Hydroxycruciforms: Amine-responsive fluorophores

Article abstract of DOI:10.1002/chem.200800296

The synthesis of three hydroxy-substituted cruciforms (XF, 1,4-bis(4′-hydroxystyryl)-2,5-bis(4″-methoxyphenylethynyl)benzene, 1,4-bis(4′-methoxystyryl)-2,5-bis(4″-hydroxyphenylethynyl)benzene, and 1,4-bis(4′-hydroxystyryl)-2,5-bis(4″-hydroxyphenylethynyl)

Full text of DOI:10.1002/chem.200800296

FULL PAPER
DOI: 10.1002/chem.200800296
Hydroxycruciforms: Amine-Responsive Fluorophores
[a]
Psaras L. McGrier,[a] Kyril M. Solntsev, Shaobin Miao,[a] Laren M. Tolbert,[a]
Oscar R. Miranda,[b] Vincent M. Rotello,[b] and Uwe H. F. Bunz*[a]
Dedicated to Prof. Klaus Müllen
Abstract: The synthesis of three hy-
droxy-substituted cruciforms (XF, 1,4-
bis(4’-hydroxystyryl)-2,5-bis(4’’-meth-
jected to photometric UV/Vis titrations
in a methanol/water mixture. The re-
spective pKa values were obtained by
data deconvolution. As the three XFs
display a significant change in emission
color upon photoinduced deprotona-
tion, the XFs were taken up in differ-
ent solvents and exposed to twelve
amines. The amine-dependent change
in emissivity of the tetrahydroxy XF is
sufficiently distinct in the eight solvents
that all of the inspected amines are dis-
cerned by a linear discriminant analy-
sis. The tetrahydroxy XF in different
solvents forms a sensor array, the re-
sponse of which is based on the excit-
ed-state proton transfer (ESPT) to
amines and mediated by the choice of
the battery of solvents that are utilized.
ACHTREUNGoxyphenylethynyl)benzene, 1,4-bis(4’-
methoxystyryl)-2,5-bis(4’’-hydroxyphe-
nylethynyl)benzene, and 1,4-bis(4’-hy-
droxystyryl)-2,5-bis(4’’-hydroxyphenyl-
ACHTREUNGethynyl)benzene) starts with a Horner
reaction followed by a Sonogashira
couplingand subsequent deprotection.
The three herein described XFs contain
either two or four free phenolic hy-
droxyl groups. All three XFs were sub-
Keywords: alkynes · amines · fluo-
rescence · sensors · solvatochrom-
ism
Introduction
imprinted polymers,[2] enzymes,[3] single-molecule and array
sensors,[4] and chromatographic methods.[5] Recently Lavinge
elegantly demonstrated that the interplay of planarization
and aggregate formation in a water soluble polythiophene
We describe the synthesis, photophysics and amine respon-
sive optical properties in absorption and emission of three
cruciform (XF) chromophores 1–3. The detection determi-
nation and quantification of low-molecular-weight amines is
critical in the medical field, in environmental science, and in
food safety. The enhanced presence of low-molecular-weight
amines in breath can mark disease states in patients and in
foods it indicates spoilage. The detection and quantification
of amines has been achieved by antibodies,[1] molecularly
derivative is a powerful colorimetric tool to detect histamine
[6]
in food, predictingspoilaeg in fish samples.
Inspired by
Lavigneꢀs work, we have tailored a novel class of cruciform
fluorophores/chromophores (XF), 1,4-distyryl-2,5-bis(aryl-
AHCTREeUNG thynyl)benzenes, carryingphenol functionalities as model
probes for amines.[7] While these small XF-fluorophores do
not display aggregation or distinct planarization behavior,[8]
their specifically engineered frontier molecular orbitals
(FMOs) should allow signal generation and amplification of
amine-probingfunctions such as phenols. The phenols will
exhibit either full or partial proton transfer to the amine ni-
trogen atom, resulting in observable spectroscopic changes.
