PELDOR on an exchange coupled nitroxide copper(II) spin pair

Article abstract of DOI:10.1016/j.jorganchem.2008.11.029

Transition metal ions play an important role in the design of macromolecular architectures as well as for the structure and function of proteins and oligonucleotides, which makes them interesting targets for spectroscopic investigations. In combination wi

Full text of DOI:10.1016/j.jorganchem.2008.11.029

Journal of Organometallic Chemistry 694 (2009) 1172–1179
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
PELDOR on an exchange coupled nitroxide copper(II) spin pair
Bela E. Bode a, Jörn Plackmeyer a, Michael Bolte b, Thomas F. Prisner a, Olav Schiemann a,1,
*
a Institute of Physical and Theoretical Chemistry and Center for Biomolecular Magnetic Resonance, Goethe University, Max-von-Laue-Strasse 7, 60438 Frankfurt am Main, Germany
b Institute of Inorganic and Analytical Chemistry, Goethe University, Max-von-Laue-Strasse 7, 60438 Frankfurt am Main, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 July 2008
Received in revised form 20 October 2008
Accepted 14 November 2008
Available online 24 November 2008
Transition metal ions play an important role in the design of macromolecular architectures as well as for
the structure and function of proteins and oligonucleotides, which makes them interesting targets for
spectroscopic investigations. In combination with site directed spin labelling, pulsed electron–electron
double resonance (PELDOR or DEER) could be a well-suited method for their characterization and local-
ization. Here, we report on the synthesis and full characterization of a copper(II) porphyrin/nitroxide
model system bearing an extended p-conjugation between the spin centres and demonstrate the possi-
Dedicated to Prof. Ch. Elschenbroich on the
occasion of his 70th birthday.
bility to disentangle the dipolar through space interaction from the through bond exchange coupling con-
tribution even in the presence of orientational selectivity and conformational flexibility. The simulations
used are based on the known experimental and spin Hamiltonian parameters and on a structural model
as previously employed for similar systems. The mean exchange coupling of +4(1) MHz (antiferromag-
netic) is in agreement with the value of |J| = 3(1) MHz determined from room temperature continuous
wave electron paramagnetic resonance (EPR). Thus, as long as the pulse excitation bandwidths are large
versus the spin–spin coupling, X-band PELDOR measurements in combination with explicit time trace
simulations allow for disentangling the sign and magnitude of through bond electron–electron exchange
from the through space dipolar interaction D.
Keywords:
EPR
DEER
Exchange coupling
Dipolar coupling
Porphyrin
Metal ions
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
linium(III) pairs [13], as well as distances between iron–sulphur
clusters and a nickel–iron centre [14] or an organic radical [15].
Transition metal ions play an important role in the design of
macromolecular architectures [1] as well as the structure and func-
tion of proteins [2] and oligonucleotides [3]. Structures of such
assemblies, and therefore a key to their function, can be obtained
via X-ray diffraction or NMR spectroscopy. However, the former
method fails if the sample is non-crystalline and the latter reaches
its limits if the metal ions are paramagnetic and/or the assembly
becomes too large. On the other hand modern pulsed EPR methods
have shown to yield not only precise electronic and geometric
information about metal ion binding pockets [4] but also to reveal
long-range arrangements on the nanometre scale via the dipolar
coupling between paramagnetic centres [5]. One of the latter
methods is called pulsed electron–electron double resonance (PEL-
DOR or DEER) [6], which has been shown to very reliably measure
distances of up to 6 nm between two organic radicals [7] including
distance distributions [8], angular information [9] and the number
of coupled spins [10]. Over the last years, this method has been
extended to paramagnetic metal centres involving copper(II)/
nitroxide [11] copper(II)/copper(II) [12], and gadolinium(III)/gado-
Such systems are more demanding, due to several reasons. First,
they usually have a large hyperfine- and g-tensor anisotropy,
which results in broad EPR spectra from which the pulses excite
only a fraction which can induce a strong orientation selection.
Second, the relaxation time might be fast, which narrows the
detection window and limits thereby the accessible distance range.
And third, the spin density might be distributed into the ligands,
leading to the breakdown of the simple point–dipole model and
the onset of an exchange coupling J.
Since copper(II) centres often occur in biological systems and
are frequently used for building supramolecular assemblies a dee-
per and quantitative understanding of PELDOR spectra from such
centres is needed. We therefore synthesized model system 1 in
which a copper(II)porphyrin is connected via a conjugated bridge
to a nitroxide as well as the reference molecule 2 in which the
paramagnetic copper(II) is exchanged for the diamagnetic nickel(II)
ion (Scheme 1). Recently, we have shown for a similar model sys-
tem 3 [16] (Scheme 1), in which the conjugation of the bridge is
disrupted by an ester linkage, that the broad copper spectrum in-
duces orientation selection, but that its effect on the PELDOR spec-
tra is weak. It could also be demonstrated that the point–dipole
model is still valid despite the considerable delocalization of spin
density from the copper into the prophyrin ring. The reason for
the former is the large hyperfine coupling of the ring nitrogens,
* Corresponding author. Tel./fax: +44 1334 46 3410.
E-mail address: os11@st-andrews.ac.uk (O. Schiemann).
1
Present address: Centre for Biomolecular Sciences, Centre of Magnetic Resonance,
University of St. Andrews, St. Andrews, Fife, KY16 9UA Scotland, United Kingdom.
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.11.029

B.E. Bode et al. / Journal of Organometallic Chemistry 694 (2009) 1172–1179
1173
O
N
I
5
N
N
N
N
Fig. 1. Pulse sequences for 4-pulse PELDOR.
PdCl2(PPh3)2/CuI
NEt3/DMF
M
pulse. The resulting PELDOR signal V(t) is a product of two
contributions
VðtÞ ¼ VintraðtÞVinterðtÞ:
ð1Þ
M = Cu
M = Ni
6
7
Vintra(t) describes the intramolecular contribution, whereas
Vinter(t) takes into account the signal decay with t caused by
intermolecular interactions. In case of a homogeneous distribution
of the molecules Vinter(t) is a monoexponential decay [18]. In the
general case of disordered samples and angular correlations
between the spin centres, the echo signal intensity of the intramo-
lecular contribution Vintra(t) is given by [19].
