity. This increase was due to an up-regulation of Pao gene expression. In contrast, red chl catabolite reductase is not regulated during leaf development and senescence. A pao1 knockout mutant was identified and analyzed. The mutant showed an age-and light-dependent cell death phenotype on leaves and in flowers caused by the accumulation of photoreactive pheide a. In the dark, pao1 exhibited a stay-green phenotype. The key role of PAO in chl breakdown is discussed.Chlorophyll (chl) degradation is an integral part of leaf senescence and fruit ripening. The fate of chl during senescence has been well established in recent years (for review, see Matile et al., 1999; Hörtensteiner, 1999; Hö rtensteiner and Kräutler, 2000;Kräutler, 2003;Eckhardt et al., 2004). Thereby, chl is converted to colorless nonfluorescent chl catabolites (NCCs; Fig. 1) in a pathway that is probably active in all higher plants (Pružinská et al., 2003;Gray et al., 2004). Structure elucidation of NCCs from different species has unraveled a common tetrapyrrolic skeleton with an oxygenolytically opened porphyrin macrocycle (Kräutler, 2003). Peripheral modifications at several side chains within different NCCs (Fig. 1, R 1 -R 3 ) are species specific (Berghold et al., 2002(Berghold et al., , 2004, and hence have been proposed to occur rather late in the pathway (Hö rtensteiner, 1999). Indeed, a primary chl breakdown product (primary fluorescent chl catabolite-1 [pFCC-1]), which exhibits a blue fluorescence, could be identified as a common product of porphyrin ring cleavage ( Fig. 1; Mü hlecker et al., 1997). Thus, the sequence of reactions is the removal of phytol and magnesium (Mg) by chlorophyllase and Mg-dechelatase, respectively, followed by the conversion of pheophorbide (pheide) a to pFCC-1, which requires the activity of two enzymes, pheide a oxygenase (PAO) and red chl catabolite (RCC) reductase (RCCR; Rodoni et al., 1997;Hö rtensteiner, 1999).PAO is a chloroplast envelope-bound Rieske-type iron-sulfur oxygenase, which is identical to lethal leaf spot 1 (LLS1) from maize (Zea mays) and accelerated cell death 1 (ACD1) from Arabidopsis (Arabidopsis thaliana; Pružinská et al., 2003;Yang e...

An apple a day keeps the doctor away: This old saying may obtain a new meaning. The degradation of chlorophyll in ripe apples and pears gives rise to so‐called nonfluorescent catabolites of chlorophyll (NCCs), which are identical to NCCs from leaves. The NCCs from fruit prove to be effective natural antioxidants.

In continuation of our work on Wanzlick/Arduengo carbenes containing redox-active ferrocenyl substituents we report on the synthesis of N,N‘-diferrocenyl imidazol(in)ium salts as precursors of imidazol(in)-2-ylidenes. The necessary starting material for this chemistry is aminoferrocene, which was prepared by an improved and large-scale synthesis by the sequence solid lithioferrocene, iodoferrocene, N-ferrocenylphthalimide, aminoferrocene. The preparation of N,N‘-diferrocenyl heterocycles involves condensation of aminoferrocene with glyoxal to afford N,N‘-diferrocenyldiazabutadiene [Fc-DAB], reduction, condensation with formaldehyde, and oxidation with trityl salts to yield N,N‘-diferrocenylimidazol(in)ium salts. In situ deprotonation and trapping with electrophiles yielded the expected metal complexes and derivatives in some cases [Ag+ or S8], but attempted reaction with other transition metals [e.g., Pd(II)] failed to give the corresponding complexes, due to (i) steric hindrance by the two N-ferrocenyl substituents, (ii) reduced acidity of the imidazol(in)ium precursors, and (iii) inaccessibility of the free carbenes. Spectroscopic [IR, Raman, UV−vis, MS, NMR (1H, 13C, 109Ag)], structural [X-ray], and electrochemical [CV] properties are reported and compared to those of other N-heterocyclic carbene derivatives.

The biological enigma of the breakdown of chlorophyll in plants only recently began to be understood. The structure elucidation of a “fluorescent” catabolite (depicted on the right) from senescent rape cotyledons provides further insight into this process. For this purpose, the catabolite was prepared from pheophorbide a with an enzyme extract from senescent choloroplasts.

Dedicated to Professor Duilio Arigoni on the occasion of his 75th birthdayThe corrinoid cofactor of the tetrachloroethene reductive dehalogenase of Dehalospirillum multivorans was isolated in its Cob-cyano form. This cofactor represents the main corrinoid found in D. multivorans cells. Analysis of the isolated cyano-corrinoid by a combination of HPLC and UV/VIS-absorbance spectroscopy revealed it to be nonidentical to a variety of known natural B 12 derivatives. From high-resolution massspectrometric analysis, the molecular formula of the corrinoid isolated from D. multivorans could be deduced as C 58 H 81 CoN 17 O 14 P. The sample of the novel corrinoid from D. multivorans was further analyzed by UV/VIS, CD, and one-and two-dimensional 1 H-, 13 C-, and 15 N-NMR spectroscopy, which indicated its structure to be closely related to that of pseudovitamin B 12 (Cob-cyano-7''-adeninylcobamide). By the same means, the corrinoid could be shown to differ from pseudovitamin B 12 only by the lack of the methyl group attached to carbon 176, and, therefore, it was named norpseudovitamin B 12 (or, more precisely, 176-norpseudovitamin B 12 ). Norpseudovitamin B 12 represents the first example of a −complete× B 12 -cofactor that lacks one of the methyl groups of the cobamide moiety, indicating that the B 12 -biosynthetic pathway in D. multivorans differs from that of other organisms. X-Ray crystal-structures were determined for norpseudovitamin B 12 from D. multivorans and the analogues pseudovitamin B 12 and factor A (Cob-cyano-7''-[2-methyl]adeninylcobamide). These first accurate crystal structures of complete corrinoids with an adeninyl pseudonucleotide confirmed the expected coordination properties around Co and corroborated the close conformational similarity of the nucleotide moieties of norpseudovitamin B 12 and its two homologues.