Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

34j 2 Physicochemical Approaches to Studying Plant Growth Promoting Rhizobacteria
Figure 2.7 Emission Mössbauer spectra of
cation-free glutamine synthetase (GS; E2.2)
from A. brasilense Sp245 incubated with 57 Co 2þ
for 60 min at ambient temperature (a) in rapidly
frozen aqueous solution and as a dried solid
(b) (measured at T = 80 K; intensities converted
to the absorption convention). For each
spectrum, the relevant subspectra (quadrupole
doublets) are shown which contributed to
the resulting spectrum (solid-line envelope)
obtained by computer fitting to the
experimental data (points with vertical error
bars). The positions of the spectral
components (quadrupole doublets) are
indicated by horizontal square brackets
above the zero lines.
The relatively low IS values for both the 57 Co II forms in GS with E2.2 (IS = 1.05 and
1.08 mm s 1 at T = 80 K; see Figure 2.8) may indicate a tetrahedral symmetry of
cobalt(II) coordination. In this case, the coordination mode of all the Glu residues
must be monodentate, which is often observed for cation-binding sites in metalloproteins
[46] (note that there is also at least one water molecule as a ligand, according
to Eads et al. [43]). Similarly, low IS values were found for EMS spectra of dilute
frozen aqueous solutions of 57 Co II complexes with amino acids (anthranilic acid and
tryptophan; IS = 1.1 and 0.9 mm s 1 , QS = 2.7 and 2.8 mm s 1 , respectively), also
assuming a tetrahedral symmetry [49]. Note that tetrahedral coordination (Td) of