O
N
N
N
N
M
N
ꢂ
ꢀ
Z
ꢁꢃ
p=2
M = Cu 1
VintraðtÞ ¼ V0 1 ꢀ
PðhÞ½1 ꢀ cosð
x
ABtÞꢁdh
ð2Þ
M = Ni
2
0
with
PðhÞ ¼ kðhÞ sin h:
ð3Þ
O
N
N
N
N
O
V0 is the spin echo intensity at t = 0, given by the fraction of spins A
excited by the detection pulses. P(h) is the distribution function of
dipolar angles h excited by the pulse sequence. It takes into account
the orientation selection of the detection pulses and of the pump
pulse and includes the dependence of the excitation efficiency
k(h) of the pump pulse on the mutual orientation of the two radicals
A and B. sin(h) is the distribution function of uncorrelated radical
pairs. In the point–dipole approximation and for small g-anisotro-
pies, such that the spin states are quantized parallel to the external
field, and in the weak coupling regime, the coupling xAB between
the two spins is described by the sum of the dipolar contribution
xDD mediated through space and the exchange coupling constant
J mediated through bond.
N
Cu
O
3
Scheme 1. Synthesis of model compounds 1 and 2. For the sake of comparison, the
scheme also shows reference molecule 3, previously reported.
leading to orientational ‘‘smearing”. The reason for the latter is the
concentric distribution of spin density resulting only in a small
orthorhombic contribution to the dipolar tensor [17].
Here, we were interested whether the conjugated bridge in 1
would go in-parallel with an increase in the exchange coupling
constant J compared to 3, which displayed a distribution in J of
xAB
¼
xDD þ J:
ð4Þ
The dipolar contribution is expressed by Eq. (5) and J is defined
D
J = 1 MHz centred around J = 0 MHz; and if so, how it effects
via the spin Hamiltonian H = JSzASzB
l0l2B gAgB
.
would be on the PELDOR time traces and whether it would be pos-
sible to disentangle the dipolar through space interaction D from
the through bond exchange coupling contribution J. Especially,
the last point is of concern, because if this is not possible the error
in mean distances and distance distributions will dramatically
increase.
xDD ¼ ꢀ
3 ð3 cos2 h ꢀ 1Þ:
ð5Þ
4pꢀh
jrAB
j
In Eq. (5), gA and gB are the g-values of the spins A and B, respec-
tively, ⁄ is the Planck constant divided by 2 , rAB is the distance
vector connecting the spins, h is the angle between rAB and the
p
external magnetic field, and l0 is the vacuum permeability.
1.1. PELDOR theory
All following PELDOR experiments were performed with the 4-
pulse sequence shown in Fig. 1.
2. Results and discussion
The pulses at the detection frequency
mA create an echo from the
2.1. Synthesis and X-ray structure
spins on resonance, named in the following A spins (the unpaired
electron centred at the Cu(II)porphyrin in this study). Introduction
Model compounds 1 and 2 were both synthesized by means of
Sonogashira cross coupling as depicted in Scheme 1. The spin la-
belled iodobiphenyl 5 is coupled to either the paramagnetic cop-
of an inversion pulse at the pump frequency
with this second frequency, here defined as B spins (here the elec-
tron spin centred at the nitroxide). The coupling AB, between the
A and B spins causes a shift in the Larmor frequency of the A spins
by AB. Therefore, after pumping the B spins the A spins accumu-
late a phase shift ABt, where t defines the time delay of the pump
mB flips spins resonant
x
per(meso-ethynyl-octaethylpophyrin)
6
or the diamagnetic
nickel(meso-ethynyl-octaethylpophyrin) 7. The final products 1
and 2 were both obtained as violet crystals after purification via
column chromatography and subsequent crystallization.
x
x

1174
B.E. Bode et al. / Journal of Organometallic Chemistry 694 (2009) 1172–1179
molecular structure is shown in Fig. 2a and selected bond lengths
and angles are given in Tables 1 and 2. The porphyrin moiety
exhibits significant deviations from planarity as commonly found
for substituted metallo-porphyrins as for example the homo- and
heterobimetallic ethene-linked bisporphyrins reported by Smith
and coworkers [20]. Fig. 2b shows the ruffling of the porphyrinato
core as formal diagram and Table 3 summarizes the angles be-
tween the least-squares planes of adjacent pyrrole rings. Despite
the ruffled conformation, the distances between the nickel ion
and the porphyrinato nitrogen atoms are all four in the range of
1.950(9) Å and 1.969(9) Å, which is in good agreement with the re-
ported values of the planar parent compound nickel(octaethylp-
ophyrin) [21]. The distances between the nickel ion and the
nitrogen or the oxygen atom of the nitroxide group amount to
20.617(11) Å and 21.565(11) Å, respectively. Since the spin density
in the N-O group is almost equally distributed between both cen-
tres, the mean distance of 21.087 Å will be used for the discussion.
Crystals of 1 were not suitable for X-ray diffraction, however, due
to their structural similarity the mean metal nitroxide distance
gathered from 2 will be used instead.
2.2. CW EPR
To estimate the magnitude of the exchange coupling J in 1 cw X-
band EPR spectra were recorded in toluene solution at room tem-
perature. The resulting nitroxide spectrum is shown in Fig. 3a
together with the spectrum of reference compound 3 (Fig. 3b).
Both spectra are very similar as expected based on the structural
similarity. However, a closer inspection of the nitroxide part re-
veals that the 1H hyperfine splitting pattern of the 12 methyl-pro-
tons is clearly visible for 3 but is unresolved for 1. The reason is a
larger line width, which is attributed to a larger exchange coupling.
Thus, simulating the spectrum of 1 with the same set of parame-
ters and values as used for 3 but increasing J from 0 to 3(1) MHz
yields the simulation shown in Fig. 3a, which is in good agreement
with the experiment. This value will be used in the following as a
measure for the quality of the separation of J and D by the PELDOR
experiment. An extraction of J and D from cw EPR spectra of 1 at
10 K (Fig. 3C) is not possible, due to the intrinsic large line width
in frozen solutions and the small values for D and J [22].
Fig. 2. (a) Molecular structure of 2. Hydrogen atoms are omitted for clarity. (b)
Formal diagram of the porphyrinato core of 2. The displacements of each ring atom
from the 24-atom core plane are illustrated. The structure is shown viewing
perpendicular to the least-square plane calculated for the 24 core carbon and
nitrogen atoms. Deviations are given in Å ꢂ 100 and calculated via
2.3. PELDOR
The experimental PELDOR time traces of 1 and 3 are depicted in
Fig. 4a. The traces have been acquired detecting the copper elec-
tron spin and pumping on the maximum of the nitroxide spectrum.
