ANALYSIS OF NICOTIANAMINE METAL COMPLEXES BY ELECTROSPRAY-MASS
SPECTROMETRY
R. Rellán-Álvarez, A. Álvarez-Fernández, J. Abadía.
Department of Plant Nutrition, Experimental Station Aula Dei-CSIC, Avd. Montañana 1005,
50059, Zaragoza, Spain.
The non-proteinogenic amino acid nicotianamine (NA) is ubiquitous among plants and is
supossed to be involved in micronutrient transport including Fe. The distances between the
groups of NA molecule are optimal for the formation of chelate rings where six functional
groups allow octahedral coordination with metals ions such as Cu, Zn, Fe or Ni. However,
most of the evidences supporting the micronutrient-NA complex formation in plant fluids are
based in indirect measurements or software predictions. Investigation in this area should
include direct determination of the possible metal-nicotianamine complex (Hider et al., 2004).
An example of this approach was the determination of the NA-Ni complex in a nickel
hypperaccumulator by mass spectrometry (Vacchina et al., 2003) or the recent determination
of metal-NA complexes by liquid chromatography mass spectrometry (Xuan et al., 2006).
The aim of our work was to study the formation of metal-NA complexes at different pH values
by electrospray-mass spectrometry (ESI-MS) determination. in order to understand the role
that NA may be playing in plant fluids micronutrient speciation.
NA complexes with Mn, Fe(II), Fe(III), Ni, Cu, Zn and Cd were prepared by mixing appropriate
amounts of NA and metal chloride solutions in ammonium acetate or ammonium bicarbonate
100 mM at different pH values. Concentration of metals and NA were between 5-150 µM.
Then, metal complexes were diluted with acetonitrile.
Samples were injected with a syringe pump in a ESIMS Time Of Flight apparatus (BiotTOF II, Bruker
Daltonics) in negative ion mode.
Metal complex can be easily observed with mass
charge ratio and isotopic distribution matching the
theoretical values (Fig 1). Metal-NA complexes are
affected by pH. Some metals like Mn is bonded to NA
above pH 7 while Cu is complexed on the whole pH
range studied (2.5-8.5). Fe(II)-NA was formed over pH
5 as it was predicted with software simulation (von
Wiren, 1999). Fe(III)-NA,is hardly ionized because the
complex is neutral while divalent metals form negative
complexes ions. On the other hand metal exchange
experimentes showed that Cu was able to break
others NA-metal complexes.
In conclusion our work sheds light about the stability of
the different NA-metal complexes, their possible
interactions and the possibility to analyze real plant
samples.
Fig 1. ESI-MS spectra of Fe(II)-NA complex.
References:
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Vacchina V, Mari S, Czernic P, Marques L, Pianelli K, Schaumloffel D, Lebrun M, Łobinski R.
Anal. Chem. 2003; 75: 2740.
von Wiren N, Klair S, Bansal S, Briat JF, Khodr H, Shioiri T, Leigh RA, Hider RC. Plant
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