[a] P. L. McGrier, Dr. K. M. Solntsev, Dr. S. Miao, Prof. L. M. Tolbert,
Prof. U. H. F. Bunz
School of Chemistry and Biochemistry
Georgia Institute of Technology, 901 Atlantic Drive
Atlanta, GA 30332-0400 (USA)
Fax : (+1)404-385-1795
E-mail: uwe.bunz@chemistry.gatech.edu
Chromophore design centers around different fundamental
paradigms: 1) Choose or construct a suitable (aromatic) car-
bocyclic or heterocyclic skeleton, then 2) attach the necessa-
ry auxochromic groups, that is, electron-accepting or elec-
tron-releasingsubstituents to the skeleton to tune absorp-
tion and emission. In most chromophores donor and accep-
[b] O. R. Miranda, Prof. V. M. Rotello
Department of Chemistry, 710 North Pleasant St.
University of Massachusetts, Amherst, MA 01003 (USA)
Supportinginformation for this article is available on the WWW
under http://www.chemeurj.org/ or from the author. Experimental de-
tails are provided for the synthesis of XFs 1–3, as well as details for
the photophysical experiments and the absorption and emission spec-
tra for all systems studied.
AHCTREtUNG or substituents are attached to the skeleton into positions
in which both FMOs have their largest orbital nodes, ensur-
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Scheme 1. Modulation of the HOMO–LUMO gap in cruciform fluorophores by interaction with metal cations and pH change.
ingthe maximum conjugative effect of the auxochromes to
the dye skeleton.[9] Auxochromes enlarge the p system and
stabilize/destabilize both the HOMO and LUMO, but to a
different degree, leading to red-shifted absorption.
upon exposure to a panel of amines in different solvents.
These studies are of interest as it is possible, just by chang-
ingthe solvent, to identify specific amines by the analysis of
the fluorescence color of a single XF, 3.
We and others have designed dyes in which the geometric
overlap of HOMO and LUMO is minimized. These dyes,
cruciforms[10] (XF), consist of two independent but centrally
connected molecular axes, which carry electron-donating
and electron-withdrawingsubstituents respectively; this ar-
rangement leads to a situation in which the HOMO is spa-
tially confined on the electron-rich branch, while the
LUMO is confined on the branch that carries the electron-
withdrawingsubstituents. Examples are 1,4-distyryl-2,5-bis-
Results and Discussion
Synthesis: A Horner reaction of 4 with the aldehydes 5a or
5b furnished the distyrylbenzene derivatives 6a and 6b in
77 and 64% yield, respectively, after chromatography
(Scheme 2). Subsequently, a Sonogashira coupling[19] with
either 7a or 7b gave rise to the formation of 8a–c in yields
from 61 to 77%. At a temperature of ꢀ788C trifluoroacetic
acid (TFA) in dichloromethane smoothly deprotected 8a–c
to give the XFs 1–3 in yields around 88–92% as air and
water-stable yellow powders.
ACHTREUNG
ACHTREUNG
tetraethynylethylenes,[15] and Galvinꢀs distyrylbenzene deriv-
atives.[16] One consequence of the spatially localized FMOs
is a surprisingly large auxochromic effect; that is, absorption
and emission are more strongly influenced by substituents
than would be generally expected, allowing to tune the
emission of a carbocyclic skeleton from blue to red. These
FMO-separated phores should allow biomolecular or envi-
ronmental sensingas XFs might be able to probe metal cat-
ions in cell compartments.[17] Outlined in Scheme 1 is a two-
stage metalloresponsive, orange-emitting model fluorophore
A. Upon exposure to magnesium or zinc ions the fluores-
cence of A changes to blue, but upon further increasing the
amount of metal salt the fluorescence color changes from
blue to yellow-green. The unusual color changes are ex-
plained by a consecutive stabilization of first the HOMO
and then the LUMO of the metal-complexed XF
(Scheme 1).