A reference mixture of the unlabelled copper-porphyrin and the
nitroxide spin-label, yielding only the typical monoexponential de-
cay of mono radicals, was measured to ensure that the observed
D = 7.949(9)X ꢀ 6.460(10)Y ꢀ 10.707(13)Z + 0.286(7), where X, Y, and
Z are the
fractional coordinates.
The crystals obtained for 2 were suitable for X-ray diffraction
and had a triclinic symmetry. A graphical representation of its
Table 1
Selected bond lengths for 2.
Distance, Å
Distance, Å
Distance, Å
Ni(1)–N(1)
Ni(1)–N(2)
Ni(1)–N(3)
Ni(1)–N(4)
N(1)–C(14)
N(1)–C(11)
N(2)–C(21)
N(2)–C(24)
N(3)–C(34)
N(3)–C(31)
N(4)–C(41)
N(4)–C(44)
C(11)–C(45)
1.958(7)
1.950(9)
1.964(7)
1.969(9)
1.364(12)
1.368(11)
1.373(12)
1.376(11)
1.356(12)
1.402(12)
1.372(12)
1.386(11)
1.452(14)
C(11)–C(12)
C(12)–C(13)
C(13)–C(14)
C(14)–C(15)
C(15)–C(21)
C(21)–C(22)
C(22)–C(23)
C(23)–C(24)
C(24)–C(25)
C(25)–C(31)
C(31)–C(32)
C(32)–C(33)
C(33)–C(34)
1.475(13)
1.366(14)
1.453(11)
1.433(14)
1.389(12)
1.485(13)
1.366(12)
1.474(15)
1.373(14)
1.357(14)
1.449(13)
1.360(14)
1.462(12)
C(34)–C(35)
C(35)–C(41)
C(41)–C(42)
C(42)–C(43)
C(43)–C(44)
C(44)–C(45)
C(45)–C(51)
C(51)–C(52)
C(67)–C(68)
C(71)–C(72)
N(74)–O(74)
Ni(1)–N(74)
Ni(1)–O(74)
1.392(14)
1.375(12)
1.473(14)
1.385(12)
1.466(14)
1.406(13)
1.445(12)
1.207(12)
1.219(12)
1.368(14)
1.271(10)
20.617(11)
21.565(11)

B.E. Bode et al. / Journal of Organometallic Chemistry 694 (2009) 1172–1179
1175
Table 2
Selected bond angles for 2.
that the relatively fast modulation damping in 3 compared to
structurally similar bisnitroxides can be attributed to a small dis-
tribution in exchange couplings
DJ = 1 MHz centred around
Angle, °
Angle, °
J = 0 MHz [12]. In principle the appearance of a small exchange
coupling does not lead to drastic changes in the modulation damp-
ing, since it only shifts the frequencies of the dipolar tensor. Yet, a
distribution in J has a similar effect as a distribution in r. Thus, the
fast modulation damping in 1 may be attributed to a larger distri-
bution in J. In order to prove that, we chose to simulate the time
domain data to extract J and r. Available programs to invert the
PELDOR time traces into the distance domain could not be used
for this, since they neglect exchange contributions in the regulari-
zation kernels. It is also not possible to extract J and r from the Pake
pattern after Fourier transformation of the time traces, as shown
previously for a bisnitroxide system [24] since the singularities of
the Pake pattern are not resolved (Fig. 4b).
N(2)–Ni(1)–N(1)
N(2)–Ni(1)–N(3)
N(1)–Ni(1)–N(3)
N(2)–Ni(1)–N(4)
N(1)–Ni(1)–N(4)
N(3)–Ni(1)–N(4)
C(14)–N(1)–Ni(1)
C(11)–N(1)–Ni(1)
C(21)–N(2)–Ni(1)
C(24)–N(2)–Ni(1)
C(34)–N(3)–Ni(1)
C(31)–N(3)–Ni(1)
C(41)–N(4)–Ni(1)
C(44)–N(4)–Ni(1)
C(14)–N(1)–C(11)
C(21)–N(2)–C(24)
C(34)–N(3)–C(31)
C(41)–N(4)–C(44)
N(1)–C(11)–C(45)
N(1)–C(11)–C(12)
N(1)–C(14)–C(15)
N(1)–C(14)–C(13)
C(45)–C(11)–C(12)
C(13)–C(12)–C(11)
C(12)–C(13)–C(14)
C(15)–C(14)–C(13)
C(21)–C(15)–C(14)
N(2)–C(21)–C(15)
91.4(4)
89.0(4)
178.8(4)
179.3(4)
89.0(3)
N(2)–C(21)–C(22)
N(2)–C(24)–C(23)
C(25)–C(24)–N(2)
C(15)–C(21)–C(22)
C(23)–C(22)–C(21)
C(22)–C(23)–C(24)
C(25)–C(24)–C(23)
C(31)–C(25)–C(24)
C(25)–C(31)–N(3)
N(3)–C(31)–C(32)
N(3)–C(34)–C(35)
N(3)–C(34)–C(33)
C(25)–C(31)–C(32)
C(33)–C(32)–C(31)
C(32)–C(33)–C(34)
C(35)–C(34)–C(33)
C(41)–C(35)–C(34)
N(4)–C(41)–C(35)
N(4)–C(41)–C(42)
N(4)–C(44)–C(45)
N(4)–C(44)–C(43)
C(35)–C(41)–C(42)
C(43)–C(42)–C(41)
C(35)–C(41)–C(42)
C(43)–C(42)–C(41)
C(42)–C(43)–C(44)
C(45)–C(44)–C(43)
C(44)–C(45)–C(11)
111.8(8)
111.9(9)
124.8(10)
122.2(10)
105.6(9)
106.1(9)
123.2(9)
123.8(9)
124.6(9)
111.0(10)
124.4(8)
111.1(9)
124.3(9)
105.9(9)
107.1(9)
124.3(10)
123.5(10)
127.0(10)
110.7(8)
124.2(10)
110.9(8)
122.3(10)
106.4(9)
122.3(10)
106.4(9)
105.9(8)
124.8(9)
121.3(9)
90.6(3)
126.2(6)
129.9(7)
127.0(6)
128.4(7)
127.6(6)
127.4(7)
125.1(6)
128.9(7)
103.8(7)
104.6(8)
104.9(8)
106.0(8)
123.3(9)
112.3(9)
125.8(8)
112.8(9)
124.4(8)
104.9(8)
106.1(9)
121.2(10)
121.8(10)
126.0(10)
The simulations were performed in analogy to the procedure
described for 3. First, the excitation profiles in the nitroxide and
Cu(OEP) molecular axis systems have been calculated using the
values for the spectral parameters as gathered from the simula-
tions of the cw EPR spectra.
