Indian Journal of Chemistry
Vol. 53A, Aug-Sept 2014, pp. 996-1000
Enhanced interaction of molecular oxygen with amino acid complexes of
silver and gold clusters
Dar Manzoor & Sourav Pal*
Theoretical Chemistry Group, CSIR-National Chemical Laboratory, Pune 411 008, India
Email: s.pal.ncl.res.in
Received 19 April 2014 ; accepted 13 May 2014
Density functional theory calculations have been carried out to study the effect of amino acid adsorption on the reactivity
of silver and gold clusters with molecular oxygen. It is found that the amino acids glycine and alanine form stable
complexes with both Ag4 and Au4 clusters. However, the extent of interaction is more in the case of the Au4 cluster, as
reflected from the increase in binding energy in the amino acid Au4 complexes. Our results confirm that the adsorption of
amino acids glycine and alanine enhances the reactivity of Ag4 and Au4 clusters towards O2 molecule. The enhanced
reactivity of O2 molecule towards the amino acid metal cluster complexes is manifested by an increase in the O2 binding
energy and a decrease in the M–O (M = Ag, Au) bond length. Moreover, it is found that the co-adsorption of amino acids
and O2 molecule on the Ag4 and Au4 clusters is cooperative.
Keywords: Theoretical chemistry, Density functional calculations, Metal clusters, Gold clusters, Silver clusters,
Amino acid complexes
During the last decade, metal clusters of silver and
gold have been the subject of great interest due to
their enhanced catalytic and optical properties1-9. The
facile synthesis and biocompatible conjugation with
various biomolecules, such as proteins and
oligonucleotides makes silver and gold clusters
materials of choice for biochemical sensing and
detection10-13. A number of experimental studies have
been devoted to the mechanism of interaction of gold
and silver nanoparticles with proteins and
oligonucleotides which finds various kinds of
applications in the growing field of nanotechnology.
The interaction of metal clusters of gold and silver
with proteins and oligonucleotides forms the basis of
a number of diagnostic techniques, such as surface
plasmon resonance spectroscopy (SPRS), surface
enhanced
Raman
spectroscopy
(SERS),
electrochemical and colorimetric detection14-25.
Protein and oligonucleotide molecules adsorbed on
gold and silver clusters, and thin films of these metal
atoms are known to have applications in the fields of
electronic chips, sensors, drug delivery and so on26-31.
Further, the amino acid molecules and peptides have
been used for the stabilization and controlled
synthesis of nanoclusters of gold and silver32-34.
The interaction of the building blocks of the
proteins, i.e., amino acids with metal clusters of gold
and silver is of essential importance keeping in mind
the diverse applications of these systems. In this
regard, recently a number of theoretical works using
density functional theory (DFT) have been carried out
to study the interaction of various amino acids with
silver and gold clusters. Pakiari et al.35 studied the
interaction of cystine and glycine amino acids with
the neutral, cationic and anionic Ag3 and Au3 clusters.
The authors found that the two major factors which
govern the bonding between the metal clusters and
amino acids are (i) the M–X (M = Au, Ag and X = N,
O, S) anchor bonds, and, (ii) the nonconventional
X–H….M (M = Au, Ag, and, X = N, O, S) hydrogen
bonds. Also, it was shown by the same authors that
the amino acids interact more strongly with anionic
Au3 and Ag3 clusters as compared to the cationic and
neutral clusters. Fang & co-workers36 elucidated the
interaction between Aun clusters (n = 3, 4) and
selected amino acids (glycine and cysteine) using first
principles calculations with particular emphasis on the
structures of the gold cluster amino acid complexes.
The authors showed using natural bond orbital (NBO)
analysis that the lone pair electrons on the sulfur,
nitrogen and oxygen atoms in the case of various
conformations are transferred to the antibonding
orbitals of the gold. Similarly, Rai et al.37 studied the
interaction of proline with the gold trimer and found
that the Au-N and a non-conventional O-H…Au
hydrogen bond leads to the stabilization of the
MANZOOR & PAL.: INTERACTION OF O2 WITH Ag4 & Au4 CLUSTER COMPLEXES OF AMINO ACIDS
complex. Recently, the effect of the size of the
clusters on the interaction of the gold clusters with
cysteine thiol and cysteine thiolate was studied by
Su & co-workers38. It was found that the cysteine thiol
interacts with one Au-S bond with all the gold
clusters, whereas, cysteine thiolate experiences
evolution from one to two Au-S bonds with increase
in the number of atoms in the cluster. Further, the
authors showed that binding strength of the cysteine
thiolate gold cluster complexes is more as compared
to that of the cysteine thiol gold cluster complexes.
