Metal Speciation, Chelation and Complexing Ligands in Plants
Most transition metal ions and some main group metal ions form complexes with small (Cl-, carboxylic acids, amino acids) and macromolecular organic substances (proteins, DNA, polysaccharides) present in the xylem and phloem of plants (Łobinski and Potin-G
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Metal Speciation, Chelation and Complexing Ligands in Plants
A.Pohlmeier Institut fur Chemie und Dynamik der Geosphare 7, Forschungszentrum D-52425 Jillich, Germany
2.1 Introduction Most transition metal ions and some main group metal ions form complexes with small (CI-, carboxylic acids, amino acids) and macromolecular organic substances (proteins, DNA, polysaccharides) present in the xylem and phloem of plants (Lobinski and Potin-Gautier 1998). In the first section of this chapter some remarks and definitions concerning the aqueous chemistry of metal ions will be given to facilitate further reading. The following sections present a brief outline of theories describing the nature of the coordination bond, the basic thermodynamics of complexation, the hydrolysis and chelation of metal ions and the reactions with important groups of ligands present in plants. The chapter will close with an overview of some special aspects relevant for the understanding of complexation reactions at macromolecules and the reaction mechanism of complex formation.
2.2 Terminology and Definitions Most of the metal ions forming complexes may exist in aqueous solution in different oxidation states. Under environmental conditions, i.e. at moderate pHvalues and under slightly oxidating conditions, + 1 to + 3 are the most usual formal oxidation states for the metal ions concerned in the context of this book. Some important exceptions are M0 6 +, which exists in the natural environment as stable MoOt ions, forming no complexes with anionic ligands present in plants, and V4+, present in most cases as V0 2 +, forming complexes with ligands. The
M. N. V. Prasad et al., Heavy Metal Stress in Plants © Springer-Verlag Berlin Heidelberg 1999
30 most stable oxidation state of chromium and iron is + 3. However, due to their strong tendency to hydrolyze at moderate pH-values (see below), these ions exist as (poly)hydroxo species or as unsoluble minerals. In the following, when no special metal ions are considered, complexforming metal ions will be termed as MX+. Metal ion complexes, often also termed as coordination compounds, are formed by at least two partners: the central metal ion, which is generally MX+ and the ligand, abbreviated in the following as U- _ In most cases one metal ion binds at least four ligands, which can be identical, as in the case of the hexaquo-complexes M(H20)/+ or different like M(H20)4CI2' The coordination number Z is the total number of ligand ions or molecules directly associated with the metal ion. Its value depends on several factors like the size of MX+ and L and the specific electron configuration of M+. Table 2.1. gives a survey of the most usual coordination numbers and geometries of some important metal ions. The charge of the complexes can be negative, zero or positive depending on the sum of charges of MX+ and all L The stability of metal ion complexes is expressed by means of the equilibrium constant K, often also termed the stability constant or complexation constant. This describes the thermodynamic equili
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