Fig. 5 reveals that mainly copper-porphyrin moieties with the
porphyrin plane parallel to the magnetic field are detected,
whereas all orientation of the nitroxide moiety are inverted by
the pump pulse. This orientation selection enters the PELDOR sim-
ulations via the form factor, which describes the excitation proba-
bility with respect to the dipolar angle h. The form factor has been
calculated for an ensemble of conformers generated from a geo-
metric model based on the crystal structure of 2. The model
(Fig. 5d) is essentially the same as used for 3 but taking into ac-
count that the mean metal/nitroxide distance equals 21 Å, instead
of 20.7 Å and that the free rotation of the nitroxide group around
the linker bond is on a cone of 39°, instead of 31.4°. A single bend-
ing motion of 15° centred at the mid-point of the biphenyl bridge
enters both models. The resulting distribution function of dipolar
angles shown in Fig. 6 displays a strong deviation from a sin(h) dis-
tribution, which describes the form factor of a system without
angular correlation. Distance vectors parallel to the magnetic field
become more probable, whereas distance vectors perpendicular to
the magnetic field are deselected, which can be rationalized by the
deselection of Cu(OEP) moieties with the porphyrin ring plane per-
pendicular to the magnetic field. Thus, the most prominent feature
of the dipolar Pake pattern is deselected, the theoretical dipolar
spectrum for this distribution of dipolar angles is shown is Fig. 6b.
Simulating with this form factor the PELDOR time trace reveals
that the data can not be satisfactorily reproduced (data not shown).
Taking a singular exchange coupling constant into account does
also not lead to better simulations (Fig. 7), but including a distribu-
PELDOR effect is solely due to intramolecular spin–spin interaction
and not to intermolecular interactions or instrumental artefacts. A
PELDOR time trace of 2 cannot be recorded detecting the metal
centre, since Ni(OEP) is diamagnetic.
The modulation depth of 1 and 3 are comparable, even though
time traces of 1 are recorded with spectrally broader detection
pulses of 16 ns instead of 32 ns. The shorter detection pulses
proved to be necessary, since data recorded under the same condi-
tions as for 3 did not bear visible oscillation and only a shallow
modulation depth (data not shown). This is attributed to the fact
that the pulse excitation bandwidth of 32 ns is not sufficiently
large versus the spin-spin coupling leading to an incomplete detec-
tion of dipolar frequencies [23]. The frequency offset (DmAB) be-
tween detection and pump frequency was set to 118 MHz. With
smaller offsets, the excitation profiles of the pump and detection
pulses overlap to such an extent that contributions from the nitr-
oxide spin enter the refocused echo. At larger offsets it was not
possible to achieve 16 ns detection pulses.
In addition, 1 and 3 show both modulations, but it is remarkable
how fast the modulation is damped in 1. Compound 3 displays
three well resolved periods of modulation, whereas the modula-
tion for 1 is barely visible. Commonly, a fast damping of a PELDOR
modulation is attributed to a large conformational flexibility. How-
ever, the structural similarity of 1 and 3 renders an increased con-
formational flexibility as unlikely. In an earlier study, it was shown
tion
DJ = 5(1) MHz centred around +4(1) MHz (antiferromagnetic)
achieves very good agreement between experiment and simulation
(see Fig. 7). A negative J (ferromagnetic) leads to completely differ-
ent results (data not shown). And indeed, the antiferromagnetic ex-
change coupling of 4(1) MHz is in agreement with the value of
3(1) MHz determined from the room temperature cw EPR spec-
Table 3
Angles (°) between the pyrrole best planes of compound 2. X, Y, and Z are in triclinic fractional coordinates.
Plane 2
15.1(6)
Plane 3
Plane 4
Plane 1
Plane 2
Plane 3
26.1(6)
20.0(6)
20.2(6)
24.6(6)
15.6(6)
Plane 1, Pyrrole ring 1. N(1), C(11)–C(14) – 6.47(6)X + 5.32(7)Y + 13.34(7)Z = 2.17(5)
Plane 2, Pyrrole ring 2. N(2), C(21)–C(24) – 5.79(6)X + 8.22(5)Y + 12.25(7)Z = 0.51(5)
Plane 3, Pyrrole ring 3. N(3), C(31)–C(34) 8.96(4)X ꢀ 7.41(5)Y ꢀ 7.52(9)Z = 1.41(5)
Plane 4, Pyrrole ring 4. N(4), C(41)–C(44) 9.74(5)X ꢀ 4.45(7)Y ꢀ 8.66(8)Z = – 0.11(2)

1176
B.E. Bode et al. / Journal of Organometallic Chemistry 694 (2009) 1172–1179
Fig. 4. (a) PELDOR time domain data of 1, 3 and a reference mixture of Cu(OEP) and
TPA. (b) Dipolar spectra obtained by Fourier transformation of the time traces
above.
dipolar through space coupling from the through bond exchange
coupling, even if the singularities of the Pake pattern are not re-
solved. This enables one to evaluate structural models on the basis
of PELDOR measurements between a copper ion and a nitroxide
even in the presence of an exchange coupling. Furthermore, not
only the magnitude, but also the sign of the exchange coupling
and the distribution in J can be gathered. It is, however, mandatory,
that the spin–spin coupling tensor is smaller than the excitation
band width of the pulses, which limits the method to small ex-
change couplings and rather long distances. Thus, PELDOR comple-
ments nicely other methods capable to measure J: Large exchange
coupling constants can be determined with their sign via temper-
ature dependent Squid measurements [26] or via temperature
dependent measurements of the intensity of the half-field EPR sig-
nal [27]. Exchange coupling constants on the order of the hyperfine
coupling can be obtained via simulations of the isotropic main field
EPR signal but without sign [28] and small exchange coupling con-
stants are accessible from PELDOR spectra.
Fig. 3. CW X-band EPR spectra of the nitroxide part of (a) model compound 1 and
(b) reference 3, both at room temperature. (c) Spectrum of the copper centre of 1
acquired at 10 K and with a microwave power of 100 mW saturating the nitroxide
centre. The solid black lines are the experimental spectra, the broken grey lines are
the simulations.
trum. Thus, 1 possesses a larger exchange coupling than 3, as antic-
ipated from the conjugated bridge and the exchange coupling is
antiferromagnetic as usually found for copper-nitroxide systems
of such geometries [25]. The reason for the distribution in J is the
ensemble of conformers. The different conformers possess different
orbital overlaps and thus different magnitudes for the through bond
exchange coupling constant. Thus, the increased modulation damp-
ing in the PELDOR time trace of 1 compared to 3, can be rationalized
3. Experimental
3.1. Synthesis
All reactions were performed with exclusion of air under argon
employing standard Schlenk techniques. Reagent-grade solvents
and chemicals were used without further purification, except where
stated otherwise. Dry solvents were purchased from Fluka or ACROS
and thoroughly degassed prior to use. Triethylamine was freshly
distilled from CaH2. D14-o-terphenyl was purchased from Cam-
bridge Isotope Laboratories and used without further purification.
