Synthesis and evaluation of the sensitivity and vibrational lifetimes of thiocyanate and selenocyanate infrared reporters

Article abstract of DOI:10.1039/c5ra27363c

Two novel 2′-deoxyadenosine (dA) analogues, Si₂-dA-SCN and Si₂-dA-SeCN, and two novel phenylalanine (Phe) analogues, Boc-Me-PheCH₂SCN and Boc-Me-PheCH₂SeCN, have been synthesized and the thiocyanate (SCN) and selenocyanate (SeCN) functional groups evaluated as vibrational reporters. The syntheses of Si₂-dA-SCN and Si₂-dA-SeCN were accomplished in three steps in 16% and 32% overall yields, respectively, and the syntheses of Boc-Me-PheCH₂SCN and Boc-Me-PheCH₂SeCN were completed in four steps in 8.9% and 2.3% overall yields, respectively. The SCN and SeCN stretch vibrational modes were shown to be sensitive to the local environment by frequency shifts and full-width half-maximum (fwhm) changes in response to tetrahydrofuran (THF) and THF/water solvent mixtures. The vibrational lifetimes of the Si₂-dA-SeCN (237 ± 12 ps) and Boc-Me-PheCH₂SeCN (295 ± 31 ps) in THF solution were determined by ultrafast infrared pump-probe spectroscopy to be 1.5 to 3 times longer than those for Si₂-dA-SCN (140 ± 6 ps) and Boc-Me-PheCH₂SCN (102 ± 4 ps). The longer lifetimes for the SeCN analogues were attributed to the better insulating effects of the heavier selenium atom compared to the sulfur atom. The solvent sensitivity and longer vibrational lifetimes compared to other vibrational reporters suggest that SCN and SeCN vibrational reporters are well suited to studying several dynamic processes including protein and nucleic acid hydration and conformational changes, however stability issues may require post-synthetic modification methods to incorporate these reporters into biomacromolecules.

Full text of DOI:10.1039/c5ra27363c

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Synthesis and evaluation of the sensitivity and
vibrational lifetimes of thiocyanate and
selenocyanate infrared reporters†
Cite this: RSC Adv., 2016, 6, 36231
Daniel E. Levin,a Andrew J. Schmitz,b Shawn M. Hines,a Kevin J. Hines,a
b
a
a
*
*
*
Matthew J. Tucker, Scott H. Brewer and Edward E. Fenlon
Two novel 20-deoxyadenosine (dA) analogues, Si2–dA–SCN and Si2–dA–SeCN, and two novel
phenylalanine (Phe) analogues, Boc–Me–PheCH2SCN and Boc–Me–PheCH2SeCN, have been
synthesized and the thiocyanate (SCN) and selenocyanate (SeCN) functional groups evaluated as
vibrational reporters. The syntheses of Si2–dA–SCN and Si2–dA–SeCN were accomplished in three steps
in 16% and 32% overall yields, respectively, and the syntheses of Boc–Me–PheCH2SCN and Boc–Me–
PheCH2SeCN were completed in four steps in 8.9% and 2.3% overall yields, respectively. The SCN and
SeCN stretch vibrational modes were shown to be sensitive to the local environment by frequency shifts
and full-width half-maximum (fwhm) changes in response to tetrahydrofuran (THF) and THF/water
solvent mixtures. The vibrational lifetimes of the Si2–dA–SeCN (237 ꢀ 12 ps) and Boc–Me–PheCH2SeCN
(295 ꢀ 31 ps) in THF solution were determined by ultrafast infrared pump-probe spectroscopy to be 1.5
to 3 times longer than those for Si2–dA–SCN (140 ꢀ 6 ps) and Boc–Me–PheCH2SCN (102 ꢀ 4 ps). The
longer lifetimes for the SeCN analogues were attributed to the better insulating effects of the heavier
selenium atom compared to the sulfur atom. The solvent sensitivity and longer vibrational lifetimes
compared to other vibrational reporters suggest that SCN and SeCN vibrational reporters are well suited
to studying several dynamic processes including protein and nucleic acid hydration and conformational
changes, however stability issues may require post-synthetic modification methods to incorporate these
reporters into biomacromolecules.
Received 21st December 2015
Accepted 4th April 2016
DOI: 10.1039/c5ra27363c
www.rsc.org/advances
and a metal–carbonyl–triazole analogue of homoalanine19
(Fig. 1a–g, respectively). With the exception of the isonitrile,
Introduction
cyanate, and selenocyanate reporters, all of these vibrational
reporters have been successfully incorporated into proteins.