Although the structure, electronic and bonding
properties of amino acid complexes of gold and silver
clusters have been studied, to the best of our
knowledge the effect of the adsorption of the amino
acids on the reactivity of gold and silver clusters has
not been explored. In the current study, we have
highlighted the adsorption effect of the alanine and
glycine amino acids on the reactivity of Ag4 and Au4
clusters with the oxygen molecule using density
functional theory.
Computational Details
All the calculations were carried by using Gaussian 09
package39. Density functional theory (DFT) with
PBEPBE functional was utilized to optimize the
geometries of the various structures. For the gold and
silver atoms, the LANL2DZ+ECPs40,41 basis set and
for the rest of the atoms the cc-pvDZ basis set was
used. The geometry optimizations were carried out
using the Berny algorithm with the default
convergence criterion. Following the geometry
optimizations, frequency calculations were performed
for all the structures to confirm that the optimized
structures correspond to the true minima. The amino
acid and O2 adsorption was studied at all the possible
sites and in various modes on the Ag4 and Au4
clusters. The amino acid and O2 binding energies
were calculated as the difference between the energies
of the constituents and their complex. The basis set
superposition (BSSE) was not considered while
calculating the binding energies as it has been shown
that the BSSE has a negligible effect on the binding
energies of small molecules on the silver and gold
clusters42,43.
Results and Discussion
Interaction of glycine and alanine with Ag4 and Au4 clusters
We begin with a discussion on the binding and
adsorption of the amino acids glycine and alanine on
the silver and gold clusters. There can be several
997
possible modes of interaction between the amino
acids glycine and alanine with various sites of Ag4
and Au4 clusters involving Ag/Au-N or Ag/Au-O
anchor bonds, and a nonconventional hydrogen bond.
We optimized a number of geometries for each amino
acid metal cluster complex and the two lowest energy
structures for each amino acid metal cluster complex
are shown in the Fig. 1. The optimized geometries
show that the lowest energy isomer in all the cases is
the one which involves a M–N bond and the first high
energy isomer is the one where the metal interacts
with carbonyl oxygen atom of the amino acids. The
computed binding energies and the relevant
geometrical parameters for the amino acid complexes
of Ag4 and Au4 clusters, such as M–X bond lengths
and M–X–C bond angles ( M = Ag, Au, and X = N, O)
are shown in the Table 1. The positive value of
binding energies for all the complexes of silver and
gold clusters shows that the complexes are stable. The
obtained binding energies of the amino acid Au4
complexes are higher as compared to amino acid Ag4
complexes. The higher value of binding energies
(see Table 1) for the amino acid Au4 complexes
indicates that the Au4 cluster has a strong affinity for
the amino acids glycine and alanine and hence forms
stable complexes as compared to the Ag4 cluster. The
small Au-X bond lengths in the amino acid Au4
cluster complexes also reflects this strong
complexation with respect to the Ag4 cluster. We have
also calculated the net NBO charges on the clusters
and the amino acid in all the complexes. It can be
seen from Table 1 that there is a charge transfer from
Table 1—Optimized geometrical parameters such as M–X bond
lengtha, M-X-C bond angleb and binding energies (Eb) of the Ag4
and Au4 cluster complexes of glycine and alanine amino acids
System
Eb
(eV)
Ag4 cluster complexes
Glycine-Ag4(I)
Glycine-Ag4(II)
Alanine-Ag4(I)
AlanineAg4(II)
1.05
0.94
1.07
0.95
M-X M-X-C
(Å)
(º)
2.31
2.32
2.30
2.32
115.2
121.0
115.0
120.3
qaac
qclusterc
+0.14
+0.07
+0.15
+0.07
-0.14
-0.07
-0.15
-0.07
+0.23
+0.14
+0.25
+0.14
-0.23
-0.14
-0.25
-0.14
Au4 cluster complexes
Glycine-Au4(I)
1.54
2.18 115.7
1.19
2.23 120.1
Glycine-Au4(II)
Alanine-Au4(I)
1.59
2.17 116.7
Alanine-Au4(II)
1.17
2.23 121.2
a
M = Ag or Au; bX =N or O;
c
qaa= net positive charge on the amino acids;
d
qcluster= net negative charge on the M4 cluster.