Octaethylporphyrin (H2(OEP)) and copper(II)-octaethylporphyrin
by the larger J, +4 MHz compared to 0 MHz, and larger
DJ, 5 MHz
compared to 1 MHz, respectively.
2.4. Conclusion
We have shown on a copper/nitroxide model system that PEL-
DOR in combination with simulations allows for separating the

B.E. Bode et al. / Journal of Organometallic Chemistry 694 (2009) 1172–1179
1177
Fig. 6. (a) P(h) of 1. A sin(h) distribution for random orientations is shown for
comparison. (b) Dipolar spectra calculated from P(h) assuming a distance of 2.1 nm
and a J of 0 MHz.
3.1.1. Synthesis of (3-(40-copper(II)-2,3,7,8,12,13,17,18-
octaethylporphyrin-5-ylethynyl-biphenyl-4-ylethynyl)-2,2,5,5-
tetramethyl-pyrrolin-1-oxyl) (1)
3-(40-Iodo-biphenyl-4-ylethynyl)-2,2,5,5-tetramethyl-pyrrolin-
1-oxyl
5
(80 mg, 0.18 mmol) and PdCl2(PPh3)2 (19 mg,
0.027 mmol) were dissolved in NEt3 (12 mL) and DMF (5 mL) by
slightly warming the mixture. CuI (19 mg, 0.100 mmol) was added,
Fig. 5. Excitation profiles of (a) the detection pulses on the Cu(OEP) and (b) the
pump pulse on the nitroxide. The molecular axis systems are defined in (c). The
geometric model is depicted in (d).
followed by
a solution of Cu(meso-ethynyl-OEP) 6 (112 mg,
0.18 mmol) and PPh3 (18 mg, 0.06 mmol) in NEt3 (8 mL) and
DMF (3 mL). The mixture was stirred for 16 h at 60 °C. The solvents
were removed in vacuo, the residue dissolved in dichloromethane
and washed three times with water. The organic phase was dried
with sodium sulphate, and the solvent was removed in vacuo.
(Cu(OEP)) were purchased from Aldrich, thin layer chromatogra-
phy plates SiO2 60 F254 and Al2O3 60 F254 neutral were bought
from Merck, SiO2 60 (70–230 mesh) and Al2O3 MP Alumina N
– Super I were used from Aldrich or MP Biomedicals, respec-
tively. 1-Oxyl-2,2,5,5-tetramethylpyrrolin-3-acetylene (TPA)
4
[29] and 3-(40-Iodo-biphenyl-4-ylethynyl)-2,2,5,5-tetramethyl-
pyrrolin-1-oxyl 5 [10] were prepared by procedures described
earlier. Cu(meso-ethynyl-OEP)
6 and Ni(meso-ethynyl-OEP) 7
were prepared according to references [16,30] and used as
obtained, due to the products susceptibility to spontaneous oxi-
dative dimerisation [31].
Analytic relied on elemental analysis and mass spectrometry
such as EI, ESI, and MALDI. EI mass spectra were recorded on a
CH7A spectrometer from MAT, ESI mass spectra on a LCQ Classic
spectrometer from Thermo Electron and MALDI mass spectra on
a
Voyager DE-Pro or STR spectrometer, both from Applied
Biosystems. Proton NMR spectra of diamagnetic molecules were
acquired at 250 MHz on a Bruker AM-250 spectrometer and
calibrated using residual non-deuterated solvents as internal
standard (d CHCl3 = 7.240). Elementary analysis was performed
on a Foss-Heraeus CHN-O-Rapid and UV–Vis spectra were recorded
on an Agilent 8453 Spectrophotometer.
Fig. 7. Background corrected PELDOR data of 1 and its simulations based on an
exchange coupling of J = +4 MHz and a
DJ of 5 MHz or 0 MHz.

1178
B.E. Bode et al. / Journal of Organometallic Chemistry 694 (2009) 1172–1179
Chromatography (4 ꢂ 20 cm, Al2O3, 4% H2O, CHCl3/hexane 1:1 to
2:1) yielded a slow moving red-brown main band after three zones
of green, pink and light green colour all three of weak intensity.
The red-brown product fraction was collected and stripped from
solvent. The residue was further purified by repeated (2ꢂ) chroma-
tography (3 ꢂ 20 cm, Al2O3, 4% H2O, CH2Cl2) to remove a green by-
product. The desired product was then recrystallised two times
from chloroform/methanol and finally from benzene layered with
petroleum ether. Compound 1 was obtained as thin violet leaflets
and needles. Yield: 65 mg (0.07 mmol, 39%). Anal. Calc. for
C60H64N5OCu: C, 77.10; H, 6.90; N, 7.49. Found: C, 77.37; H, 6.89;
R indices [I > 2r(I)]: R1 = 0.0869, wR2 = 0.1232, R indices (all data):
R1 = 0.2730, wR2 = 0.1862, largest difference in peak/hole: 0.404/
0.583 e Å3. Crystallographic data for the structural analysis has been
deposited with the Cambridge Crystallographic Data Centre, CCDC
No. 614808 for compound 2 ꢃ C6H6.
3.3. EPR measurements
Samples of 1 and 2 were prepared as a d8-toluene and d14-o-ter-
phenyl (both 200 lM, 80 lL) solutions. Five equivalents of 1-meth-
ylimidazole were added to samples of 1, to prevent aggregation.
The d8-toluene sample was degassed by several freeze–pump cy-
cles and frozen in liquid nitrogen prior to sealing the sample tube.
The cw X-band EPR spectra were acquired on a Bruker ELEXSYS
E500 cw X-band EPR spectrometer equipped with a standard rect-
angular Bruker EPR cavity (ER4102T) equipped with an Oxford he-
lium cryostat (ESR900). The microwave frequency was measured
by use of a Systron Donner (6054D) frequency counter. The mag-
netic field was measured with a Bruker gaussmeter (ER035M).
