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B. Thermodynamics and Kinetics of Ligand-Exchange Reactions with Phytosiderophores as Receiving Ligands

B. Thermodynamics and Kinetics of Ligand-Exchange Reactions with Phytosiderophores as Receiving Ligands

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DFO‐B compete strongly with high concentrations of DMA for iron complexation independent of the presence or absence of calcium. The much higher

aYnity of DFO‐B for iron compared to phytosiderophores was experimentally verified in a ligand‐exchange experiment where 100 mM epi‐HMA did

not bind significant concentrations of iron in the presence of 200 mM Fe–

DFO‐B at pH 6 (Hoărdt et al., 2000). This is consistent with equilibrium

calculations using the set of equilibrium constants listed in Tables IV and V.

At millimolar phytosiderophore concentrations that may be realized in the

apoplast or very close to the root surface, ligand exchange may lead to the

formation of low concentrations of iron–phytosiderophore complexes in a

range that may partially satisfy iron requirements. However, slow ligand

exchange rates may further reduce the eYciency of this reaction for iron

acquisition as discussed later.

Due to large variations in the stability of Fe–siderophore complexes, it

is diYcult to draw generalizations on their availability for plant uptake.

For example, DMA is able to eYciently sequester iron from the iron–

rhodotorulic acid (Fe2RA3) complex (Fig. 8B). Rhodotorulic acid is a tetradentate siderophore forming binuclear complexes with somewhat lower

stability compared to the other siderophore complexes shown here. Zhang

et al. (1991a) found higher iron uptake via an apoplastic pathway in the

presence of the rhodotorulic acid complex compared to the DFO‐B complex.

However, they did not report if rhodotorulic acid was present in (molar)

excess over iron. Due to the stoichiometry of the dominant complex, the

Table IV

pKa of Organic and Inorganic Ligands, Corrected to Zero Ionic Strength

with the Davies Equation T ¼ 298.15 K













Rhodotorulic acidd










































von Wire´n et al. (2000).

Murakami et al. (1989).


Sugiura et al. (1981).


Martell et al. (2001).




Table V

Thermodynamic Formation Constants, Corrected to Zero Ionic Strength

with the Davies Equation T ẳ 298.15 K


Complex formation constants

Ca2ỵ ỵ CO32 ẳ CaCO3

Ca2ỵ ỵ HCO3 ẳ CaHCO3

MA4 ỵ Fe3ỵ ẳ FeMA

MA3 ỵ Fe3ỵ ỵ H2O ẳ

Fe(OH)MA ỵ Hỵ

MA3 ỵ Ca2ỵ ẳ CaMA

DMA4 þ Fe3þ ¼ FeDMẦ

DMẦ3 þ Fe3þ þ H2O ¼

Fe(OH)DMẦ þ Hỵ

DMA4 ỵ Ca2ỵ ẳ CaDMA

EpiHMA3 ỵ Fe3ỵ ỵ H2O ẳ

Fe(OH)EpiHMA þ Hþ

Epi‐HMẦ3 þ Ca2þ¼ CaEpi‐HMẦ

HDFO‐B2– þ Fe3þ¼ FeHDFO‐Bþ





Ferrichrome3 ỵ Fe3ỵ ẳ FeFerrichrome

FeFerrichrome ỵ Hỵ ẳ


log K



















Coprogen3 þ Feþ ¼ FeCoprogen

FeCoprogen þ Hþ ¼ FeHCoprogenþ

RA2– þ Fe3þ ẳ FeRAỵ

3RA2 ỵ 2 Fe3ỵ ẳ Fe2RA3

Fe hydrolysis constants

Fe3ỵ ỵ OH ẳ FeOH2ỵ

Fe3ỵ ỵ 2OH ẳ Fe(OH)2ỵ

Fe3ỵ ỵ 4OH ẳ Fe(OH)4

2Fe3ỵ ỵ 2OH ẳ Fe2(OH)24ỵ

3Fe3ỵỵ 4OHẳ Fe3(OH)45ỵ

Solubility constants calcite

CaCO3 ẳ Ca2ỵ ỵ CO32


Fe(OH)3 ỵ 3H ẳ Fe3ỵỵ 3H2O

soil iron oxide

Fe(OH)3 þ 3HÃ ¼ Fe3þ þ 3H2O


FeOOH þ 3HÃ ¼ Fe3þ þ H2O

Henry’s law constant

CO2(g) þ H2O ¼ H2CO3Ã

log K
















Martell et al. (2001).