998
INDIAN J CHEM, SEC A, AUG-SEPT 2014
Fig. 1—Optimized geometries of amino acid (glycine and alanine) complexes of Ag4 and Au4 clusters.
the amino acids to the Ag4 and Au4 clusters. Further,
the amount of net charge transfer is almost two times
more in the case of Au4 cluster as compared to the
Ag4 cluster. This again shows the strong interaction
between the amino acids, glycine and alanine, and the
Au4 cluster as compared to the Ag4 cluster. Thus, the
binding energies, M–X bond lengths and NBO charge
analysis confirm that the amino acids glycine and
alanine undergo stronger complexation with Au4
cluster as compared to the Ag4 cluster.
Reactivity of molecular oxygen with the amino acid complexes
of Ag4 and Au4 clusters
In this section, we have looked at the reactivity of
the amino acid metal cluster (Ag4 and Au4) complexes
with molecular oxygen and compared our results
with the parent metal clusters (i.e. Ag4 and Au4).
The optimized geometries of the O2 adsorbed lowest
energy structures of the parent metal clusters and the
amino acid metal cluster complexes are shown in the
Fig. 2. Looking at Fig. 2, we note that the O2
molecule adsorbs in atop mode with a single M–O
bond in all the structures. Table 2 enlists the binding
energies and relevant geometrical parameters such as
M–N, M–O and O–O bond lengths (M = Ag, Au).
It can be seen from Table 2 that the binding energy of
the O2 molecule increases in the case of the amino
acid metal cluster complexes with respect to the
Table 2—Optimized geometrical parameters such as M–N, M–O
and O–O bond lengthsa and O2 binding energies (Eb) of the Ag4
and Au4 cluster complexes of glycine and alanine amino acids
System
Eb
(eV)
M-N
(Å)
M-O
(Å)
O-O
(Å)
qO2
0.23
2.32
1.26
-0.22
Ag4O2
Au4O2
0.31
2.40
1.24
-0.10
0.48
2.29
2.38
1.27
-0.27
Glycine-Ag4O2
Alanine-Ag4O2
0.45
2.29
2.40
1.27
-0.26
Glycine-Au4O2
0.36
2.18
2.31
1.25
-0.17
0.40
2.17
2.29
1.26
-0.16
Alanine-Au4O2
a
M = Ag or Au; bqO2= net negative charge on the O2 molecule in
the various complexes.
parent Ag4 and Au4 clusters. The decrease in the M–O
and increase in the O–O bond lengths also depicts the
strong interaction of molecular oxygen with the amino
acid metal cluster complexes. The NBO charge
analysis shows an increase in the charge transfer from
amino acid metal cluster complexes to the O2
molecule and hence shows the same trend as the
binding energy and structural analysis. Also
interestingly, it can be seen from Table 2 that there is
a decrease in the M–N bond distances upon O2
adsorption, thereby suggesting that the binding of O2
and the amino acids glycine and alanine can be
cooperative rather than competitive on the silver and
MANZOOR & PAL.: INTERACTION OF O2 WITH Ag4 & Au4 CLUSTER COMPLEXES OF AMINO ACIDS
999
glycine and alanine amino acids is stronger with the
gold tetramer than with the silver tetramer. The strong
interaction of the amino acids with the Au4 cluster is
confirmed by an increase in the binding energy of
amino acid gold cluster complexes and a decrease in
the Au-N distances as compared to the amino acid
silver cluster complexes. To highlight the effect of
amino acid complexation on the reactivity of the Ag4
and Au4 clusters, we explored the interaction of the O2
molecule with these complexes. Interestingly, it was
found that the binding energy of both the amino acids
and molecular oxygen show an increase when
adsorbed on the clusters together. Thus, the
co-adsorption of the amino acids (glycine and alanine)
and molecular oxygen is cooperative rather
competitive on the Ag4 and Au4 clusters.
Acknowledgement
The authors acknowledge the grant from CSIR
XIIth Five Year Plan project on Multi-scale
Simulations of Material and facilities at the Centre of
Excellence in Scientific Computing at NCL, Pune.
DM thanks the University Grants Commission
(UGC), India, for a Senior Research Fellowship. SP
acknowledges the DST J C Bose Fellowship and
CSIR SSB grant towards completion of the work.
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Fig. 2—Optimized geometries for O2 adsorption on the Ag4 and
Au4 clusters and their complexes with glycine and alanine amino
acids.
gold clusters. In order to confirm this, we computed
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increase by 0.10–0.15 eV, thus confirming the
cooperative adsorption of the amino acids and
molecular oxygen on the Ag4 and Au4 clusters.
Conclusions
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