Synthesis of N,N-dimethylaminopyrene-modified short peptides for chemical photocatalysis

Article abstract of DOI:10.1002/psc.2966

The synthesis of peptide-based photocatalysts that use 1-N,N-dimethylaminopyrene as chromophore and their application in photocatalysis is reported. The copper(I)-catalyzed alkyne-azide cycloaddition was applied as key step to prepare the peptide–pyrene c

Full text of DOI:10.1002/psc.2966

Rapid Communication
Received: 3 November 2016
Revised: 14 December 2016
Accepted: 15 December 2016
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/psc.2966
Synthesis of N,N-dimethylaminopyrene-modified
short peptides for chemical photocatalysis
Sergej Hermann and Hans-Achim Wagenknecht*
The synthesis of peptide-based photocatalysts that use 1-N,N-dimethylaminopyrene as chromophore and their application in
photocatalysis is reported. The copper(I)-catalyzed alkyne-azide cycloaddition was applied as key step to prepare the peptide–
pyrene conjugates in quantitative yields for different short peptide sequences. The photocatalysts were evaluated for the
nucleophilic addition of methanol to 1,1-diphenylethylenes to products with Markovnikov-type orientation The short peptides
contain arginine as substrate binding site during photocatalysis, and thus, the reaction was performed without the additive
triethylamine that was previously applied as electron shuttle. Full conversion of the substrate and good yields for the addition
product were achieved. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.
Keywords: arginine; cycloaddition; irradiation; Styrene; UV light
Chemical photocatalysts are organic dyes or transition metal
complexes that efficiently couple the physical process of light
absorption to a chemical reaction by means of time, space, and
energetics, especially by photoinduced transfer of energy or
electrons [1–8]. The principal problem for this type of photochemis-
try was the use of visible light, provided by sunlight as an essentially
unlimited and thereby ‘green’ natural light source or LEDs as cheap
and energy-saving artificial light sources. This problem has been
solved by photoinduced electron transfer instead of energy transfer
processes, commonly named as photoredox catalysis, a research
field that has been established over the past decade [1]. The ‘work-
ing horse’ for photoredox catalysis is mainly [Ru(bpy)3]2+ [9,10], but
nowadays, also organic compounds like eosin Y [11] and 9-mesityl-
10-methyl-acridiniumperchlorat [8,12] are applied to strengthen
the sustainability by avoiding transition metal complexes. We
recently published the photocatalysis of the nucleophilic addition
of methanol and other alcohols to 1,1-diphenylethylene (1) and
other styrene derivatives to products with Markovnikov-orientation
and anti-Markovnikov-orientation by 1-(N,N-dimethylamino)pyrene
(Py) and 1,7-dicyano-perylene-3,4:9,10-tetracarboxylic acid
bisimide, respectively [13,14]. Especially with this perylene bisimide
derivative as photocatalyst, the yields of nucleophilic methanol ad-
dition to styrene derivatives were higher than those obtained with
9-mesityl-10-methyl-acridiniumperchlorat as photocatalyst [12].
One of the major drawbacks of these photocatalytic conversions
was the use of additives, especially triethylamine, as electron shuttle
to promote the efficiency of forward and backward electron trans-
fers. In order to avoid these additives, it looked reasonable to design
short peptides with binding sites that fix the substrates for the
electron transfer process during photocatalysis. Short peptides
have been successfully applied for enantioselective catalysis, mainly
by Miller et al. [15] and by Wennemers et al. [16] but not yet for
chemical photocatalysis. Moreover, short peptides have the advan-
tage that they are soluble both in polar organic solvents and in
aqueous solutions. Herein, we report the synthesis of short peptides
modified with Py and the first elucidation of their photocatalytic
activity with respect to nucleophilic additions to styrenes in the
Markovnikov orientation.
Py as photocatalyst has an oxidation potential of E = 0.91 V
(vs. NHE), together with the singlet state energy of E00 = 3.1 V; the
excited state has a potential of approximately E* = À2.2 V, which
is sufficiently high to reduce styrene derivatives [17]. Moreover, Py
has the highest extinction in the range around 350–370 nm and
hence allows to apply 369 nm LEDs as cheap and reliable light
source. In order to attach this pyrene chromophore to short
peptides, it looked reasonable to apply the copper(I)-catalyzed
alkyne-azide cycloaddition (CuAAC) because this gives an easy
and flexible access to a variety of peptide conjugates. A commonly
applied amino acid for this type of bioconjugation is β-azido-L-
alanine (1, in the Boc-protected form) that can be synthesized from
L-serine in two steps according to literature [18]. The chromophore
building block 2 for the CuAAC was designed as the pyrene
derivative with N,N-dimethylamino and ethynyl substituents in the
1 and 6 positions, respectively. The synthesis of this building block
2 (Scheme 1) started with methylation of 1-aminopyrene (3) by
methyl iodide in 98% yield. Treatment with N-bromosuccinimide
gave bromination of 4 at the 6-position in 96% yield. Pyrene 5
was coupled to trimethylsilyl-acetylene by
a Pd-catalyzed,
Sonogashira-type reaction. After immediate cleavage of the
trimethylsilyl group by tetrabutylammonium fluoride, the building
block 2 was achieved in 96% yield over two steps.
Initially, we synthesized the Boc-Arg(Tos)-Arg(Tos)-N3Ala-OMe
and Boc-Arg(Tos)-Phe-N3Ala-OMe as precursors for the potential
photocatalytic peptides using the β-azido-L-alanine building block
2 (=‘N3Ala’) and the Boc strategy for solution peptide synthesis. It
was tried to subsequently functionalize these peptides by the
pyrene building block 2 in the presence of Cu(I). We found MS
evidence for Boc-Arg(Tos)-Arg(Tos)-PyAla-OMe (8) and Boc-Arg
(Tos)-Phe-PyAla-OMe (9) (Supporting Information); however, the
*
Correspondence to: Hans-Achim Wagenknecht, Institute of Organic Chemistry,
Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 6, 76131 Karlsruhe,
Germany. E-mail: wagenknecht@kit.edu
Institute of Organic Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-
Haber-Weg 6, 76131, Karlsruhe, Germany
J. Pept. Sci. 2017
Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