All room temperature spectra were recorded with a sampling time
of 40 ms, a microwave power of 1 mW and a modulation ampli-
N, 7.52%. UV–Vis (benzene) kmax (loge): 427 nm (5.389), 554
(4.274), 594 (4.273). MALDI-TOF-MS m/z Calc. for C60H64N5OCu:
934.746; found: 936.6 ((M + 2 ꢂ H)+, 100%), 919.6 ((M + 2 ꢂ H – CH3)+,
32%), 875.7 ((M + 2 ꢂ H – 4 ꢂ CH3)+, 28%).
3.1.2. Synthesis of (3-(40-nickel(II)-2,3,7,8,12,13,17,18-
octaethylporphyrin-5-ylethynyl-biphenyl-4-ylethynyl)-2,2,5,5-
tetramethyl-pyrrolin-1-oxyl) (2)
To a suspension of 5 (27 mg, 0.06 mmol) and PdCl2(PPh3)2 in
5 mL NEt3
a solution of Ni(meso-ethynyl-OEP) 7 (37 mg,
0.06 mmol) in 10 mL NEt3 was added at room temperature via syr-
inge. The resulting red solution was heated to gentle reflux for
3.5 h and stirred for additional 16 h at ambient temperature. From
the red-brown solution a yellowish precipitate was formed. The
solvent was removed in vacuo, the residue dissolved in 2–4 mL
dichloromethane/hexane (1:1) and subjected to column chroma-
tography (3 ꢂ 25 cm, Al2O3, 3% H2O). A dark green band was eluted
first, followed by a broad red band of the main product. The main
fraction was brought to dryness and the residue washed with a
small amount of hexane. Recrystallisation from chloroform/meth-
anol yielded 32 mg (0.03 mmol, 57%) of 2 as thin, lustrous violet
platelets and needles. Suitable crystals for x-ray diffraction were
obtained from benzene solution layered with petroleum ether.
Anal. Calc. for C60H64N5ONi: C, 77.50; H, 6.94; N, 7.53. Found: C,
tude of 1 lT at a modulation frequency of 100 kHz. Low tempera-
ture spectra were recoded at 10 K, with a microwave power of
100 mW and a modulation amplitude of 0.5 mT at a modulation
frequency of 100 kHz. All cw X-band spectra simulations were per-
formed with EasySpin [36]. The room temperature nitroxide spec-
tra were simulated using gxx = 2.0095, gyy = 2.0071, gzz = 2.0036
and Axx = 0.4 mT, Ayy = 0.3 mT, Azz = 3.1 mT for the 14N hyperfine
coupling. The isotropic 1H hyperfine coupling of the 12 equiv.
methyl-protons was set to 0.65 MHz and to 1.3 MHz for the vinylic
proton. An isotropic 13C hyperfine coupling constant of 16.5 MHz
was assumed for the a- and methyl-carbons all with natural abun-
dance. The residual Lorentzian line width is 0.3 MHz and an axial
diffusion tensor with Dxy = 1 GHz and Dzz = 100 GHz was assumed.
All values are in good agreement with literature values [27,37].
The low temperature spectrum of the copper(II) centre has been
77.30; H, 6.98; N, 7.48%. UV–Vis (benzene) kmax (loge): 432 nm
(5.326), 557 (4.227), 592 (4.260). MALDI-TOF-MS (matrix, DHB)
m/z Calc. for C60H64N5NiO: 929.901; Found: 930.3 (M+, 100%),
914.3 ((M – CH3)+, 12%), 899.2 ((M – 2 ꢂ CH3)+, 12%).
simulated using gxx = gyy = 2.047, gzz = 2.17 and a g-strain of
gxx = gyy = 0.005, gzz = 0.015 and Axx = Ayy = 3 mT, Azz = 21 mT for
the Cu hyperfine coupling. The 14N hyperfine coupling of the four
porphyrin nitrogen atoms was assumed to be isotropic and set to
1.7 mT. The residual line width was 1.3 mT.
3.2. X-ray crystallography
All PELDOR spectra were recorded on a Bruker ELEXSYS E580
pulsed X-band EPR spectrometer with a standard flex line probe
head housing a dielectric ring resonator (MD5 W1) equipped with
a continuous flow helium cryostat (CF935) and temperature con-
trol system (ITC 502), both from Oxford instruments. The second
microwave frequency was coupled into the microwave bridge by
a commercially available setup (E580-400U) from Bruker. All
pulses were amplified via a pulsed travelling wave tube (TWT)
amplifier (117X) from Applied Systems Engineering. The resonator
was over-coupled to a quality factor Q of about 50. PELDOR exper-
Data of 2 ꢃ C6H6 were collected at 173 K on a STOE IPDS II two-
circle diffractometer using Mo K
a radiation. The structure was
solved using direct methods and refined on F2 values using the
full-matrix least-squares method. Programs used include X-AREA
(STOE & CIE GmbH, Darmstadt, Germany, 2001), SHELXS-97 [32], SHELXL-97
(G.M. Sheldrick, University of Göttingen, Germany, 1997), SHELXTL
Plus (Release 4.1, Siemens Analytical X-ray Instruments Inc., Mad-
ison, Visconsin, USA). The SHELXL-97
-
,
PLATON [33] and MERCURY [34] soft-
ware has been used to prepare material for publication. Graphical
representations of the crystal structure were produced using the
WinGX software [35].
iments were performed with the pulse sequence
A)–( 1 + t)– A)– 2-echo. The detection pulses
B)–(s2 ꢀ t)–
A) were set to 16 ns for both and /2-pulses and applied at a
p/2(mA)–s1–
p(m
s
p(m
p
(m
s
Crystal data: Empirical formula: C66H70N5NiO, formula weight:
1007.98, temperature: 173(2) K, wavelength: 0.71073 Å, crystal sys-
(m
p
p
frequency 118 MHz higher than the resonance frequency of the
resonator. The pulse amplitudes were chosen to optimize the refo-
ꢀ
tem: triclinic, space group: P1, unit cell dimensions: a = 12.639(4) Å,
a
= 73.80(3)°, b = 13.134(4) Å, b = 82.56(3)°, c = 17.540(6) Å,
87.32(3)°, volume: 2772.3(15) Å3, Z = 2, Dcalc: 1.207 Mg/m3, absorp-
tion coefficient: 0.397 mmꢀ1
F(000): 1074, crystal size:
c
=
cused echo. The
offsets. The pump pulse (
p
/2-pulse was phase-cycled to eliminate receiver
mB) with a length of 12 ns was set at the
,
resonance frequency of the resonator. The field was adjusted such
that the pump pulse is applied to the maximum of the nitroxide
spectrum, where it selects the central mI = 0 transition of Azz to-
gether with the mI = 0, 1 transitions of Axx and Ayy. The pulse
amplitude was optimized to maximize the inversion of a Hahn-
echo at the pump frequency. All PELDOR spectra were recorded
at 20 K with an experiment repetition time of 3 ms, a video ampli-
0.42 ꢂ 0.14 ꢂ 0.03 mm3, h-range for data collection: 3.61°–25.12°,
hkl index ranges: 14/14, 15/15, 20/20, reflections collected: 16491,
independent reflections: 9042 [Rint = 0.1828], completeness to
h = 25.12°: 91.3%, absorption correction: semi-empirical from
equivalents, maximum and minimum transmission: 0.9882 and
0.8510, refinement method: full-matrix least-squares on F2, data/re-
straints/parameters: 9042/0/658, goodness-of-fit on F2: 0.841, final
fier bandwidth of 25 MHz and an amplifier gain of 63 dB. s1 was set

B.E. Bode et al. / Journal of Organometallic Chemistry 694 (2009) 1172–1179
1179
[11] E. Narr, A. Godt, G. Jeschke, Angew. Chem., Int. Ed. 41 (2002) 3907.