Parker and Khodakovskii (1995).


Murakami et al. (1989).


Schindler et al. (1963).


Lindsay (1979).


RA: Rhodotorulic acid.


addition of 1:1 mole ratio of Fe(III) and rhodotorulic acid could lead

to oversaturation of ferrihydrite near neutral pH, depending on the total

Fe(III) concentration.

An example for a fungal siderophore with relatively low aYnity and

specificity for iron is rhizoferrin. In a calcareous soil, rhizoferrin will be even

less competitive for iron complexation due to its relatively high aYnity for

calcium. Ferrichrome, DFO‐B, and coprogen are trihydroxamate siderophores.

Coprogen has a higher aYnity for iron compared to DFOB. Despite its high

stability, Hoărdt et al. (2000) found higher uptake of iron in the presence of the

coprogen complex compared to the DFO‐B complex by cucumber (strategy I)

and maize (strategy II). They hypothesized that the higher uptake was due to

faster ligand exchange kinetics of the coprogen complex. Unfortunately, the

corresponding ligand exchange rates are unknown.

Iron complexes with humic substances are also important sources for

strategy II iron acquisition. Cesco et al. (2000) showed qualitatively that



Figure 8 Equilibrium concentration of Fe–DMA complexes as a function of DMA concentrations in the presence of 1 mM Fe(III) and various ligands. Ligand concentrations are suYcient

to prevent ferrihydrite precipitation. (A) Competing ligands: 1 mM DFO‐B, 2 mM EDTA, or

1000 mM citrate. All calculations in the absence of calcium or in equilibrium with calcite at

elevated rhizosphere CO2 partial pressure. (B) Competing ligands (all bacterial and fungal

siderophores): 1 or 2 mM DFO‐B, 1 mM coprogen, 1 mM desferrichrome, 1 mM rhizoferrin, or

1.5 mM rhodotorulic acid. No calcium present. Equilibrium constants and solubility products

are listed in Tables IV and V. All calculations at pH ¼ 6.8, MPCO2 ¼ 8%, and I ¼ 0.1 M.

HMA sequestered iron from a water extractable humic substances–Fe complex by ligand exchange. Solinas (1994) reacted the microbial siderophore

DFO‐B with Fe(III)–saturated soil humic acid. After 24 h reaction time, he

found all soluble DFO‐B as iron complex. However, significant partitioning of

DFO‐B into humic substances decreases the mobility of the siderophore in soil

solution (Higashi et al., 1998; Powell et al., 1982; Solinas, 1994). Unlike most

known siderophores, free DFO‐B and the Fe–DFO‐B complex are positively



charged below pH 10 so that an electrostatic driving force may contribute to

the interaction of this particular siderophore with humic substances.


Ligand Exchange Kinetics

To evaluate the degree of kinetic limitation of a process, it is useful to

compare the half‐life of the process in question with a reference timescale. In

the case of ligand exchange between an iron shuttle and phytosiderophores,

the reference timescale is given by the diurnal exudation cycle of siderophores. It has been demonstrated that iron‐uptake rates are high during the

period of high siderophore exudation and low otherwise (Cesco et al., 2002;

Yehuda et al., 1996), although the eYciency of the Fe–phytosiderophore

uptake system remains constant over the day (Alam et al., 2004).

Assuming that the diurnal exudation cycle provides a reference timescale

for iron acquisition processes, the half‐life of the exchange reaction should

not significantly exceed the period of elevated phytosiderophore release.

Very slow metal‐ and ligand‐exchange reactions exceeding this reference

timescale could limit iron acquisition.

A classic study of kinetic control of a geochemical process in this context is the double‐exchange reaction of copper between EDTA and humic

substances in the presence of calcium (Hering and Morel, 1989).

