Multistep electron transfer in oligopeptides: Direct observation of radical cation intermediates

Article abstract of DOI:10.1002/anie.200500391

Hopping holes: Oligopeptides with a photosensitive charge-injection system were synthesized and irradiated by a nanosecond laser. A multistep electron-transport process occurred which uses aromatic side chains as hole carriers (see scheme). The rates of t

Full text of DOI:10.1002/anie.200500391

Angewandte
Chemie
[
5]
negative charge (“excess-electron transport”). A similar
hopping model was also applied to explain long-distance
[
6]
charge transport through proteins. Hopping is especially
favored if the amino acids carry side chains that can be easily
oxidized (tyrosine, tryptophan, and histidine). For instance, a
multistep electron-hole transport cascade involving three
tryptophan units was proposed for the photoactivation
[
6a]
process in the enzyme photolyase. Presumably, not only
tryptophan but also tyrosine and histidine are involved in the
mechanism of radical initiation in ribonucleotide reductase,
where an electron hole is transported over a distance of
[
6e]
Electron Transfer in Peptides
35 ꢀ. In these systems, transient intermediates occurring in
the multistep charge transport have not yet been detected,
and it is not clear whether the electron transfer is best
described as proceeding through space or through the peptide
Multistep Electron Transfer in Oligopeptides:
Direct Observation of Radical Cation
Intermediates**
[
7]
bonds.
In order to examine electron-hole transport in peptides,
we synthesized the series of model systems 1a–d bearing three
different redox sites, that is, a modified tetrahydrofuran as a
Bernd Giese,* Matthias Napp, Olivier Jacques,
Hassen Boudebous, Alexander M. Taylor, and
Jakob Wirz
[
8]
[9]
charge-injection system, 2,4,6-trimethoxyphenylalanine as
an intermediate charge carrier, and tyrosine as a final electron
donor. An oligoproline spacer of variable length (n = 0, 1, 3,
5) was chosen to separate the tyrosine residue from the 2,4,6-
trimethoxyphenylalanine moiety. As is known from related
studies of electron-hole transfer in DNA, irradiation of the
pivaloylated tetrahydrofuran unit in compounds like 1a–d
will generate the dihydrofuranosyl radical cations, 2a–d,
through Norrish a cleavage and subsequent b elimination of
For the transport of electrons or electron holes over long
distances, nature often uses DNA double strands or proteins.
[
1]
[2]
With DNA, experiments and calculations have clearly
shown that these processes can occur by a hopping mecha-
nism, where the charge transport takes place in a multistep
[
3]
[4]
reaction. Purines act as carriers of positive charge
“electron-hole transport”), and pyrimidines are carriers of
[
10]
(
the phosphate leaving group. These radical cations have
Scheme 1. Mechanism for the photolysis of 1a–d and the subsequent electron-transfer reactions.
[
*] Prof. Dr. B. Giese, Dr. M. Napp, Dr. O. Jacques, A. M. Taylor
Department of Chemistry, University of Basel
St. Johanns-Ring 19, 4056 Basel (Switzerland)
Fax: (+41)61-267-1105
[11]
strong oxidizing properties and can oxidize the neighboring
2
[12]
,4,6-trimethoxyphenylalanine side chain (Scheme 1).
Compounds 1a–d were dissolved in acetonitrile/water
3:1) and irradiated with 308-nm pulses from a XeCl excimer
(
E-mail: bernd.giese@unibas.ch
[
13]
laser,
which allowed selective excitation of the pivaloyl
Dipl.-Chem. H. Boudebous, Prof. Dr. J. Wirz
Department of Chemistry, University of Basel
Klingelbergstrasse 80, 4056 Basel (Switzerland)
chromophore. The transient absorption spectra observed
after a delay of 50 ns with respect to the laser pulse indicated
the presence of radical cations 3a–d with an absorption
maximum at 550 nm (Figure 1a shows the spectrum for
[
**] This work was supported by the Swiss National Science Foundation.
Supporting Information for this article is available on the WWW
under http://www.angewandte.org or from the author.
[
14,15]
system 1c as a typical example).