[12] (a) I.M.C.v. Amsterdam, M. Ubbink, G.W. Canters, M. Huber, Angew. Chem.,
Int. Ed. 42 (2003) 62;
to 136 ns and s2 to 1200 ns. Usually 300 scans were accumulated
with 294 data points and time increments t of 4 ns giving an
approximate measurement time of 18 h necessary to obtain a sig-
nal-to-noise ratio >200:1. Proton modulation was suppressed by
D
(b) C.W.M. Kay, H.E. Mkami, R. Cammack, R.W. Evans, J. Am. Chem. Soc. 129
(2007) 4868;
(c) Z. Yang, J. Becker, S. Saxena, J. Magn. Reson. 188 (2007) 337.
[13] A.M. Raitsimring, C. Gunanathan, A. Potapov, I. Efremenko, J.M.L. Martin,
D. Milstein, D. Goldfarb, J. Am. Chem. Soc. 129 (2007) 14138.
[14] C. Elsässer, M. Brecht, R. Bittl, J. Am. Chem. Soc. 124 (2002) 12606.
[15] A.V. Astashkin, J. Seravalli, S.O. Mansoorabadi, G.H. Reed, S.W. Ragsdale, J. Am.
Chem. Soc. 128 (2006) 3888.
addition of eight spectra of variable s1 with a
Ds1 of 8 ns [38].
For comparison with simulations the time traces were divided by
a monoexponential decay and normalized to the point t = 0.
3.4. PELDOR simulations
[16] B.E. Bode, J. Plackmeyer, T.F. Prisner, O. Schiemann, J. Phys. Chem. A 112 (2008)
5064.
[17] S.O. Mansoorabadi, G.H. Reed, in: J. Telser (Ed.), Paramagnetic Resonance of
Metallobiomolecules, American Chemical Society, Washington, 2003, p. 82.
[18] W.B. Mims, in: S. Geschwind (Ed.), Electron Paramagnetic Resonance, Plenum,
New York, 1972, p. 263.
[19] (a) R.G. Larsen, D.J. Singel, J. Chem. Phys. 98 (1993) 5134;
(b) A.D. Milov, A.G. Maryasov, Y.D. Tsvetkov, Appl. Magn. Reson. 15 (1998)
107.
Commonly PELDOR data are analyzed by using data inversion
methods like Tikhonov regularization [39]. However, up to now
all these methods are based on the assumption of negligible angu-
lar correlations and exchange coupling contributions, which can
lead to erroneous results for systems like 1 and 3. We have, there-
fore, chosen to simulate the experimental PELDOR time traces of 1
with a home-written MatlabÒ program, in analogy to the proce-
dure as described for 3 in references [16,40].
[20] (a) M.O. Senge, M.G.H. Vicente, K.R. Gerzevske, T.P. Forsyth, K.M. Smith, Inorg.
Chem. 33 (1994) 5625;
(b) T.E. Clement, D.J. Nurco, K.M. Smith, Inorg. Chem. 37 (1998) 1150.
[21] D.L. Cullen, E.F. Meyer Jr., J. Am. Chem. Soc. 96 (1974) 2095.
[22] Low temperature cw X-band EPR spectra of 1 and 3 showed strong half-field
signals at g = 4 indicative of close spin–spin contacts. These signals
disappeared completely upon adding 1-methylimidazole pointing to
intermolecular stacking of these compounds as the origin for this strong
spin–spin coupling which can be disrupted by coordination of 1-
methylimidazole to copper(II) as known from the literature.
[23] (a) A.G. Maryasov, Yu.D. Tsvetkov, Appl. Magn. Reson. 18 (2000) 583;
(b) A.D. Milov, B.D. Naumov, Y.D. Tsvetkov, Appl. Magn. Reson. 26 (2004) 587.
[24] A. Weber, O. Schiemann, B. Bode, T.F. Prisner, J. Magn. Reson. 157 (2002)
277.
Acknowledgements
We thank Dr. Ute Bahr, Goethe-University, for recording mass
spectra. This work has been supported by the Deutsche Fors-
chungsgemeinschaft (SFB 579 ‘‘RNA-ligand interactions” and the
Center for Biomolecular Magnetic Resonance (BMRZ) Frankfurt.
[25] (a) O. Kahn, Molecular Magnetism, VCH, New York, 1993;
(b) S.S. Eaton, K.M. More, B.M. Sawant, P.M. Boymel, G.R. Eaton, J. Magn. Reson.
50 (1983) 435.
References
[26] (a) C. Elschenbroich, O. Schiemann, O. Burghaus, K. Harms, J. Pebler,
Organometallics 18 (1999) 3273;
[1] M. Ruben, J. Rojo, F.J. Romero-Salguero, L.H. Uppadine, J.-M. Lehn, Angew.
Chem., Int. Ed. 43 (2004) 3644.
[2] (a) A. Messerschmidt, R. Huber, T. Poulos, K. Wieghardt (Eds.), Handbook of
Metalloproteins, vols. 1–2, John Wiley & Sons, Ltd., Chichester, 2001;
(b) W.F. DeGrado, C.M. Summa, V. Pavone, F. Nastri, A. Lombardi, Ann. Rev.
Biochem. 68 (1999) 779.
[3] (a) F. Eckstein, D.M. Lilley, Nucleic acids and molecular biology, Catalytic RNA,
vol. 10, Springer, Berlin, 1995;
(b) R.F. Gesteland, T.R. Cech, J.F. Atkins, The RNA World, third ed., Cold Spring
Harbor, New York, 2006.