CaEDTA ỵ Cuhumate $ CuEDTA ỵ CaÀhumate


The rate of this double‐exchange reaction was significantly slower than

the rate of the corresponding ligand‐exchange reaction in the absence of

calcium (Hering and Morel, 1989):

EDTA ỵ Cuhumate $ CuEDTA ỵ humate


In contrast to EDTA, phytosiderophores and most microbial siderophores (with the known exception of carboxylate siderophores such as

rhizoferrin) (Shenker et al., 1996) have a low aYnity for calcium complexation. Equilibrium calculations of DFO‐B and phytosiderophore speciation

in equilibrium with calcite at high PCO2 and in the absence of competing ions

(other than Hỵ) predict that the calcium complexes are minor species compared to (partially) protonated free siderophore species. In the light of the

observations by Hering and Morel (1989), it appears that the specificity of

siderophores for iron is a prerequisite for their utility in biological iron

acquisition not only from a thermodynamic standpoint but also from a

kinetic perspective.



In the context of strategy II iron acquisition, we will therefore limit the

discussion on exchange reactions of the type:

PS ỵ FeL $ FePS ỵ L


where PS is the phytosiderophore and L stands for organic acids, humic

substances, and microbial siderophores. EDTA is a synthetic aminocarboxylate ligand (like mugineic acids) with similar aYnity if not specificity for

iron as phytosiderophores. In contrast to phytosiderophores, EDTA has

been the subject of many detailed studies of ligand‐exchange reactions.

Therefore, we consider EDTA a useful model compound for phytosiderophores as long as competing transition metal ions or alkaline‐earth metal

ions are not present.

Reaction 15 may proceed via an adjunctive or a disjunctive pathway

(Hering and Morel, 1990):


FeL ! Fe ỵ L

PS ỵ Fe ! FePS


PS ỵ FeL ! PSFeL

PSFeL ! FePS ỵ L

where the disjunctive pathway is initiated by the complete dissociation of

the initial complex and the adjunctive pathway involves the formation of

a ternary complex of the outgoing and incoming ligands with iron.

The rate of ligand‐exchange reactions depends on the nature and concentrations of the incoming and outgoing ligands. Bell et al. (2005a) used

ethylenediamine‐di(o‐hydroxyphenylacetic acid) (EDDHA) as a model

ligand for phytosiderophores to exchange various ligands from their corresponding iron complexes. They found increasing exchange rates in the order

of EDTA < HEDTA < citrate < NTA as outgoing ligand. While exchange

reactions between EDDHA and the Fe–NTA complexes reached equilibrium within 4 h (which approximately corresponds to the timescale of diurnal

phytosiderophore exudation), the reaction with Fe–EDTA complexes was

still far from equilibrium after 2 days of reaction time.

3. Ligand Exchange Between Phytosiderophores

and Microbial Siderophores

Very slow exchange kinetics are observed in reactions involving the release

of iron from microbial siderophores. For example, iron exchange between

equimolar concentrations (1 mM) of DFO‐B and ferrichrome A (both microbial siderophores) at pH 7.4 with a 5% excess of DFO‐B over iron had a half‐

life of more than 1000 h (Tufano and Raymond, 1981)! While the rates



and mechanisms of ligand exchange between DFO‐B and phytosiderophores

are not known, it is instructive to consider the corresponding reactions

involving EDTA. The iron transfer from DFO‐B to EDTA was extensively

studied, as a model for intracellular iron release from siderophores in microbial iron acquisition (Albrecht‐Gary et al., 1995; Birus et al., 1998; Hoărdt

et al., 2000; Monzyk and Crumbliss, 1981, 1983; Tufano and Raymond, 1981)

or for ligand exchange by phytosiderophores (Hoărdt et al., 2000).