Spectroscopic properties of the cruciforms 1–3: Figure 1 dis-
plays the absorption and emission spectra of 1–3 in different
solvents (see also Tables 1–3). The spectra of tetra-ether 8c
are given for comparison. Solvatochromic behavior of 1–3
and 8c was investigated.[20] Kamlet–Taft solvent parame-
ters[21] can account for the contribution of selective (such as
point-to-point hydrogen-bonding interactions) versus non-
selective (solute–solvent dipole interactions) solvation to the
electronic spectra of the hydroxyaromatic molecules. For 8c
the absorbance spectra depend weakly on solvent polarity,
indicatinga small ground-state dipole moment. The emis-
sion spectra of 8c exhibit stronger bathochromic shifts in
polar solvents due to the increase in dipole moment upon
excitation (Table 4). All four compounds (8c, 1–3) are iso-
While we have investigated the stabilization of the FMOs,
we can also induce destabilization of the HOMO by intro-
duction of negative charges as in the hydroxy XFs B
(Scheme 1), and indeed, deprotonation led to a red-shift in
emission.[18] The LUMO of B is not affected accordingto
DFT calculations. In this contribution we examine the emis-
sive properties of three different hydroxy-substituted XFs 1–
3 and their emissive properties upon deprotonation and
ACHTREeUNG lectronic. We assume that their dipole moments in the
ground and the excited states are similar. Therefore, addi-
tional bathochromic shifts in the di- and tetrahydroxy cruci-
forms are related to hydrogen-bonding between the hydroxy
groups of the chromophores and basic solvents, such as
DMSO or DMF. However, these shifts are small, indicating
weak acidity of phenol moieties in both ground and excited
states. A weak shoulder located at 530 nm in the emission of
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Hydroxycruciforms
FULL PAPER
3 in 90% water/methanol sol-
vent mixture might be associat-
ed with the excited-state
proton-transfer product.[22] For
all dyes the fluorescence quan-
tum yield (F) in methanol was
in the range of 16–37%
(Table 5). Compound 2 has the
highest quantum yield and the
longest emissive lifetime (t=
1.6 ns). It is not clear what the
reason is for the differences in
the structurally similar XFs.
The compounds 1 and 2 were
poorly soluble in pure water at
neutral pH, demonstratingfor-
mation of red-shifted aggre-
gates in the absorbance spectra
(see SupportingInformation).
A
comparative study of the
acid–base behavior of 1–3 was
performed in a 2:1 volume ratio
of methanol/water (Figure 2).
However, the absorbance spec-
tra of 2 in this solvent at neu-
tral pH differed from that in
various organic solvents. Thus,
the absorbance titration data of
2 should be evaluated with cau-
tion, since it reflects not only
deprotonation, but also the dis-
solution of its aggregates. This
aggregation phenomenon at
neutral pH is observed also for
1, but to a much lesser extent.
The three compounds respond
differently towards hydroxide
ions in absorption and emission.
In the case of 1 a new band
emerges (416 nm, UV/Vis),
which is fully developed at
pH 12. Visually, the almost col-
orless solution turns yellow. Si-
multaneously, a new band at
600 nm appears in the emission
spectra, similar to that de-
scribed by us for B (Scheme 1),
while the emission at 460 nm
dissapears due to full ground-
state deprotonation. At higher
pH the long-wavelength emis-
sion exhibits a small hypsochro-
mic shift. The titration of XF 2
did not lead to the formation of
the characteristic red-shifted
phenolate band in the absorb-
ance spectra, and the fluores-
Scheme 2. Synthesis of cruciforms 1–3 by a combination of Horner and Sonogashira methods.
Figure 1. Absorption and emission spectra of 8c and 1–3 in different solvents. The color codingis the identical
for all graphs. MW is 1:9 vol. methanol/water.
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U. H. F. Bunz et al.
Table 1. Absorption and emission maxima for 1 in different solvents.
cence spectra were quenched without appearance of a new
low-energy band. Similar effects were observed by us earli-
er[18] for some other dihydroxycruciforms. On the basis of
MO calculations we have demonstrated that the HOMO
and LUMO orbitals of such molecules have a vanishing
overlap, which makes the S0–S1 electronic transition forbid-
den. As a result, bathochromic shifts in the absorbance spec-
tra were not observed upon deprotonation; the emission
from the deprotonated species is so weak that it can not be
recorded. The XF 3, with four hydroxy groups, features a
diffuse isosbestic point around 346 nm; upon deprotonation
a prominent feature develops at 370 nm. We assumed that
the terminal spectra at the highest pH value for each XF
belongto the fully deprotonated form of the respective
chromophore. The absorbance spectra of the tetra-anion of
3 can be viewed as a superposition of 1 and 2 dianion bands.
Solvent
lmaxabs
lmaxem
Stokes shift
[cmꢀ1
Vibronic
[nm]
[nm]
G
]
progression
AHCTREUNG ]
[cmꢀ1
methanol
acetonitrile
DMF
DMSO
THF
CH2Cl2
diethyl ether
toluene
336, 365 sh
337, 370 sh
340, 375 sh
343, 380 sh
339, 370 sh
342, 380 sh
336, 370 sh
339, 375 sh
451
451
463
468
431, 453
428, 451
423, 447
424, 449
7589, 5224
7501, 4854
7813, 5068
7787, 4948
6297, 3825
5875, 2951
6121, 3386
5914, 3082
-
-
-
-
1127
1192
1269
1313
Table 2. Absorption and emission maxima for 2 in different solvents.