This signal decayed
Angew. Chem. Int. Ed. 2005, 44, 4073 –4075
DOI: 10.1002/anie.200500391
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4073

Communications
Table 1: Rate constants for intramolecular (kintra) and intermolecular
kinter) electron transfer, determined from the intercepts and slopes of
(
rate-versus-concentration plots. The number of proline units between
the aromatic side chains is given by n, and s is the length of the through-
l
bond pathway, based on the number of covalent bonds between the
[6f]
aromatic redox centers.
ꢁ1
ꢁ
ꢁ1
n
kintra [s
]
kinter [m 1s
]
sl [ꢁ]
6
6
5
5
8
1
1
1
1
a
b
c
0
1
3
5
8.6(ꢂ0.6)ꢀ10
1.1(ꢂ0.1)ꢀ10
5.1(ꢂ0.3)ꢀ10
2.8(ꢂ0.3)ꢀ10
ꢃ5ꢀ10
9.8
14.0
22.4
30.8
8
8
8
4.3(ꢂ0.4)ꢀ10
3.0(ꢂ0.1)ꢀ10
3.2(ꢂ0.1)ꢀ10
d
Figure 1. Transient absorption spectra observed after a delay of
a) 50 ns and b) 1 ms with respect to the irradiation of 1c in degassed
acetonitrile/water (3:1) at a concentration of 8 mm.
[
19]
flexibility of the aromatic amino acids.
The rate does
within 1 ms and was replaced by the characteristic signal of the
increase considerably when the proline spacer is absent, as in
1a, where the aromatic side chains are in close contact with
each other.
tyrosyl radical with a maximum absorbance at 410 nm
[
16]
(
Figure 1b).
Kinetic traces of the absorbance decay at 550 nm and of
[
12]
The radical cation of 1,3,5-trimethoxybenzene should
the growth observed at 410 nm obeyed the first-order rate law
well, and the rate constants determined by least-squares
fitting at the two wavelengths were the same within the limits
of error. A typical example is shown in Figure 2.
not be capable of oxidizing phenol to the phenol radical
+
cation (oxidation potential E (PhOH/PhOHC ) = 1.50 V
ox
[
20]
versus a NHE). But oxidation of phenol to the phenoxy
radical in aqueous solution (pH 7) has a much lower potential
[
21]
of E (PhOH/PhOC) = 0.86 V versus a NHE. Thus, oxida-
ox
tion of tyrosine in the systems 3a–d can easily occur, if the
electron transfer is coupled with proton transfer to the
solvent. We measured a kinetic deuterium isotope effect k /
H
[
22]
kD of 1.8(ꢂ0.2). This value is in good agreement with the
isotope effect of 2.0–2.5 for the oxidation of tyrosine by a
ruthenium–tris(bipyridine) complex in aqueous media, where
[
23]
a proton-coupled electron transfer occurs.
In summary, electron-hole transport in peptide radical
cations 3a–d occurs by a hopping mechanism between the
aromatic side chains of the amino acids, which is similar to the
[
1–4]
Figure 2. Kinetic traces of the transient absorptions at 410 and 550 nm
for 1c (7 mm) in degassed acetonitrile/water (3:1). The decay rate
long-distance hole transport through DNA.
6
ꢁ1
constant at 550 nm is 3.1(ꢂ0.2)ꢀ10 s and the absorbance growth
Received: February 1, 2005
Published online: May 25, 2005
6
ꢁ1
constant at 410 nm is 3.5(ꢂ0.3)ꢀ10 s
.
Keywords: charge transport · electron transfer · flash photolysis ·
peptides · radicals
However, the rate constants observed with model pep-
tides 1a–d increased linearly with the concentration of these
compounds. This pointed to competition of intra- and
intermolecular electron transfer. The slopes of these plots
correspond to the intermolecular rate contributions kinter, and
.
[
1] E. Meggers, M. E. Michel-Beyerle, B. Giese, J. Am. Chem. Soc.
1998, 120, 12950.
8
ꢁ1 ꢁ1
the values so obtained (kinter ꢀ 4 ꢁ 10 m s ) agree well with
the rate constants of diffusion expected for compounds of this
size. The intramolecular contributions, namely the rate
constants kintra, for electron transfer from the tyrosine side
chain to the 2,4,6-trimethoxyphenylalanine radical cation
moiety (3a–d!4a–d) were determined from the intercepts
[2] J. Jortner, M. Bixon, T. Langenbacher, M. E. Michel-Beyerle,
Proc. Natl. Acad. Sci. USA 1998, 95, 12759.