FeHDFOBỵ ỵ H2 EDTA2 ỵ Hỵ ! FeEDTA ỵ H4 DFOBỵ


The ligandexchange reaction proceeds via an adjunctive pathway involving the successive exchange of hydroxamate groups and formation of an

intermediate ternary complex, followed by complete exchange of coordination (Birusˇ et al., 1993). Tufano and Raymond (1981) have parameterized a

rate law for this exchange reaction:



k1 k2 FeHDFOỵ ẵHỵ H2 EDTA2


k1 ỵ k2 H2 EDTA2


where Vinitial is the initial rate (M sÀ1), k1 (MÀ1 sÀ1) and kÀ1 (sÀ1) are the

forward and reverse rates of unwrapping/protonation of the first hydroxamate

group of the Fe–DFO‐B complex, and k2 (MÀ1 sÀ1) is the overall rate constant

of the ligand‐exchange reaction. At I ¼ 0.2 M (KNO3), 25  C, and pH 5.4, they

found k–1 ¼ 34 (sÀ1), k–1 ¼ 3.4 (sÀ1), and k2 ¼ 200 (MÀ1 sÀ1).

Emulating rhizosphere conditions, we can use the rate law to calculate

initial ligand exchange rates of approximately 0.01 mM hÀ1, assuming initial

concentrations of 1 mM Fe–DFO‐B complex and 500 mM EDTA at pH 5.4.

Noting that the observed initial rates decreased with increasing pH (Tufano

and Raymond, 1981), we can conclude that the ligand exchange process is

slow compared to timescales of diurnal exudation of phytosiderophores.

Somewhat faster ligand exchange kinetics have been observed for siderophores having catecholate and a‐hydroxycarboxylate functional groups

(Albrecht‐Gary et al., 1995).

Low‐molecular weight ligands, such as mono‐hydroxamates (Monzyk

and Crumbliss, 1983) and oxalate (Birusˇ et al., 1998), can catalyze the

ligand‐exchange reaction between Fe–DFO‐B and EDTA. It may casually

be noted that such low‐molecular weight ligands are present in the rhizosphere and their exudation by iron‐stressed strategy II plants (Fan et al.,

1997, 2001) may serve to increase the rates of ligand‐exchange reactions.

In summary, not only the thermodynamics, but also the rates of ligand‐

exchange reactions under rhizosphere conditions need to be considered in

plant iron acquisition.



Mono‐ and dihydroxamate siderophores can undergo ligand‐exchange

reactions via several pathways (Boukhalfa and Crumbliss, 2002). Hoărdt

et al. (2000) observed fast iron transfer from iron coordinated by a mixture

of mono‐ and dihydroxamate degradation products of coprogen to EDTA

or epi‐HMA, demonstrating that biodegradation of microbial siderophores

facilitates ligand exchange from a thermodynamic and a kinetic standpoint.

4. Ligand Exchange Between Phytosiderophores

and Humic Substances

As discussed earlier, iron bound to water‐soluble humic substances is

taken up by strategy I and II plants (Cesco et al., 2002). Observations of

diurnal variations in uptake by strategy II plants indicate that iron is released

from the NOM by a ligand‐exchange reaction. This is supported by direct

observations of ligand exchange to mugineic acid (Cesco et al., 2000).

Rocha et al. (2002) observed ligand exchange between humic substances

at natural Fe‐concentration levels (350–450 mmol gÀ1) with 1.27 mM diethylenetetraaminepentaacetic acid (DTPA) or 1.71 mM EDTA at pH 4.5. They

found that 55% or 51% of the total iron was DTPA or EDTA available and

more than 8 h were required to reach exchange equilibrium but less than 1 h

was required to exchange 50% of the available pool. Burba and Van den

Berg (2004) used humic substances collected at the same site and used the

same experimental protocol as Rocha et al. (2002) and found EDTA availability of 98% and a release kinetic that was characterized by a fast release

with a half‐life of 50 min in which 90% of the available iron was released, and

a slow step with a half‐life of 650 min. Gu et al. (1995) reacted humic

substances at natural Fe concentration levels (185 mmol gÀ1) with 100 mM

EDTA at pH 4 and found that exchangeable Fe was released in a ligand‐

exchange reaction with a half‐life of 8.2 min.

Considerably slower exchange kinetics were observed at pH 8 using an

immobilized chelator (Burba, 1994). The Fe exchange kinetics was characterized by a fast exchange reaction in which a small percentage of iron was

exchanged during the first 3 h, which was followed by a slow exchange

reaction with a half‐life of about 24 h. About 86% of the total iron was

exchangeable. Nevertheless, these results indicate that ligand‐exchange reaction between humic substances and other ligands is fast enough to make at

least a fraction of this iron pool available for plant uptake.