[4] (a) M. Ubbink, J.A.R. Worrall, G.W. Canters, E.J.J. Groenen, M. Huber, Ann. Rev.
Biophys. Biomol. Struct. 31 (2002) 393;
(b) C. Elschenbroich, M. Wolf, O. Schiemann, K. Harms, O. Burghaus, J. Pebler,
Organometallics 21 (2002) 5810;
(c) C. Elschenbroich, J. Plackmeyer, K. Harms, O. Burghaus, J. Pebler,
Organometallics 22 (2003) 3369;
(d) C. Elschenbroich, J. Plackmeyer, M. Nowotny, K. Harms, J. Pebler, O.
Burghaus, Inorg. Chem. 44 (2005) 955;
(e) C. Elschenbroich, O. Schiemann, O. Burghaus, K. Harms, Chem. Commun.
(2005) 2149.
[27] J. Fritscher, M. Beyer, O. Schiemann, Chem. Phys. Lett. 364 (2002) 393.
[28] C. Elschenbroich, O. Schiemann, O. Burghaus, K. Harms, J. Am. Chem. Soc. 119
(1997) 7452.
[29] (a) T. Kálai, M. Balog, J. Jekö, K. Hideg, Synthesis (1999) 973;
(b) O. Schiemann, N. Piton, J. Plackmeyer, B.E. Bode, T.F. Prisner, J.W. Engels,
Nat. Protoc. 2 (2007) 904.
(b) C. Calle, A. Sreekanth, M.V. Fedin, J. Forrer, I. Garcia-Rubio, I.A. Gromov,
D. Hinderberger, B. Kasumaj, P. Léger, B. Mancosu, G. Mitrikas, M.G. Santangelo,
S. Stoll, A. Schweiger, R. Tschaggelar, J. Harmer, Helv. Chim. Acta 89 (2006)
2495;
[30] N. Hayashi, M. Murayama, K. Mori, A. Matsuda, E. Chikamatsu, K. Tani,
K. Miyabayashi, M. Miyake, H. Higuchi, Tetrahedron 60 (2004) 6363.
[31] (a) D.P. Arnold, D.A. James, J. Org. Chem. 62 (1997) 3460;
(b) D.P. Arnold, R.D. Hartnell, Tetrahedron 57 (2001) 1335.
[32] G.M. Sheldrick, Acta Crystallogr. A 46 (1990) 467.
[33] A.L. Spek, J. Appl. Crystallogr. 36 (2003) 7.
[34] (a) I.J. Bruno, J.C. Cole, P.R. Edgington, M.K. Kessler, C.F. Macrae, P. McCabe,
J. Pearson, R. Taylor, Acta Crystallogr. B 58 (2002) 389;
(b) C.F. Macrae, P.R. Edgington, P. McCabe, E. Pidcock, G.P. Shields, R. Taylor,
M. Towler, J. van de Streek, J. Appl. Crystallogr. 39 (2006) 453.
[35] L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837.
(c) N. Kisseleva, S. Kraut, A. Jäschke, O. Schiemann, HFSP J. 1 (2007) 127;
(d) N. Kisseleva, A. Khvorova, E. Westhof, O. Schiemann, RNA 11 (2005) 1.
[5] (a) G. Jeschke, Y. Polyhach, Phys. Chem. Chem. Phys. 9 (2007) 1895;
(b) O. Schiemann, T.F. Prisner, Quart. Rev. Biophys. 40 (2007) 1.
[6] (a) A.D. Milov, K.M. Salikhov, M.D. Shirov, Fiz. Tverd. Tela 23 (1981) 975;
(b) R.E. Martin, M. Pannier, F. Diederich, V. Gramlich, M. Hubrich, H.W. Spiess,
Angew. Chem., Int. Ed. 37 (1998) 2833.
[7] (a) M. Bennati, A. Weber, J. Antonic, D.L. Perlstein, J. Robblee, J. Stubbe, J. Am.
Chem. Soc. 125 (2003) 14988;
(b) G. Jeschke, Macromol. Rapid Commun. 23 (2002) 227;
(c) O. Schiemann, N. Piton, Y. Mu, G. Stock, J.W. Engels, T.F. Prisner, J. Am.
Chem. Soc. 126 (2004) 5722;
[36] S. Stoll, A. Schweiger, J. Magn. Reson. 178 (2006) 42.
[37] (a) R.W. Kreilick, J. Chem. Phys. 46 (1967) 4260;
(d) S.-Y. Park, P.P. Borbat, G. Gonzalez-Bonet, J. Bhatnagar, A.M. Pollard,
J.H. Freed, A.M. Bilwes, B.R. Crane, Nat. Struct. Mol. Biol. 13 (2006) 400.
[8] A. Godt, M. Schulte, H. Zimmermann, G. Jeschke, Angew. Chem., Int. Ed. 45
(2006) 7560.
[9] (a) V.P. Denysenkov, T.F. Prisner, J. Stubbe, M. Bennati, Proc. Natl. Acad. Sci.
USA 103 (2006) 13386;
(b) G.F. Hatch, R.W. Kreilick, J. Chem. Phys. 57 (1972) 3696.
G. Jeschke, G. Panek, A. Godt, A. Bender, H. Paulsen, Appl. Magn. Reson. 26
[38]
(2004) 223.
[39] (a) G. Jeschke, V. Chechik, P. Ionita, A. Godt, H. Zimmermann, J. Banham,
C.R. Timmel, D. Hilger, H. Jung, Appl. Magn. Reson. 30 (2006) 4733;
(b) M.K. Bowman, A.G. Maryasov, N. Kim, V.J. DeRose, Appl. Magn. Reson. 26
(2004) 23;
(c) Y.-W. Chiang, P.P. Borbat, J.H. Freed, J. Magn. Reson. 172 (2005) 279.
[40] D. Margraf, B.E. Bode, A. Marko, O. Schiemann, T.F. Prisner, Mol. Phys. 105
(2007) 2153.
(b) Y. Polyhach, A. Godt, C. Bauer, G. Jeschke, J. Magn. Reson. 185 (2007) 118;
(c) A. Savitsky, A.A. Dubinskii, M. Flores, W. Lubitz, K. Möbius, J. Phys. Chem. B
111 (2007) 6245.
[10] B.E. Bode, D. Margraf, J. Plackmeyer, G. Dürner, T.F. Prisner, O. Schiemann,
J. Am. Chem. Soc. 129 (2007) 6736.