[
[
[
3] Recent reviews: “Long-Range Charge Transfer in DNA”: Top.
Curr. Chem. 2004, 236 and 237.
4] B. Giese, J. Amaudrut, A.-K. Kꢂhler, M. Spormann, S. Wessely,
Nature 2001, 412, 318.
5] a) S. Breeger, U. Hennecke, T. Carell, J. Am. Chem. Soc. 2004,
126, 1302; b) T. Ito, S. E. Rokita, Angew. Chem. 2004, 116, 1875;
Angew. Chem. Int. Ed. 2004, 43, 1839.
[
17]
(
Table 1).
The rate constants kintra (Table 1) decrease by a factor of
only about two when the spacer between the aromatic redox
centers is elongated by addition of two proline units (compare
[6] a) C. Aubert, M. H. Vos, P. Mathis, A. P. M. Eker, K. Brettel,
Nature 2000, 405, 586; b) J. R. Winkler, Curr. Opin. Chem. Biol.
1
b with 1c and 1c with 1d). For an electron transfer through
2000, 4, 192; c) E. G. Petrov, V. May, J. Phys. Chem. A 2001, 105,
the peptide bonds one would expect a rate decrease of about
10176; d) C. C. Page, C. C. Moser, P. L. Dutton, Curr. Opin.
[
18]
5
00 for two additional proline units. This indicates that the
Chem. Biol. 2003, 7, 551; e) J. Stubbe, D. G. Nocera, C. S. Yee,
M. C. Y. Chang, Chem. Rev. 2003, 103, 2167; f) H. B. Gray, J. R.
Winkler, Q. Rev. Biophys. 2003, 36, 341; g) T. Morita, S. Kimura,
J. Am. Chem. Soc. 2003, 125, 8732.
electron transfer does not proceed by a through-bond
mechanism. We attribute the small influence of additional
proline spacers upon the kintra value to the conformational
4
074
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Angew. Chem. Int. Ed. 2005, 44, 4073 –4075

Angewandte
Chemie
[
7] The question of whether the electron travels through the peptide
bonds by a bridge-mediated superexchange mechanism or by a
hopping process has been the subject of several investigations:
a) A. K. Mishra, R. Chandrasekar, M. Faraggi, M. H. Klapper, J.
Am. Chem. Soc. 1994, 116, 1414; b) K. Bobrowski, J. Poznanski,
J. Holcman, K. L. Wierzchowski, J. Phys. Chem. B 1999, 103,
photosensitized electron transfer with 1,4-dicyanonaphthalene
as a sensitizer and biphenyl as a codonor is similar to the
spectrum shown in Figure 1a (see the Supporting Information).
Furthermore, a compound with the pivaloylated tetrahydrofuran
unit directly connected to N-acetyl-2,4,6-trimethoxyphenylala-
nine was synthesized (6). Laser flash photolysis of this com-
pound again gave a spectrum that was practically identical to the
one shown in Figure 1a (see the Supporting Information).
[16] K. M. Bansal, R. W. Fessenden, Radiat. Res. 1976, 67, 1.
[17] The irradiations were carried out at concentrations of 0.3–8 mm,
which provided kintra values from the intercepts of rate-versus-
concentration plots with a standard error of about 10% (see the
Supporting Information). The decay of the radical cation of
2,4,6-trimethoxyphenylalanine in the absence of tyrosine was
10316; c) R. A. Malak, Z. Gao, J. F. Wishart, S. S. Isied, J. Am.
Chem. Soc. 2004, 126, 13888; d) K. Yanagisawa, T. Morita, S.
Kimura, J. Am. Chem. Soc. 2004, 126, 12780; e) F. Polo, S.
Antonello, F. Formaggio, C. Toniolo, F. Maran, J. Am. Chem.
Soc. 2005, 127, 492.
[
8] The synthesis of the charge-injecting system was performed
following procedures reported earlier: a) R. Glatthar, M.
Spichty, A. Gugger, R. Batra, W. Damm, M. Mohr, H. Zipse,
B. Giese, Tetrahedron 2000, 56, 4117; b) A. Marx, P. Erdmann,
M. Senn, S. Kꢂrner, T. Jungo, M. Petretta, P. Imwinkelried, A.
Dussy, K. J. Kulicke, L. Macko, M. Zehnder, B. Giese, Helv.
