Highlights
dium surface. In contrast, when strongly binding cysteine is lo-
cated at the end of the chain (C11 and A6-C11, entries 3 and
5
) the rest of the peptide may freely bind to the surface.
Indeed, the shapes of C6 and C6-A11, as well as C11 and A6-
C11, are very similar. Finally, C6,11 has a hybrid structure with
characteristics of C6 and C11.
Catalyst performance is in correlation with the structural fea-
tures of catalysts modulated with varied peptides. Activities of
samples modified with C6, C6,11, and C6-A11 in Stille cou-
plings were found to be approximately twice that of the native
Pd/Pd4 (compare entries 2, 4, and 6 with entry 1 in Table 2). In
all three cases strongly binding cysteine occupies position 6 in
the peptide. Pd catalysts capped with C11 and A6-C11 exhibit
an even more remarkable three-fold increase (Table 2, entries 3
and 5). Notably, catalyst Pd/A6,11, with alanine at positions 6
and 11, afforded the lowest TOF value of 361Æ21 (Table 1,
entry 4). Clearly, there is a trend in catalytic activities depend-
ing on the position of cysteine. Namely, modification of pep-
tides with cysteine in position 11 with a weakly binding alanine
or histidine residue at position 6 provides the highest activities,
followed by peptides with cysteine in position 6 (Table 2,
Figure 2). It is worth noting that there is no correlation be-
Figure 1. Stille reaction of 4-iodobenzoic acid and phenyltin trichloride.
a) Proposed binding of peptides Pd4, A6, and A11 to a spherical Pd particle.
Black spheres represent histidine. b) TOF of Stille coupling for catalysts pre-
pared with selected peptides.
sample (Pd/A6), the peptide may expose an excessively large
open metal surface, thereby promoting oxidative addition (Fig-
II
ure 1a). As a consequence of the fast release of Pd ions faster
reactions are achieved. Samples with altered amino acid resi-
dues that stabilize particles are characterized by modified (de-
creased) electronic character. This disfavors oxidative addition
and, consequently, hinders the release of active metal spe-
[
8]
cies.
The idea of modulating catalyst performance was further
tested by synthesizing a new set of peptides with cysteine (C)
[9]
substituted for histidine in peptide Pd4 (Table 2). Interesting-
[
a]
Table 2. Peptide sequences and characteristic data.
Entry Peptide Peptide
sequences
DG
[kJmol
Particle size TOF
À1
À1 [b]
]
[nm]
[h ]
1
2
3
4
5
6
Pd4
C6
C11
C6,11
TSNAVHPTLRHL À33.0Æ0.5 2.0Æ0.4
TSNAVCPTLRHL À35.9Æ0.9 2.2Æ0.3
TSNAVHPTLRCL À35.1Æ1.2 2.4Æ0.4
TSNAVCPTLRCL À35.5Æ1.2 2.3Æ0.4
2234Æ99
3963Æ28
6138Æ55
3974Æ280
6097Æ65
4147Æ340
A6-C11 TSNAVAPTLRCL À32.7Æ0.8 2.4Æ0.4
C6-A11 TSNAVCPTLRAL À36.5Æ1.3 2.4Æ0.4
[
a] Reaction conditions: see Scheme 1. [b] Calculated with the amount of
Pd in solution.
Figure 2. Reactivity of Pd/peptide catalysts in the Stille reaction modulated
by peptide sequences. Reprinted (adapted) from Ref. [9]. Copyright (2015)
American Chemical Society.
ly, the particle size of the six peptide-capped catalysts showed
only minimal changes (2.0–2.4Æ0.4 nm) including samples A6-
C11 and C6-A11 with both histidine residues replaced
(
Table 2). This finding is unexpected because significantly dif-
ferent binding affinities were measured for these two peptides
Table 2, DG, entries 5 and 6). Binding affinities, consequently,
tween peptide–Pd affinity, particle size, and catalytic reactivity.
Consequently, the results summarized herein suggest that cat-
alyst performance is controlled by peptide sequence and sur-
face binding. In other words, peptides modulate catalytic activ-
ities and the major difference between samples lies in the
biotic–abiotic (biomolecule–nanoparticle) interface. Specifically,
both CD and computational modeling indicated increased sur-
face flexibility for peptides C11 and A6-C11. This certainly con-
tributes to enhanced reactivity by exposing more surface
atoms to the reacting haloarene. Flexibility facilitates the
atom-leaching process, thereby increasing the concentration of
active metal species in solution. Overall, localized binding ef-
fects, rather than global peptide interactions, seem to have
a determining role in catalytic activity.
(
do not appear to be the determining factor in the size of Pd
particles. Rather, the function of peptide binding motifs and
surface-recognition elements, for example, interaction between
the metal and the sulfur atom of cysteine, may play the domi-
nant role. Indeed, additional crucial pieces of information ac-
quired by circular dichroism (CD) spectroscopy and molecular
modeling revealed the conformation of peptides upon deposi-
tion of palladium depends on the position of cysteine. In Pd/
C6 and Pd/C6-A11 the thiol group in the cysteine residue in
the middle of the peptide anchors the peptide to the surface
(
Table 2, entries 2 and 6). This, together with proline (P) incor-
porated at position 7 with increased steric demand results in
a significantly decreased flexibility of the peptides on the palla-
The focal point of these investigations and a clear highlight
of the efforts made by Knecht and co-workers is the synthesis
ChemCatChem 2015, 7, 2025 – 2027
2026
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