Soft ligands--biodegradable complexants in plant cultivation and environmental protection.
In this paper, the place and role of biodegradable complexants in light of modern plant cultivation criteria is discussed. From this point of view, micro- and macroelement (metal) chelators play a remarkable role. They are widely used in a variety of consumer products and processes, especially in agriculture. The great majority of the traditional chelators (e.g., EDTA) are, however, essentially non-biodegradable. We have systematically investigated some new biodegradable ligands with polyaminopolycarboxylic structure. We conclude that these compounds may be considered as a special group of the soft (safer) chemicals proposed first by Bodor based on retrometabolic concepts. We studied the structural conditions of biodegradability, especially regarding the new ligands, by the physicochemical characterization of their complexes formed with micro- and macroelements. The study of the complicated equilibrium conditions required the prior determination of the stability constants of the metal aqua/hydroxo complexes and the protonation constants of the ligands as well. Knowledge of the pH-dependence of the proper competitive equilibriums allowed the optimization of the investigated complexation reactions. The protonation and stability constants obtained were compared with the adequate values of the traditional, but weakly biodegradable EDTA. Excellent or very good linear correlations have been found between the logarithms of the different ligands' protonation constants, but the slopes of the straight lines were different and generally smaller than EDTA's corresponding values. Presumably, the log stability constants (logK(st)) of complexes formed by different ligands (X(m), X(n)) having analogue structures with a given metal-ion series (M(i), i = 1, 2, ...) are linearly correlated [i.e., log K(X(m)M(i)) = a log K(X(n)M(i)) + b], just as the stability constants of complexes formed by different ions (M(k), M(l)) with a given ligand series (X(j), j = 1, 2, ...). Based on the rich data basis of EDTA, these regression equations allow the calculation of some still unmeasured stability constants of the new biodegradable complexes. In the analysis of structural effects we would like to especially highlight the role of stereochemical relations in addition to the usual structural elements. From this point of view, the eventual presence of chiral nitrogen atoms is of particular importance. The stability of these compounds increases in their complexed form. As a special research field, we have examined the possibility of obtaining metal ion buffers. These systems may get an important role both from theoretical and practical points of view.