Ligand Exchange in the Apoplasm

Solutes can freely diVuse into the outer layers of the root cortex, the

apoplasm. DiVusion proceeds in the pore space between plant cells, the



apoplastic space. The central part of the root, including the xylem and

phloem are isolated from the apoplasm by the impermeable casparian

strip. The main route of transport through the casparian strip is by crossing

cell cytoplasmic plasmalemma membrane through membrane‐bound transport systems. Alternatively, diVusion through breaks of the casparian strip

at budding lateral routes or damaged roots may occur.

The apoplasm has a net negative charge and possesses ligating functional

groups resulting in an electrostatic and directional binding of iron. Iron carriers

can release iron to the apoplasm, creating an apoplastic iron pool that can serve

as an iron source to strategy II iron acquisition (Zhang et al., 1991a, 1999). The

apoplastic iron pool decreases during the time of maximum siderophore exudation (in the morning) and recovers until the next morning (Alam et al., 2004).

This indicates that the reference timescale for ligand‐exchange reactions

between iron carriers and the apoplasm is essentially 24 h, whereas the reference

timescale for ligand exchange between phytosiderophores and the apoplasm

corresponds to the period of maximum siderophore release.

A study that has been already discussed earlier investigated the ligand‐

exchange rates between an aquatic Fe–humic acid complex and a receiving

chelating ion exchanger on cellulose basis (Burba, 1994). This system can

serve as a model for apoplastic iron loading by humic substances. They

found that the ligand‐exchange reaction proceeds by a disjunctive mechanism where the release of iron by the humic acid complex was rate determining. The Fe exchange kinetics was dominated by a slow exchange reaction

with a half time of about 24 h. This result seems to indicate that humic

substances may facilitate apoplasm loading from a kinetic standpoint.

Zhang et al. (1991a) investigated apoplasm loading (wheat) by addition

of 5 mM iron to nutrient solutions for 5 h during the dark period. They applied

inorganic iron (i.e., ferrihydrite), and iron complexed to rhodotorulic acid (see

Fig. 5 and Table IV), DFO‐B, and EDDHA, respectively. The resulting

apoplastic iron concentrations and corresponding translocation rates during

the day were high in experiments where iron was supplied in the form of

hydroxide or rhodotorulic acid complex, and low when it was supplied as

either the DFO‐B or EDDHA complex. The relative low eYciency of DFO‐B

and EDDHA complexes for iron nutrition via the apoplasm may reflect their

slow exchange kinetics and/or their high thermodynamic stability.


The exudation of siderophores by chlorotic graminaceous plants is part of

a highly eYcient iron acquisition process. The presence of siderophores in

the rhizosphere triggers a range of complex soil chemical processes including



ligand exchange, dissolution, and transport. In the past, most studies on iron

dissolution have considered the reactions of iron minerals with individual

ligands. New research, however, shows that organic acids and siderophores

may interact synergistically to cause iron dissolution. Equilibrium models

have been very useful for understanding the driving forces and direction for

dissolution and precipitation reactions. Nonetheless, kinetic processes are a

major constraint in the mobilization of iron and exchange between diVerent

ligands. A full understanding of plant iron acquisition requires the detailed

knowledge of the rates and mechanisms of these processes.

In recent years, much progress has been achieved in the understanding of

the molecular biology of iron acquisition. Various transporters for root iron

uptake have been identified and their regulation is a subject of active

research (Bauer et al., 2004; Curie and Briat, 2003; Grotz and Guerinot,

2003; Reid and Hayes, 2003; Schaaf et al., 2004; Schmidt, 2003). Concurrently, applied research in Fe‐deficiency diagnosis and remediation considering agricultural and horticultural management practices have become

critical to the eYciency and economic viability of food production (Abadı´a

et al., 2004; Jolley et al., 2004). The soil is the ultimate target of important

plant physiological iron‐deficiency responses and is a mediator for many

agricultural remediation strategies for Fe deficiency. Therefore, we hope that

this overview of soil chemical aspects of plant iron acquisition will serve as a

useful reference for scientists working in these exciting fields.


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