Chelating Agent Structure-Reactivity Relationships Pertaining to Metal Ion Uptake by Grasses

Graminaceous crops (grasses) exude phytosiderophore chelating agents under iron stress conditions to facilitate metal ion acquisition from mineral surfaces and from complexes with natural organic matter (NOM). The nine known phytosiderophores all possess three carboxylate groups and between one and three alcoholate groups. Avenic acid A and two of the other phytosiderophores possess two secondary amines that allow free bond rotation. Mugineic acid and five of the other phytosiderophores possess one secondary amine of this kind, plus a tertiary amine that is held within a rigid azetidine ring. Because of this rigidity, the orientation in three dimensional space of the electron pair available for bonding is restricted. Rigidity is also relevant when selecting synthetic chelating agent agricultural amendments. CDTA, for example, possesses a rigidity-inducing cyclohexane linkage between its two tertiary amine groups that EDTA lacks. Our interest is in the effect of rigidity on metal ion exchange reactions (ML + Y → L + MY) and chelating agent-assisted metal ion desorption from mineral surfaces (>S-M + Y → >S + MY). The contaminant metal nickel(II) is employed in our experiments because rates of exchange are slow enough for aqueous speciation to be conveniently monitored by capillary electrophoresis. Iron(III) has a higher charge and a smaller radius, so the effects of rigidity on its reactions are expected to play out differently.