S. HERMANN AND H.-A. WAGENKNECHT
Scheme 1. Synthesis of pyrene building block 2 and CuAAC with Boc-protected β-azido-L-alanine (1) to the Boc-protected β-pyrenyl-L-alanine building block
6 for synthesis of the functionalized peptides 10–12: (A) MeI, K2CO3, DMF, 2 h 120 °C; 98%; (B) NBS, CHCl3, 16 h, r.t.; 96%; (C) trimethylsilyl-acetylene, Pd
(PPh3)2Cl2, Pd(dppf)Cl2, CuI, NEt3, tetrahydrofuran, 16 h, 80 °C; then, tetrabutylammonium fluoride (1 M in tetrahydrofuran), CH2Cl2, 1 h, r.t.; 96%; (D)
sodium ascorbate, Cu(MeCN)4PF6, TBTA, CH2Cl2:DMF:MeOH = 1:1:3, 1 h, r.t.; quant.; (E) HCl (4 M in dioxane), 1 h, r.t.; quant.
yields were very low and the purity not satisfying. Hence, the
synthetic strategy was changed, and the CuAAC was performed
on the level of the Boc-protected peptide building block 1.
Accordingly, the reaction of 1 with 2 in CH2Cl2/dimethylformamide/
MeOH solvent mixture in the presence Cu(MeCN)4PF6 and sodium
ascorbate for 1 h at room temperature gave the corresponding
pyrene-modified alanine building block 6. The yield of the
β-pyrenyl-L-alanine (7, gives later ‘PyAla’ in peptides) after removal
of the t-butyloxycarbonyl (BOC) group by treatment with 4 M HCl
in dioxane was quantitative. This building block was successfully
applied to synthesize the dipeptides Boc-Arg(Tos)-PyAla-OMe (10),
Boc-Arg(Tos)-Gly-PyAla-OMe (11), and Boc-Arg(Tos)-Pro-PyAla-OMe
(12) in 65–70% yield. The tosyl protecting group was left on the
peptides to enhance their solubility in organic solvents during
photocatalysis. All peptides were purified by column chromatogra-
phy and fully characterized by nuclear magnetic resonance spectros-
copy and high-resolution mass spectrometry.
were applied as substrates for photocatalytic and nucleophilic
addition of MeOH (Scheme 2 and Table 1). The conversion of 13
and formation of 15 could be identified and quantified by gas
chromatography mass spectrometry, the conversion of 14 and
formation of 16 by nuclear magnetic resonance spectroscopy. As
previously reported for substrate 13 [13], the photocatalytic conver-
sion of both 13 and similarly of 14 can only be completed with 39%
yield of product 15 and 23% of product 16, respectively, if Et3N is
added to the solution. Side products are the adduct of Et3N (17)
and 13, which we both identified recently [14]. This is the starting
point for photocatalytic improvement.
A more detailed look on the mechanism (Scheme 2) and the
problem of inefficient back electron transfer indicates that loss of
polar attraction after rapid protonation of the substrate radical
anion may lead to diffusion and separation of the photocatalyst
from the intermediate product-forming radical cation. As it can be
assumed that back electron transfer is a strongly distance
dependant process, the photocatalyst may not be regenerated
and removed from the catalytic cycle. This scenario could
potentially be improved if a substrate binding site is available on
the photocatalyst that keeps the substrate in the vicinity of the
pyrene chromophore as long as it is required for forward and back
The photocatalytic activity of the three peptides 10–12 in
comparison with Py was determined using a high-power LED for
irradiation at 369 nm, a Peltier temperature control element and a
stirrer. In accordance with our previous publications, 1,1-
diphenylethylene (13) and 4-(phenyl-1-vinyl)-benzoic acid (14)
Scheme 2. Photocatalytic Markovnikov-type nucleophilic addition of MeOH to substrates 13 and 14, proposed photocatalytic mechanism for the conversion
of 13 using Et3N as electron shuttle (Pc = photocatalyst Py, or peptides 10, 11, or 12) and proposed binding of substrate 14 to peptide 11.
wileyonlinelibrary.com/journal/jpepsci
Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.
J. Pept. Sci. 2017