When selecting a reporter the goals of the specic experi-
ment and the relative strengths of the reporter need to be
carefully considered. Important parameters of vibrational
reporters include the magnitude of perturbation it will cause,
the strength of the signal (transition dipole), and the vibrational
lifetime. These parameters will determine, respectively, how
closely the experiment represents the native system, how easily
the signal can be detected under physiologically relevant
conditions, and the duration of dynamic processes that can be
observed. Table 1 displays an approximate ranking of these
vibrational reporters with respect to each of these parameters. It
is clear that the ideal vibrational reporter has not been devel-
oped to date. For example, while the C–D bond vibrational
reporters15,20,21 are the least likely to perturb the native protein
structure, they have some of the shortest vibrational lifetimes.22
On the other hand, metal carbonyls5,19,23,24 have the largest
transition dipole strengths and intermediate values for vibra-
tional lifetimes, but are also the bulkiest reporters sterically and
therefore the most likely to cause perturbations of the native
Reporter groups with unique spectroscopic signatures have
been successfully employed to probe local environments,
measure distances, and study dynamic processes in proteins
and nucleic acids. Spectroscopic reporters that absorb in the
infrared are termed vibrational reporters and have become
increasingly important because of advances in two-dimensional
IR (2D-IR) spectroscopic techniques.1–7 Vibrational reporter
functional groups that absorb in an open window of the IR
spectrum (ca. 2000–2500 cmꢁ1
)
are especially useful.8–11
Specically, in terms of unnatural amino acid (UAA) vibrational
reporters, important examples include cyanophenylalanine;12
azidomethylphenylalanine;13,14 methionine-d3;15 an isonitrile
analogue of alanine;16 a thiocyanate analogue of cysteine;17
cyanate, thiocyanate, and selenocyanate analogues of proline;18
aDepartment of Chemistry, Franklin & Marshall College, Lancaster, PA 17604, USA.
E-mail: sbrewer@fandm.edu; efenlon@fandm.edu
bDepartment of Chemistry, University of Nevada at Reno, Reno, NV 89557, USA.
E-mail: mtucker@unr.edu
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra27363c
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Fig. 1 Vibrational reporters (in blue) with absorptions in an open window of the infrared (a) cyanophenylalanine;12 (b) azidomethylphenylala-
nine;13,14 (c) methionine-d3;15 (d) isonitrile analogue of alanine;16 (e) thiocyanate analogue of cysteine;17 (f) cyanate (X ¼ O), thiocyanate (X ¼ S),
selenocyanate (X ¼ Se) analogues of proline;18 (g) metal–carbonyl–triazole analogue of homoalanine.19 The proline cyanate (OCN) analogue was
too unstable to be studied experimentally.
Table 1 Relative ranking of vibrational reportersa
The syntheses of the two phenylalanine (Phe) analogues
began with the palladium-catalyzed carbonylation of known
Steric perturbative
effects
Transition
dipole strength
Vibrational
lifetime
iodo-Phe 4 to give the aldehyde, which was then reduced with
sodium borohydride to give alcohol 5 in 28% for the two steps
(Scheme 2). Conversion of alcohol 5 to mesylate 6 proceeded
smoothly with an 88% yield. As with the dA analogues, the
syntheses diverged in the last step where SN2 displacement of
the mesylate with thiocyanate provided Boc–Me–PheCH2SCN
(39%) and displacement with selenocyanate provided Boc–Me–
PheCH2SeCN (9.9%). Attempts to hydrolyze the ester of Boc–
Me–PheCH2SeCN led to the loss of the selenocyanate group.
Mass spectra evidence suggests that in this analogue the nitrile
group was lost and a diselenide product formed, i.e. a mass of
517 Da was observed with an isotope pattern that is consistent
with a structure represented by PheCH2Se–SeCH2Phe (see ESI†).
Preliminary results suggest that these Phe analogues containing
SCN and SeCN groups are stable to slightly acidic conditions.
b
Most favorable
CD
CN, NC
CpM(CO)3
SeCN
SCN
CpM(CO)3
NC
CN, N3, CD
N3
NC
N3
SCN
SeCN
CpM(CO)3
CN, SCN, SeCN
CD
Least favorable
a
b
References: CD;22 CN, NC, N3;16 SCN, SeCN;18 CpM(CO)3.24 Cp ¼
cyclopentadienyl.
structure. Isonitriles are small and have intermediate values
for transition dipoles and vibrational lifetimes, however it is
unknown if these reporters will be able to be effectively incor-
porated into protein and nucleic acid systems.16 Finally, thio-
cyanates17,25–28 and especially selenocyanates29,30 have, based
upon initial literature reports,18,31 the longest vibrational life-
times and intermediate size making them promising candi-
dates in need of further investigation. Herein, the utility of
these reporters will be expanded upon.
Linear infrared spectroscopy
Using this set of unnatural nucleosides and amino acids, the IR
extinction coefficients for the SCN or the SeCN stretch vibration
were determined to be in the range of ꢂ50–120 Mꢁ1 cmꢁ1. FTIR
spectroscopic studies were performed to evaluate the solvent
sensitivity of the thiocyanate and selenocyanate moieties as IR
reporters. Fig. 2 shows the IR absorbance bands resulting from
the SCN or the SeCN stretch vibration of Si2–dA–SCN, Si2–dA–
SeCN, Boc–Me–PheCH2SCN, and Boc–Me–PheCH2SeCN in
tetrahydrofuran (THF) solution in the 2120–2200 cmꢁ1 region.
Results and discussion
Synthetic chemistry
The syntheses of the two 20-deoxyadenosine (dA) analogues The SCN vibrational stretching frequencies of Si2–dA–SCN and
began with the simultaneous protection of the 30- and 50- Boc–Me–PheCH2SCN are 2157.0 cmꢁ1 and 2154.6 cmꢁ1
,
hydroxyl groups of arabinosyl adenine 1 with the tetraisopro- respectively, while the SeCN vibrational stretching frequencies
pyldisiloxyl group (78%) and conversion of the 20-hydroxyl are 2154.1 cmꢁ1 and 2151.2 cmꢁ1 for Si2–dA–SeCN and Boc–
group to the triate (65%) following literature procedures Me–PheCH2SeCN, respectively. These vibrational frequencies
(Scheme 1). The syntheses diverged in the last step where SN2 are similar to those previously reported for proline analogues
displacement of the triate with thiocyanate provided Si2–dA– containing SCN and SeCN vibrational reporters (Fig. 1f).18
SCN (32%) and displacement with selenocyanate provided Si2– Overall, the SCN stretch vibrations are blue-shied relative
dA–SeCN (64%). Attempts to remove the silicon protecting to the corresponding SeCN stretch vibrations. The IR absor-
groups under acidic or basic conditions led to the decomposi- bance bands for these vibrational reporters are relatively
tion of the thiocyanate and selenocyanate groups. A possible symmetrical and narrow as illustrated by the full width half
cause for the instability of these groups in the nucleoside maximum (fwhm) values of 7.1 cmꢁ1 for Si2–dA–SCN, Si2–dA–
analogues is the presence of the adjacent 30-hydroxyl group as SeCN, and Boc–Me–PheCH2SeCN and the fwhm of 6.6 cmꢁ1 for
previously observed in related systems.32
Boc–Me–PheCH2SCN. These features of the IR bands
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Scheme 1
(symmetrical and narrow) are benecial in the correlation of
absorption frequency and local environment. This is in stark
contrast to the complicated absorption proles observed for
other vibration reporter modied molecules such as phenyl
cyanate and 3-azido-pyridine.33–35
FTIR solvent studies in mixtures of THF and water were
conducted to test the sensitivity of these vibrational reporters to
their local hydration environment. The SCN and SeCN IR
absorbance bands in Si2–dA–SCN and Si2–dA–SeCN undergo
a monotonic increase in position and fwhm with increasing
percent water relative to pure THF as shown in Fig. 3. For Si2–
dA–SCN, the central frequency of the SCN vibrational transition
blue shis from 2157.0 cmꢁ1 in THF to 2158.9 cmꢁ1 in 40% (v/v)
H2O/THF and broadens from 7.1 to 9.6 cmꢁ1 over the same
range. The SeCN vibrational transition in Si2–dA–SeCN shis
similarly from 2154.1 cmꢁ1 in THF to 2155.6 cmꢁ1 in 40% (v/v)
H2O/THF. This band broadens from 7.1 to 10.9 cmꢁ1 over the
same range, which is similar to Si2–dA–SCN although the
broadening is slightly more pronounced in the SeCN analogue.
Solubility limitations prevented investigation at higher
percentages of water, however the observed trends are similar to
Fig. 2 FTIR absorbance spectra of Si2–dA–SCN, Si2–dA–SeCN, Boc–
Me–PheCH2SCN, and Boc–Me–PheCH2SeCN dissolved in THF at
a concentration of 40 mM. The spectra were recorded at 25 ꢃC,
baseline corrected, and intensity normalized.
previous studies of vibrational reporters such as N3, CN, SCN,
and SeCN in terms of the impact hydrogen bonding (H-
bonding) with H2O has on the observed frequency and fwhm
Scheme 2
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Fig. 3 FTIR absorbance spectra of (A) Si2–dA–SCN and (C) Si2–dA–SeCN dissolved in THF and 40% (v/v) H2O/THF. FTIR absorbance spectra of
(B) Si2–dA–SCN and (D) Si2–dA–SeCN dissolved in H2O/THF mixtures from 0 to 40% (v/v) in 10% intervals. The spectra were recorded at 25 ꢃC,
baseline corrected, and intensity normalized. The concentration of the samples was 40 mM except for the 40% (v/v) H2O/THF samples, which
were 30 mM due to solubility limitations.
of the vibrational reporter absorbance band.17,36,37 Extrapolating dependent shi is determined by the geometry of the
the observed solvent-induced frequency shis to pure water as hydrogen bonding interactions between the nitrile group and
the solvent for Si2–dA–SCN and Si2–dA–SeCN suggests a slightly H2O.36,37 Specically, s-bonding with the lone-pair of electrons
smaller, but similar dynamic range of the SCN and SeCN on the nitrogen atom leads to the typical blue-shi of the
reporters when compared to CN reporters, and a smaller range absorbance band, while p-bonding leads to red-shis, which
than those reported for N3 reporters in similar molecules.38
was observed here for the SCN and SeCN bands of Boc–Me–
In the Phe analogues, the SCN and SeCN IR absorbance PheCH2SCN and Boc–Me–PheCH2SeCN.11 The apparent differ-
bands undergo a small decrease in frequency when going from ence in the hydrogen bonding geometry between the
THF to 10% H2O/THF, and then the frequency remains constant Phe analogues and the dA analogues demonstrates that struc-
with increasing percent water up to 40% H2O/THF (Fig. 4). tural features of the molecule can inuence the solvent-
However, there is a signicant and monotonic increase in the dependency of the stretch vibrational mode and further high-
fwhm with increasing percent water that is more substantial lights the sensitivity of these vibrational reporters to their local
than the broadening effect observed in the SCN and SeCN bands environment.
in the dA analogues described above. For Boc–Me–PheCH2SCN,
the central frequency of the SCN band red-shis from 2154.6
cmꢁ1 in THF to 2154.1 cmꢁ1 in 40% (v/v) H2O/THF and the
Vibrational lifetimes
fwhm increases from 6.6 to 10.3 cmꢁ1 over the same range. The
The vibrational lifetimes of CN for Si2–dA–SCN and Si2–dA–
SeCN absorbance band of Boc–Me–PheCH2SeCN follows
a similar trend over the 0–40% (v/v) H2O/THF range, shiing
from 2151.2 to 2150.7 cmꢁ1, and broadening from 7.1 to 11.0
cmꢁ1. Although the red-shi with increasing water percentages
is somewhat unusual, it is not unprecedented as a similar small
red-shi along with a signicant increase in fwhm has been
observed in other nitrile vibrational reporters such as a 5-cya-
notryptophan analogue.39 Cho et al. observed a similar red-shi
with acetonitrile and proposed that, in general, the solvent-
SeCN along with Boc–Me–PheCH2SCN and Boc–Me–PheCH2-
SeCN in THF were measured using ultrafast IR pump-probe
spectroscopy. The transient absorption spectra of Si2–dA–
SeCN and Boc–Me–PhCH2SCN are shown in Fig. 5. These
spectra are representative of the quality of data measured for
acquiring the kinetic traces (see ESI† for Si2–dA–SCN and Boc–
Me–PheCH2SeCN). The ground state bleach (n ¼ 0 / 1 transi-
tion) for Si2–dA–SCN and Si2–dA–SeCN are at 2156 cmꢁ1 and
2150 cmꢁ1, respectively, with their corresponding excited state
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Fig. 4 FTIR absorbance spectra (A) Boc–Me–PheCH2SCN and (C) Boc–Me–PheCH2SeCN dissolved in THF and 40% (v/v) H2O/THF. FTIR
absorbance spectra of (B) Boc–Me–PheCH2SCN and (D) Boc–Me–PheCH2SeCN in H2O/THF mixtures from 0 to 40% (v/v) dissolved in 10%
intervals. The spectra were recorded at 25 ꢃC, baseline corrected, and intensity normalized. The concentration of the samples was 40 mM.
absorptions (n ¼ 1 / 2) shied to 2135 cmꢁ1 and 2136 cmꢁ1
,
normalized intensity decay is shown in Fig. 6. The observed
respectively. Similarly, the ground state bleach transitions of vibrational population decays for Si2–dA–SCN and Boc–Me–
Boc–Me–PheCH2SCN and Boc–Me–PheCH2SeCN occur at 2153 PheCH2SCN were 140 ꢀ 6 and 102 ꢀ 4 ps, respectively. The
cmꢁ1 and 2149 cmꢁ1 and their corresponding excited state population decays of the SeCN vibrational modes were 1.5 to 3
absorption bands are observed at 2132 cmꢁ1 and 2121 cmꢁ1
.
times longer than that of SCN, having a value of 237 ꢀ 12 and
The t ¼ 0 and the t ¼ 80 ps transient absorption spectra are 295 ꢀ 31 ps for Si2–dA–SeCN and Boc–Me–PheCH2SeCN
presented illustrating the decrease in the intensity of the n ¼ 1 compounds, respectively. The observed lifetimes are similar to
/ 2 transition observed as function of time. The spectra the timescales observed by Cho et al. for comparable proline
collected prior to t ¼ 0 are also included for each case to analogues (Fig. 1f)18 and by Bredenbeck et al. for methyl thio-
demonstrate the baseline associated with the each measure- cyanate in THF.31
ment. It is expected that the heating effects in THF do not play
These types of site-specic IR reporters have lifetimes greater
a signicant role in the kinetic data, indicated by the lack of than an order of magnitude larger than typical azide and nitrile
solvent contributions observed in the transient absorption reporters,40,41 where the population decays are between 1 and 5
spectra. It should be noted that some stray light is likely ps. Although azide and nitrile vibrational reporters are quite
detected in the signal direction resulting in small artifacts useful for detecting water dynamics, hydrogen-bond making/
within the pump probe spectra. However, the overall values of breaking, and small motions,41–43 the observation window is
the vibrational lifetimes should not be signicantly affected as limited and oen cannot detect dynamics on longer timescales.
indicated by our global analysis approach (see ESI†). The errors However, by adding a bridging atom between the nitrile and the
associated with these lifetimes based on the quality of t can compound of interest, it has been shown here, as well as by
slightly be affected by the presence of small amounts of scat- others,18 that the lifetimes can be extended into the tens to
tered light.
hundreds of picoseconds regime. This increase in lifetime is
Using a global analysis approach, the vibrational lifetimes at due to the decrease in intramolecular energy transfer to other
multiple frequencies within the excited state absorption band of vibrational modes of the compound that the localized nitrile
each sample were measured. The determined decay values were transition is attached. Acting as barrier to the ow of vibrational
similar to those measured by taking the trace of the peak energy transfer, the heavy S and Se atoms and the somewhat
maximum of the SCN or SeCN signals as a function of time. The bent conguration limit the possible relaxation pathways.18
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Fig. 5 Transient absorption spectra of (A) Si2–dA–SeCN and (B) Boc–
Me–PhCH2SCN at t ¼ 0 and 80 ps. A single linear baseline correction
was applied to each spectrum for every compound. The spectra
collected prior to t ¼ 0 is also shown for each measurement.
Fig. 6 Population relaxation decays, normalized to the intensity at t ¼
0, of transient absorption signals for SeCN (empty circles) and SCN
(empty squares) in (A) Si2–dA–SeCN and Si2–dA–SCN and (B) Boc–
Me–PheCH2SeCN and Boc–Me–PheCH2SCN at the peak maximum
from 0 to 80 ps.
Although the intramolecular energy sink of the vibrational
motions is hindered, energy transfer to the solvent molecules is
still a viable pathway.
Conclusion
Since the vibrational lifetimes represent when the signal
strength has decayed by approximately 1/e, it is expected that
the observation window will be opened for hundreds of pico-
seconds to even longer timescales. By reaching these time-
scales, the vibrational dynamics of these reporters can be
utilized to detect motions beyond simple bond rotations to
larger scale structural changes, such as folding–unfolding
dynamics in RNA molecules or protein–protein binding equi-
librium dynamics. However, it should be noted that the tran-
sition dipole strength also plays a signicant role in the
observation windows. Since the oscillator strength of these
SCN and SeCN reporters are typically an order of magnitude
less than N3, it makes nonlinear spectroscopy experiments
more challenging to obtain an adequate signal. The quartic
dependence of 2D IR spectroscopy on the transition dipole
strength in combination with the intrinsically narrow transi-
tions can offer an avenue to detect these typically weak tran-
sitions. Despite these advantages, the transition dipole
strength still creates challenges in detecting these signals,
even in 3rd order spectroscopy. In general, the specic reporter
must be chosen based upon what best suits the desired
application and scientic question being studied.11
In this work two novel dA analogues and two novel Phe
analogues containing SCN and SeCN reporters were synthe-
sized. Although the ideal vibrational reporter has not yet been
developed, SCN and SeCN groups are relatively small in size,
have similar sensitivities to hydration as CN groups, and offer
signicant advantages over all other current reporters in terms
of vibrational lifetimes. The vibrational lifetimes of the SCN
analogues were determined by ultrafast IR pump-probe spec-
troscopy to be in the 102–140 ps range, which was 1.5 to 3 times
shorter than those for the SeCN analogues, which were in the
237–295 ps range. With the observed lifetimes, these vibrational
reporters are expected to measure dynamics events out to
approximately 500 ps for thiocyanate reporters and approxi-
mately 1.4 ns for selenocyanate reporters, based on the decay of
the signal strength to ꢂ1% of its maximum amplitude.
However, in practice, the signal-to-noise ratio can create chal-
lenges for the detection of these weak signals at much longer
timescales. Nevertheless, these reporters still have the potential
to measure dynamics on timescales two orders of magnitude
greater than those observed using traditional nitrile and azide
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reporters. The enhanced dynamic range measured for the SeCN 18 K. H. Park, J. Jeon, Y. Park, S. Lee, H. J. Kwon, C. Joo, S. Park,
analogues is attributed to the superior insulating effects of the H. Han and M. Cho, J. Phys. Chem. Lett., 2013, 4, 2105–2110.
heavier selenium atom compared to the sulfur atom. Thiocya- 19 I. Peran, T. Oudenhoven, A. M. Woys, M. D. Watson,
nate reporters have been routinely incorporated into proteins
but selenocyanate reporters have not yet been incorporated. The
T. Q. O. Zhang, I. Carrico, M. T. Zanni and D. P. Raleigh, J.
Phys. Chem. B, 2014, 118, 7946–7953.
problems encountered herein with deprotection of the SeCN 20 C. S. Kinnaman, M. E. Cremeens, F. E. Romesberg and
analogues suggest that incorporation of these vibrational S. A. Corcelli, J. Am. Chem. Soc., 2006, 128, 13334–13335.
reporters into proteins may require posttranslational modi- 21 J. Zimmermann and F. E. Romesberg, Protein Dynamics,
cation to account for the lower stability of this group.
Humana Press, 2014.
22 S. R. G. Naraharisetty, V. M. Kasyanenko, J. Zimmermann,
M. C. Thielges, F. E. Romesberg and I. V. Rubtsov, J. Phys.
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Acknowledgements
23 J. T. King and K. J. Kubarych, J. Am. Chem. Soc., 2012, 134,
18705–18712.
24 A. M. Woys, S. S. Mukherjee, D. R. Skoff, S. D. Moran and
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25 M. G. Maienschein-Cline and C. H. Londergan, J. Phys.
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26 H. A. McMahon, K. N. Aleri, C. A. A. Clark and
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We thank Rui Song and Hongyi Kyle Pan for initial synthetic
work on dA analogues; Ken Hess for mass spectral (ESI) anal-
yses; Lisa Mertzman for ordering supplies; and Beth Buckwalter
for acquiring NMR spectra. This work was supported by F&M
Hackman funds, F&M Eyler Funds, NSF (CHE-1053946) to SHB
and NIH (R15GM093330-02) to EEF and SHB.
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