Chim. Acta 1996, 79, 1980. (See the Supporting Information for
further details.)
4
ꢁ1
much slower (dissociation constant k = 1.1·10 s ) than the
d
intramolecular electron-transfer reaction 3a–d!4a–d (k
=
intra
6
6
ꢁ1
0.28 ꢁ 10 –8.6 ꢁ 10 s ). These intramolecular rates are 10–
100 times faster than the observed electron-transfer rates
[
7a,b]
between a tryptophan radical cation and tyrosine.
This is in
[
9] We used the racemate: D. Dauzonne, R. Royer, Synthesis 1987,
accord with the higher oxidation potential (E ) of trimethoxy-
ox
[
12]
399.
phenylalanine compared to tryptophan (Eox = 1.02 V versus a
[
21]
[
10] A. Gugger, R. Batra, P. Rzadek, G. Rist, B. Giese, J. Am. Chem.
Soc. 1997, 119, 8740.
NHE).
[18] By addition of two proline units, the length of the through-bond
[
11] When 2,3-dihydrofuran is used as a simple model system, the
reduction potentials of radical cations like 2a–d are estimated to
be larger than 1.27 V versus a normal hydrogen electrode
pathway (s , 1.4 ꢀ times the number of covalent bonds in the
l
effective path) is increased by 8.4 ꢀ. With a distance-decay
ꢁ1
constant of b = 0.73 ꢀ , the kintra value is expected to drop by a
factor of about 500: D. R. Casimiro, L. L. Wong, J. L. Colon,
T. E. Zewert, J. H. Richard, I.-J. Chang, J. R. Winkler, H. B.
(
NHE): K. Bernhard, J. Geimer, M. Canle-Lopez, J. Reynisson,
D. Beckert, R. Gleiter, S. Steenken, Chem. Eur. J. 2001, 7, 4640.
[
12] The half-wave oxidation potential of 1,3,5-trimethoxybenzene is
Gray, J. Am. Chem. Soc. 1993, 115, 1485.
1
1.25 V versus a NHE: A. Zweig, W. G. Hodgson, W. H. Jura, J.
[19] By using H-NOESY experiments in [D ]acetonitrile, conforma-
3
Am. Chem. Soc. 1964, 86, 4124.
13] Solutions of 1a–d (0.3–8 mm) were degassed in a quartz cell of
tion studies of 1c were performed. The proline spacer adopts the
all-trans conformation in about 80%. Subsequent molecular-
modeling analysis (with the MakroModel 7.2 program) of
compounds 1a–d in a fixed all-trans conformation revealed
that, due to the conformational flexibility of the aromatic side
chains, different distances exist between the redox centers.
[20] S. Canonica, B. Hellrung, J. Wirz, J. Phys. Chem. A 2000, 104,
1226.
[
4
.5-cm path length and irradiated with laser pulses (Lambda-
Physik COMPex 205, l = 308 nm, pulse width of 25 ns, 100–
50 mJ per pulse) at RT (22–248C). An iStar 720 ICCD camera
Andor) was used for the detection of transient absorption
1
(
spectra. Transient kinetics were recorded with a 1P28 photo-
multiplier (Hamamatsu). For a detailed description, see: a) J.
Wirz, Pure Appl. Chem. 1984, 56, 1289; b) E. Leyva, M. S. Platz,
G. Persy, J. Wirz, J. Am. Chem. Soc. 1986, 108, 3783; c) S. Gerber,
J. Wirz, EPA Newsl. 1989, 36, 19.
[21] A. Harriman, J. Phys. Chem. 1987, 91, 6102.
[22] The rate of intramolecular electron transfer in compound 1c
5
ꢁ1
(acetonitrile/D O (3:1)) is 2.9(ꢂ0.2) ꢁ 10 s
.
2
[
14] P. OꢃNeill, S. Steenken, D. Schulte-Frohlinde, J. Phys. Chem.
[23] M. Sjꢂdin, S. Styring, B. ꢀkermark, L. Sun, L. Hammarstrꢂm,
1975, 79, 2773.
Philos. Trans. R. Soc. London Ser. B 2002, 357, 1471.
[
15] The transient absorption spectrum recorded after oxidation of
N-acetyl-2,4,6-trimethoxyphenylalanine methyl ester (5) by
Angew. Chem. Int. Ed. 2005, 44, 4073 –4075
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