Peptide Photocatalysis
Table 1. Photocatalytic nucleophilic addition of MeOH to 13 and 14 by photocatalysts (Pc) Py, or peptides 10–12a
Line
Substrate
Pc (mol%)
Time (h)
Additive
Conversion %
Yield %
1
2
3
4
5
6
7
8
9
13
13
14
14
13
13
14
14
14
Py (100)
3
3
—
Et3Nc
—
Et3Nc
17b
100b
9d
15: 17b
Py (100)
15: 39b
Py (100)
3
16: 9d
16: 23d
15: —[b,g]
15: —[b,g,h]
16: 100f
16: —[e,h]
16: 99–100f
Py (100)
3
79d
10, 11, or 12 (100)e
10, 11, or 12 (100)
10, 11, or 12 (100)
10, 11, or 12 (100)e
10, 11, or 12 (25)
12
12
12
12
12
—
—
58–100b
b
Et3Nc
—
Et3Nc
100f
f
—
99–100f
—
aReaction conditions: 13 or 14 (2 mM) in MeCN:MeOH = 7:3 (4 ml), argon atmosphere, 12 h, 25 °C, 369-nm high-power LED.
b13 and 15 identified and quantified by gas chromatography mass spectrometry.
c5 vol%.
d14 and 16 identified by high-performance liquid chromatography mass spectrometry and quantified by high-performance liquid chromatography
ultraviolet.
eDestruction of Pc.
f14 and 16 identified and quantified by nuclear magnetic resonance spectroscopy.
gOnly traces of product 15.
h17 was obtained as product.
electron transfer process in the time course of the complete
photocatalytic cycle. Accordingly, the peptides 10–12 as
photocatalyst bear an arginine side chain that served as binding site
especially for the substrate 14 carrying the complementary
carboxylic acid, presumably by hydrogen bonding. The photocata-
lytic experiments (Table 1) revealed that especially the MeOH
addition to substrate 14 photocatalyzed by all three peptides
10–12 run with quantitative conversions and also quantitative
yields of 16 (after 12 h irradiation). The comparable irradiations with
14 in the presence of Et3N as additive lead to photodestruction of
the photocatalysts 10–12 and thus only little conversions. The
question if the binding of the carboxylic acid of substrate 14 to
the peptidic photocatalysts 10–12 was required can be answered
by the corresponding irradiations with substrate 13. Without Et3N,
only traces of 15 were detected, whereas in the presence of the
Et3N additive, the Et3N-adduct 17 was observed as the main but
undesired product. That showed clearly that the binding site
interactions between the peptidic photocatalysts and the substrate
14 played a central role for successful photocatalysis and that the
short peptides 10–12 represent significantly improved photocatalyst
for this type of reaction as they allow to avoid Et3N as additive.
Concerning the stoichiometry, we reported that Py can be applied
in catalytic amounts (10 mol%) for this type of nucleophilic addition
to styrene derivatives [14]. We performed similar experiments with
25 mol% of the peptides 10, 11, and 12; the conversions and yields
were similarly good as with stochiometric amounts.
In conclusion, we showed that the electron-rich Py chromophore
could be covalently conjugated to short peptides using the CuAAC.
However, this copper(I)-catalyzed ‘click’ functionalization had to be
performed on the level of the amino acid building block, in our case
using β-azido-L-alanine (1) because this gave an easy and flexible
access to a variety of peptide conjugates. Corresponding conjuga-
tion attempts with short peptides bearing the β-azido group as side
chain failed presumably because of the instability of the azido
functionality under the acidic conditions of Boc deprotection. The
photocatalytic activities of the three peptides 10–12 were
determined in comparison with Py for the nucleophilic addition
of MeOH to the diphenylethylene substrates (13 and 14) yielding
the corresponding products in Markovnikov orientation (15 and
16). The peptides 10–12 bear an N-terminal arginine side chain that
served as potential binding site by hydrogen bonding especially for
the substrate 14 carrying the complementary carboxylic acid. As a
result, the functionalized and synthesized peptides represent
significantly improved photocatalysts in comparison with Py for this
type of reaction as they allow avoiding Et3N as additive. These re-
sults underscore the significant potentials of short peptides for
chemical photocatalysis.
Acknowledgements
Financial support by the Deutsche Forschungsgemeinschaft (Wa
1386/16-1), the GRK 1626 (DFG and University of Regensburg),
and KIT is gratefully acknowledged.
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