Solvent
lmaxabs
lmaxem
Stokes shift
[cmꢀ1
Vibronic
[nm]
[nm]
G
]
progression
AHCTREUNG ]
[cmꢀ1
To understand these effects, we analyzed the photometric
methanol
acetonitrile
DMF
DMSO
THF
CH2Cl2
diethyl ether
toluene
338, 370 sh
337, 370 sh
344, 375 sh
346, 380 sh
341, 370 sh
339, 375 sh
338, 370 sh
339, 380 sh
428, 450
433, 452
437, 458
441, 462
428, 453
431, 453
422, 448
426, 452
6221, 3663
6579, 3932
6186, 3783
6226, 3640
5961, 3663
6297, 3465
5889, 3330
6024, 2842
1142
971
[22b]
absorption data usingthe SPECFIT software.
Figure 3
1049
1031
1289
1127
1375
1350
displays the relative amounts of the correspondingdepro-
tonated species present and the deconvoluted spectra of the
neutral compound and all of the phenolate anions up to the
tetra-anion for 3.
Upon increasingthe pH the absorption profiles of the
formed mono-, di-, tri-, and tetra-anion are deconvoluted.
Their absorption maximum is consecutively red-shifted from
335 to 370 nm.
Table 3. Absorption and emission maxima for 3 in different solvents.
When traversingfrom pH 7 to pH 10 we observe a signifi-
cantly red-shifted (588 nm) emission band of lower intensity
in 3. Upon further increase of the pH, the fluorescence in-
tensity of 3 increases again and the emission maximum blue-
shifts to 565 nm. The results of the fit demonstrate the coex-
Solvent
lmaxabs
lmaxem
Stokes shift Vibronic
[cmꢀ1
progression
[cmꢀ1
[nm]
[nm]
G
]
U
]
ꢀ
90:10 H3C OH/H2O 334
458
8106
-
methanol
acetonitrile
DMF
DMSO
THF
CH2Cl2
diethyl ther
toluene
337
335
343
345
340
332
435, 456 6685
438, 450 7020
464
467
432, 456 6264
428, 450 6756
1059
609
-
istence of several polyanions of 3 at different pH regimes. It
7602
7572
[23]
is surprisingthat in contrast to polyphenols
the pKaꢀs of
-
four hydroxyl groups differ by not more than one unit. A
possible explanation for this phenomenon is the weak elec-
tronic interaction between two pairs of hydroxyls located on
the distyrylbenzene and arylethynyl axes of the molecules.
Another important observation from the data fittingis the
pH mismatch in the existence areas of the different acid–
base species for the ground- and the excited-state titrations.
Such spectral behavior is a classical example of the photo-
1218
1142
1313
1263
337, 370 sh 424, 449 6089, 3442
336 424, 448 6177
Table 4. Absorption and emission maxima for 8c in different solvents.
Solvent
lmaxabs
lmaxem
Stokes shift
[cmꢀ1
Vibronic
[nm]
[nm]
G
]
progression
AHCTREUNG
acidity in the hydroxyaromatic molecules.[24] We note,
AHCTREUNG ]
[cmꢀ1
though, that the apparent shifts between the ground- and
excited-state pKaꢀs are only about one unit, demonstratinga
small but detectable increase of the photoacidity in aqueous
solutions.
methanol
acetonitrile
DMF
DMSO
THF
CH2Cl2
diethyl ether
toluene
insoluble
427, 448
433, 451
436, 455
441, 458
427, 451
428, 450
421, 446
426, 451
–
–
331, 370 sh
338, 385 sh
332, 370 sh
338, 375 sh
336, 370 sh
335, 375 sh
340, 380 sh
7116, 4854
6650, 4689
7445, 5193
6167, 4494
6397, 4805
6098, 4245
5938, 4143
922
958
842
1246
1142
1331
1301
Amine sensing using the cruciforms 1–3: With these results
in hand we set out to explore the fluorescence change of the
XFs 1–3 upon exposure towards different amines. We pre-
pared 10 mm solutions of the respective XFs in eight differ-
ent solvents. These were then distributed into 13 vials each
to obtain a matrix of 12 amines plus reference in 8 solvents
to give 104 samples per XF. The amine (0.1 mL per sample,
an excess, correspondingto a 0.7–7.2 m m concentration
range) was then added and a picture of the 13 samples with
12 different amines for each of the eight solvents was taken
Table 5. Photophysical data of 1–3 in methanol.
