Angewandte
Chemie
The catalytic reactivity of the biomimetic materials likely
follows an atom-leaching mechanism (Scheme 1); during the
initial oxidative addition step, Pd2+ is abstracted from the
surface to drive the reaction, which is controlled by the
peptides.[13–15] Under this process, as discussed by Astruc and
co-workers, if the peptides played no role in the reactivity,
regardless of slight particle size differences, the TOF values
should be constant for the different particles;[13,14] however,
they are drastically different, which suggests that the surface
peptides modulate the reactivity. For instance, when dendri-
mer-based palladium nanoparticles of similar sizes to the
peptide-capped particles were used, equivalent TOF values
were observed, which is attributed to the lack of involvement
of the dendrimer passivant in the reaction.[13,14] For the
biomimetic materials, the peptide sequence is critically
important to the overall TOF value. The replacement of
histidine at position 6 improves the catalytic activity while
histidine at position 11 is required for generating highly
reactive nanoparticles. Furthermore, since nanoparticles of
similar sizes were prepared, the number of moles of surface
palladium atoms is similar, thus suggesting that the major
difference between the particles is the biotic/abiotic interface,
which is controlled by different peptide binding motifs. As the
atom-leaching method would likely alter particle morpholo-
gies, we attempted to observe the materials by using TEM
after the reaction. Unfortunately, at the very dilute nano-
catalyst concentrations that were employed in these reactions,
we were unable to observe any nanoparticles on the TEM
grid. From this, two peptide-mediated events are possible to
modulate the ability to abstract palladium atoms from the
nanoparticle surface: First, the surface structure may be such
that the orientation of the A6 peptide maximally exposes the
palladium surface, thus enhancing the initial oxidative
addition step and releasing Pd2+ faster. As a result, more
palladium would react in a shorter time to result in higher
TOF values. Second, by removal of specific histidine residues,
other residues are likely to bind to the surface to maintain
particle stability. As such, the electronic character of the
palladium surface could vary based upon the individual
binding, which is known to inhibit the initial oxidative
addition at the particle surface.[13] This would result in
varied TOF values as a function of the electronic effects of
the binding motifs of the peptides. At present, we are unable
to fully distinguish between these events; however, both are
controlled by the peptide.
Figure 3. Effect of the different peptides on the reactivity of the
nanocatalyst for a) palladium loading versus product yield and b) turn-
over frequency (TOF; y-axis units=mol BPCA (molPd)À1 hÀ1
.
Analysis of the material prepared with the A6 peptide found
that the loading to achieve quantitative yield shifted to
0.001 mol%. Interestingly, with the other nanoparticles
passivated with the A11 and A6,11 peptides, higher catalyst
loadings of 0.01 mol% palladium were required for the
reaction to reach completion in 24 hours.
Studies probing the turnover frequency (TOF) of the four
systems were also performed. For these studies, the reactions
were scaled up and then aliquots were extracted and
quantified at various time intervals; the average of triplicate
samples are shown in Figure 3b. For the materials prepared
using the Pd4 peptide, a TOF value of 2234 Æ 99 mol BPCA
(molPd)À1 hÀ1 was observed. The TOF values are calculated
using the total palladium concentration, consistent with
previous studies.[12–14] A TOF of 2234 is slightly lower than
previously reported;[3] this is likely due to changes in the
reaction conditions. Surprisingly, analysis of the A6-based
materials demonstrated a TOF of 5224 Æ 381 mol BPCA
(molPd)À1 hÀ1, which corresponds to a greater-than-twofold
increase in reactivity. When the A11 sample was probed, a
decrease in the TOF was observed to 1298 Æ 107 mol BPCA
(molPd)À1 hÀ1, which was further decreased to 361 Æ 21 mol
BPCA (molPd)À1 hÀ1 for the A6,11-derived nanoparticles.
The catalysis and TEM data suggest that the peptide
sequence on the particle surface controls both the structure
and reactivity of the palladium nanoparticles. The Pd4
sequence was optimized for palladium surface binding,
which has been suggested to occur mainly through the
histidine residues.[9] Alanine substitution of either of the
histidine units demonstrated minimal changes in particle size,
within the error of the measurement, which indicates that
other amino acids in the sequence, asparagine and arginine,
may be involved in surface binding to maintain the particle
size and stability. When both histidine residues are replaced
with alanine, the particle size marginally increased, which also
supports binding through the other residues. Once the
nanoparticles are decorated with the different sequences on
the surface, different confirmations and arrangements are
possible based upon the individual binding capabilities. As
such, changes to the palladium surface may occur, which
could result in the different degrees of catalytic reactivity
observed. This theory is supported by the CD spectra of the
different peptides on the nanoparticle surface, which adopted
different structures based upon the individual sequence.
In summary, we have demonstrated significant modula-
tions in the reactivity of biomimetic nanomaterials by subtle
modifications to the peptide sequence. This suggests that the
peptide and its binding effects control the functionality of the
nanomaterials. Peptides that are isolated by phage display[1]
are optimized for binding. Whilst this is useful for structural
stability, such attributes may cause a decrease in activity. The
results presented here indicate that by altering the sequence,
particle stability may be maintained with desirable increases
in nanoparticle reactivity, which could be used as a basis for
the rational design of optimized peptide sequences.
Received: December 9, 2009
Published online: April 15, 2010
Angew. Chem. Int. Ed. 2010, 49, 3767 –3770
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim