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CATTOD-8845; No. of Pages6
ARTICLE IN PRESS
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Z. Csendes et al. / Catalysis Today xxx (2014) xxx–xxx
2. Experimental
2.5. FT-IR spectroscopy
2.1. Materials and method of synthesis
Structural information on each step of the synthesis proce-
dure was obtained by far- and mid-range infrared spectroscopy.
Mid-range spectra were recorded with a BIO-RAD Digilab Divi-
sion FTS-65A/896 FT-IR spectrophotometer with 4 cm−1 resolution,
measuring diffuse reflectance. The 3800–600 cm−1 wavenumber
range was investigated. 256 scans were collected for each spec-
trum.
For the syntheses, C-protected (in form of methylester)
l-histidine, l-cysteine and l-cystine, MnCl2·5H2O and chloropropy-
lated silica gel (SG – particle size: 230–400 mesh, BET surface area:
as the 2-propanol solvent were the products of Aldrich Chemical Co.
All the chemicals were of analytical grade and were used without
further purification.
Even though the preparation method of these types of anchored
complexes have been described previously [13,14], for the sake of
easier tracking the followings, let us briefly repeat the main points,
and give the notations.
First, the support was covalently grafted via an N-alkylation
reaction by the C-protected amino acids. Then, it was treated
with Mn2+-containing solution and the solid material was isolated.
At his point, a substance was in our hands that only contained
surface-anchored ligands. This is called a surface-grafted complex
synthesised under ligand-poor conditions, and will be referred
as SG–C-protected amino acid–Mn(II). If this material was fur-
ther treated in a solution containing excess amount of amino
acids, then a surface-grafted complex was obtained under ligand-
excess conditions, and will be referred as SG–C-protected amino
acid–Mn(II)–C-protected amino acid.
Far-range spectra were recorded with
a BIO-RAD Digilab
Division FTS-40 vacuum FT-IR spectrophotometer with 4 cm−1 res-
olution. 256 scans were collected for each spectrum. The Nujol mull
technique was used. The 500–200 cm−1 wavenumber range was
investigated. Spectra were evaluated by the Win-IR package. They
was helped either by published mid IR results [19,20] and our own
freshly published experimental work concerning the far IR spectra
of purpose-built manganese complexes having uniform coordinat-
ing groups [21].
2.6. Testing the catalytic activity
Superoxide dismutase activity was tested by the Beauchamp-
Fridovich reaction [22]. For this biochemical test reaction riboflavin,
l-methionine, and nitro blue tetrazolium (NBT) were applied as
was described previously [13] with the exception that in this case
HEPES buffer was used. During their reaction, superoxide radi-
cal anions are formed, which gives blue adducts with NBT. When
the enzyme mimic is present, it dismutates the superoxide radical
anion, the photoreduction of NBT is inhibited (its measure is IC50
in mM), i.e. the enzyme mimic works the better when the colour
change (measured by the absorbance), i.e. the value of IC50, is the
smaller.
2.2. Analytical measurements
The amount of metal ions on the surface modified silica gel was
measured by an Agilent 7700× ICP-MS. Before measurements, the
anchored complexes were digested by adding cc. H2SO4 then they
were diluted with distilled water and filtered.
The nitrogen content of the samples was determined by the
Kjeldahl method.
2.7. Catalytic oxidation of cyclohexene
2.3. EPR measurements
In the reaction a vial capped with septum was loaded with
the catalyst (25 mg), acetone (10 cm3), cyclohexene (5 mmol) and
peracetic acid (1 mmol). After 1 h continuous stirring at room tem-
perature (298 K), the mixture was analysed quantitatively by GC
using an Agilent HP-1 column. The products were identified by
GC–MS.
microwave power 12 mW, modulation amplitude 5 G, modula-
tion frequency 100 kHz) in quartz EPR tubes. All recorded EPR
by the EPR [15] computer program. Analysis of the obtained
spectra was significantly helped by the information published con-
cerning the EPR spectra of various Mn(II)–amino acid complexes
[16].
3. Results and discussion
3.1. Analytical measurements
2.4. X-ray absorption measurements
The results of the Kjeldahl method and the ICP–MS measure-
ments are shown in Table 1.
The measurements were carried out on the K-edge of the man-
multipole wiggler beamline equipped with a water-cooled chan-
nel cut Si(1 1 1) double crystal monochromator delivering at 10 keV,
approximately 2 × 1015 photons/s/0.1% bandwidth with horizontal
and vertical FWHM of 7 and 0.3 mrad, respectively [17]. A beam-
size of 0.5 mm × 1.0 mm (width × height) was used. The incident
beam intensity (I0) was measured with an ionisation chamber
filled with a mixture of He/N2. Higher order harmonics were
reduced by detuning the second monochromator to 70% of the
maximum intensity. Data collection was performed in transmis-
sion mode. The samples were contained in Teflon spacers with
Kapton tape windows according to the manganese concentra-
tion. The data were treated by the Demeter program package
[18].
Covalent grafting of the amino acids was successful in all cases.
The conversion of the immobilisation is ranged from 57 to 86%,
since the chloropropylated silica gel contained 0.705 mmol/g active
chlorine. The metal ion to amino acid ratio under ligand-poor
conditions is 1:1.5 for SG–His-OMe–Mn(II) suggesting that com-
plexes formed on the surface are not uniform, there are complexes
having 1:1 as well as 1:2 metal ion to ligand ratios. For SG–Cys-
OMe–Mn(II) this ratio is 1:1 and for SG–(Cys-OMe)2–Mn(II) is
1:1.1, meaning that mainly 1:1 complexes are formed. Under
ligand-excess conditions, this ratios are 1:5.4, 1:3.2 and 1:2.5
for SG–His-OMe–Mn(II)–H-His-OMe, SG–Cys-OMe–Mn(II)–H-Cys-
OMe and SG–(Cys-OMe)2–Mn(II)–H-(Cys-OMe)2, respectively. It
seems that in all cases mixture of complexes with varying coor-
dination numbers were formed on the surface of the support.
Please cite this article in press as: Z. Csendes, et al., Synthesis, structural characterisation, and catalytic activity of Mn(II)–protected amino acid