1
2
3
Abs [nm]
Em [nm]
F
336, 365
433, 451
0.17
338, 370
430, 451
0.37
337
435, 456
0.16
t [ns]
0.80
1.60
0.89
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Hydroxycruciforms
FULL PAPER
While the XF 1 gives color
changes in emission, the XF 2
mostly experiences quenching
(see the SupportingInforma-
tion), similar to the titration in
a methanol/water mixture. The
XF 3, however, experienced
spectacular changes in fluores-
cence upon the combination of
amines, with the emission
colors ranging from blue to red
traversingyellow and rgeen,
coveringthe full visible spectral
range.
Ground- and excited-state
acid–base interactions between
dihydroxycruciforms and vari-
ous amines in dichloromethane
were studied[18] and the fluoro-
phores exhibit emission from
the fully deprotonated (ion
pair) state. From these observa-
tions we concluded that in di-
chloromethane the difference in
pKa (or DG of the proton trans-
fer) between the excited dihy-
droxycruciforms and amines is
sufficient to produce the sol-
vent-separated ion pair with the
emission around 550 nm.[25] In
the ground state the observed
DpKa results in the formation
Figure 2. Absorption and emission spectra of 1–3 in 2:1 vol. methanol/water mixtures at different pH. The
band at 690 nm in the emission titration of 3 is a scatteringpeak and represents double the excitation wave-
length.
(Figure 4). These real-color photographs were taken in the
dark upon irradiation with a hand-held UV-lamp at an emis-
sion wavelength of 366 nm.
of the hydrogen-bonded complexes.
Generally, a similar behavior of 1 and 3 is observed in the
present work for an array of solvents and amines. The only
amine that quantitatively deprotonates the XFs in the
ground state is the most basic DBU; significant changes
occur both in absorption as well as in emission (Figure 5).
While only DBU leads to a significant shift in absorption,
almost all amines lead to a red-shift in the emission of 3.
[21]
We were successful in utilizingthe Kamlet–Taft method
to analyze the solvatochromic behavior of the ESPT product
emission maxima, that is, the vertical columns in Figure 4.
For instance, for 3 the solvent dependence of the emission
maxima (n) of the long-wavelength band in the presence of
ethylenediamine can be presented as Equation (1):
nð103 cmꢀ1Þ ¼ 19:8ꢀ2:7p ꢀ0:9b þ 0:7a
ðr ¼ 0:95Þ
*
ð1Þ
in which p*, b, and a are Kamlet–Taft solvent parameters
reflectingthe polarity, basicity, and acidity, respectively, of
the solvent (r=residual). From this analysis one can see that
the increase of solvent polarity and basicity causes the bath-
ochromic shift of the emission, while the acidity of the sol-
vent works in opposite direction. The magnitudes of the co-
Figure 3. Deconvoluted absorption and emission spectra of the anions of
3 with relative pKa values: pKa1 =9.2
10.6 (ꢁ0.2).
(ꢁ0.3); pKa4 =11.3
A
R
=
A
ACHTREUNG
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4507

U. H. F. Bunz et al.
maxima) did not have a straigh-
forward correlation with the
pKa.
