Untitled

Abstract

The in vivo substitution of magnesium, the central atom of chlorophyll, by heavy metals (mercury, copper, cadmium, nickel, zinc, lead) leads to a breakdown in photosynthesis and is an important damage mechanism in heavy metal-stressed plants. In this study, a number of methods are presented for the efficient in situ detection of this substitution (i.e. in whole plants or in chloroplasts). While macroscopic observations point to the formation of heavy metal chlorophylls at higher concentrations, fluorescence microscopy enables the detection of this reaction at very low substitution rates. Therefore, the course of the reaction can be followed by continuously measuring the fluorescence of whole plants. Furthermore absorbance spectroscopy of whole cells or isolated chloroplasts also enables the in situ detection of heavy metal chlorophylls. These methods provide practicable approaches in detecting the formation of these compounds in situ, avoiding artefacts that might occur using extraction methods based on polar solvents. In addition to the new methods for in situ detection, an extreme heterogeneity in the reaction of cells in the same tissue upon heavy metal stress was observed: while some cells are already disintegrating, others still show normal fluorescence and photosynthetic activity. Measurements of fluorescence kinetics gave a further hint that in high light intensity a substitution of Mg by heavy metals might take place specifically in PS II reaction centres.Abbreviations: chl -chlorophyll; DCMU -3-(3,4 Dichlorophenyl)-1,1-Dimethylurea; F 0 -basic ('dark') fluorescence; F m -maximum fluorescence; F v -variable fluorescence; F steadystate -steady state fluorescence, i.e. after fluorescence induction; hm 2+ -heavy metal 2+ ; hms -heavy metal-substituted; Mg-substitution -substitution of the natural central ion of chlorophyll, Mg 2+ , by heavy metals

Mechanistic Information on CuII Metalation and Transmetalation of Chlorophylls

“…A slight blue-shift of the absorption maximum (not shown) points to dimer formation in reaction (5), although the coordination of a second Ac -by CuAc + to form CuAc 2 in reaction (4) can also be expected to take place. In both solvents, and in the absence of other potential ligands, Cu II exists usually as [Cu(Solv) 6 ] 2+ [29,30] but CuAc 2 forms dimeric species, both in the crystalline state [31] and in solution. [32][33][34] + species [35,36] as the positive charge facilitates electrophilic attack on the pyrrolenine nitrogens, and the lone electron pairs on Ac oxygens can participate in hydrogen bonding, and hence, in deprotonation of the tetrapyrrole core.…”

“…For instance, a spontaneous insertion of transition metals, such as Cu and Ni (soil pollutants), may occur in vivo and lead to the irreversible impairment of photosynthetic activity in plants. [6,7] Recently, metallochlorophylls and metallobacteriochlorophylls came into focus for being interesting model pigments [8][9][10][11] and for their excellent photosensitizing properties. [12][13][14][15][16] For decades already, metalation [Equation (1)] and transmetalation [Equation (2)] of porphyrins and Chls, shown in Equations (1) and (2), respectively:…”

Fine tuning of copper(ii)–chlorophyll interactions in organic media. Metalation versus oxidation of the macrocycle

“…5 The latter effect, observed clearly for Cu 2+ , Zn 2+ , Cd 2+ and Hg 2+ , results in the irreversible loss of photosynthetic abilities. [5][6][7] In recent years, deeper insight into the mechanisms of metal ion -Chl interactions has become important also for the development of procedures for modification and synthesis of new tetrapyrrolic complexes in terms of their potential biomedical applications. [8][9][10] Although the primary function of chlorophylls is related to electron transfer processes, their occurrence in biological systems is not due to pigment interactions with redox-active metal ions.…”

“…1) at the stage of enzyme-driven Mg 2+ incorporation into protoporphyrin IX (3,7,12,17-tetramethyl-8,13-divinyl-2,18-porphinedipropionic acid), [1][2][3][4] but also in the spontaneous substitution of metal ions occurring in plants growing on soils contaminated by heavy metals. 5 The latter effect, observed clearly for Cu 2+ , Zn 2+ , Cd 2+ and Hg 2+ , results in the irreversible loss of photosynthetic abilities. [5][6][7] In recent years, deeper insight into the mechanisms of metal ion -Chl interactions has become important also for the development of procedures for modification and synthesis of new tetrapyrrolic complexes in terms of their potential biomedical applications.…”

Nickel in the Environment and Its Role in the Metabolism of Plants and Cyanobacteria

“…This reaction has been named the 'sun reaction' by Küpper et al [98]. After inhibition of photosynthesis, under high irradiance the antenna pigments are degraded [98,128]. However, bleaching during sun reaction is slower than the Mg substitution during shade reaction, so that the very low ratio of Mg substitution during sun reaction cannot be caused by competition with the bleaching process [98].…”