While the photographs give a
good indication to discern 12
amines by 3, we converted the
color into RGB values and sub-
tracted the RGB value of the
reference usingthe prorgam
Contrast Analyzer.[26] Two inde-
pendent readings yielded RGB
values for the XF 3 that were
subjected to an LDA analysis
with the program SYSTAT
(Figure 6).[27] With 24 different
data points for each amine,
SYSTAT reduces the data into
a 2D LDA plot containingonly
two factors. The 12 amines are
cleanly separated accordingto
the analysis of their RGB
values, allowingus to discern
diethylamine and triethylamine
or diethylamine and diisopro-
pylamine. Interestingly, the
amines are not grouped in this
LDA plot accordingto their
pKa values; however, the di-
Figure 4. Photographs of solutions of 1 (left), and 3 (right) upon addition of amines 2–13 (left to right) 1) XF
reference, 2) histamine (6.9), 3) imidazole (6.9), 4) morpholine (8.3), 5) piperazine (9.8), 6) putrescine (9.9), 7)
1,3-diaminopropane (10.5), 8) ethylenediamine (10.7), 9) piperidine (10.8), 10) triethylamine (10.8), 11) dieth-
ylamine (11.0), 12) diisopropylamine (11.1), 13) 1,8-diazabicyclo
A
parentheses are the pKa values of the correspondingammonium ions in water] in different solvents (top to
bottom): A) methanol, B) acetonitrile, C) DMF, D) DMSO, E) THF, F) dichloromethane, G) diethyl ether,
and H) toluene. The samples were excited by usinga hand-held UV-lamp at an emission wavelentgh of
366 nm.
AHCTREaGNU mines (green) with exception
of piperazine are grouped to-
gether, and secondary amines
such as piperidine, diethyl-
amine, and diisopropylamine
(yellow-orange) are also group-
ed together at the bottom of
the plot.
Figure 5. Absorption and emission of XF 3 in acetonitrile upon addition of different amines. Note that only
DBU gives a significant red shift in absorption, while almost all amines give a significant shift in emission.
Conclusion
In conclusion, we have synthe-
sized three phenolic XFs 1–3.
XFs 1 and 3 display red-shifted
absorption and emission upon
deprotonation in methanol/
water mixtures and were inves-
tigated for amine sensing. A
series of 12 different amines
could be discerned by the spe-
cific fluorescence response of 3
based on excited-state proton
transfer in eight different sol-
vents. These experiments imply
that one can create a “chemical
Figure 6. Linear discriminant analysis (LDA) of the differential RGB values (left) and ratio intensities (right)
of 3 obtained from the right-hand side of Figure 4. The data on the left were extracted from the matrix gener-
ated by the RGB values measured for the photographs of the XF 3 dissolved in eight solvents in the presence
of each different amine. The data on the right were extracted from the lmax of emission and the relative fluo-
rescence intensities of 3 in the presence of each amine. All of the amines are separated in the 2D LDA-plot.
The two factors do not represent a specific chemical property of the amines, such as pKa value, chemical struc-
ture or other evident chemical properties in either case.
efficients demonstrate the dominatingrole of the polarity in
the solvatochromic behavior of the fluorescence. Interesting-
ly, the data from the horizontal rows in Figure 4 (emission
nose” by usingonly one sensor molecule, but in different
environments, that is, solvents. The emission wavelength of
XF 3 is exquisitely sensitive towards different amines, and
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Hydroxycruciforms
FULL PAPER
C. OꢀSullivan, J. J. Lavigne, Org. Lett. 2007, 9, 3217–3220; c) T. L.
Nelson, C. OꢀSullivan, N. T. Greene, M. S. Maynor, J. J. Lavigne, J.
Am. Chem. Soc. 2006, 128, 5640–5641.
that in a solvent dependent fashion. The selectivity and re-
sponsivity of one fluorophore suffices to constitute a small
sensor array just by changing the solvent. Using solutions of
3 would not be the most effective way to design a strip
sensor or a similar application-oriented gadget, but the
proof of principle is important, as XFs could easily be incor-
porated into grafted, conjugated polymers, in which the ap-
pended, non-conjugated polymer chains should be able to
substitute for the solvent. Such materials could be spin cast
onto silanized silica gel and their color response be observed
upon exposure towards amines in air or water. The herein
described experiments serve as a valuable guide for the
design and execution of such polymeric materials, upon
which we will report in the future. The colorful hydroxy XFs
1 and 3 display large and unique ratiometric shifts upon ex-
posure to amines and are fascinatingobjects, fit for further
evaluation exploitingthe principles of spatial separation of
FMOs and the mechanisms of the photoinduced proton
transfer.
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The experimental details for the synthesis of XFs 1–3, as well as details
for the photophysical experiments and the absorption and emission spec-
tra for all systems studied are available in the SupportingInformation.
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We thank the National Science Foundation for financial support (CHE-
0456892 for K.S. and L.T., CHE-0750275 for U.B. and P.M.), Prof. Chris-
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Received: February 17, 2008
Published online: April 18, 2008
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