Heavy-Atom-Substituted Nucleobases in Photodynamic Applications: Substitution of Sulfur with Selenium in 6-Thioguanine Induces a Remarkable Increase in the Rate of Triplet Decay in 6-Selenoguanine

Article abstract of DOI:10.1021/jacs.8b07665

Sulfur substitution of carbonyl oxygen atoms of DNA/RNA nucleobases promotes ultrafast intersystem crossing and near-unity triplet yields that are being used for photodynamic therapy and structural-biology applications. Replacement of sulfur with selenium or tellurium should significantly red-shift the absorption spectra of the nucleobases without sacrificing the high triplet yields. Consequently, selenium/tellurium-substituted nucleobases are thought to facilitate treatment of deeper tissue carcinomas relative to the sulfur-substituted analogues, but their photodynamics are yet unexplored. In this contribution, the photochemical relaxation mechanism of 6-selenoguanine is elucidated and compared to that of the 6-thioguanine prodrug. Selenium substitution leads to a remarkable enhancement of the intersystem crossing lifetime both to and from the triplet manifold, resulting in an efficiently populated, yet short-lived triplet state. Surprisingly, the rate of triplet decay in 6-selenoguanine increases by 835-fold compared to that in 6-thioguanine. This appears to be an extreme manifestation of the classical heavy-atom effect in organic photochemistry, which challenges conventional wisdom.

Full text of DOI:10.1021/jacs.8b07665

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Communication
Heavy-Atom Substituted Nucleobases in Photodynamic Applications:
Substitution of Sulfur with Selenium in 6-Thioguanine Induces a
Remarkable Increase in the Rate of Triplet Decay in 6-Selenoguanine
Kieran M. Farrell, Matthew M Brister, Michael Pittelkow, Theis I. Sølling, and Carlos E. Crespo-Hernández
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b07665 • Publication Date (Web): 26 Aug 2018
Downloaded from http://pubs.acs.org on August 27, 2018
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Heavy-Atom Substituted Nucleobases in Photodynamic Applica-
tions: Substitution of Sulfur with Selenium in 6-Thioguanine Induces
a Remarkable Increase in the Rate of Triplet Decay in 6-Selenogua-
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Kieran M. Farrell,†,§ Matthew M. Brister,†,¶ Michael Pittelkow,‡ Theis I. Sølling,‡ Carlos E. Crespo-
,
Hernández† *
† Department of Chemistry, Case Western Reserve University, 10900 Euclid Ave., Cleveland, Ohio 44106
‡ University of Copenhagen, Department of Chemistry, Universitetsparken 5, DK-2100 Copenhagen, Denmark
Supporting Information Placeholder
redshift the absorption spectra into the visible with the potential
additional benefit of maximizing their triplet yields. Recent quan-
tum-chemical calculations predict that selenium and tellurium sub-
stituted nucleobases are highly promising photosensitizers.20,21 By
using femtosecond broadband transient absorption spectroscopy,
we demonstrate that selenium substitution of guanine leads to a re-
markable enhancement of the intersystem crossing (ISC) lifetime
both to and from the triplet manifold. In particular, the rate of ISC
to the ground state in 6-selenoguanine increases by 835-fold, or
more than two orders of magnitude, compared to that of the 6-thi-
oguanine prodrug. A strategy for harvesting the use of 6SeGua for
photodynamic therapy and structural-biology applications is pre-
sented, which might enable heavy-atom substituted nucleobases to
reach their full potential.
ABSTRACT: Sulfur substitution of carbonyl oxygen atoms of
DNA/RNA nucleobases promotes ultrafast intersystem crossing
and near-unity triplet yields that are being used for photodynamic
therapy and structural-biology applications. Replacement of sulfur
with selenium or tellurium should significantly redshift the absorp-
tion spectra of the nucleobases without sacrificing the high triplet
yields. Consequently, selenium/tellurium-substituted nucleobases
are thought to facilitate treatment of deeper tissue carcinomas rela-
tive to the sulfur-substituted analogues, but their photodynamics
are yet unexplored. In this contribution, the photochemical relaxa-
tion mechanism of 6-selenoguanine is elucidated and compared to
that of the 6-thioguanine prodrug. Selenium substitution leads to a
remarkable enhancement of the intersystem crossing lifetime both
to and from the triplet manifold, resulting in an efficiently popu-
lated, yet short-lived triplet state. Surprisingly, the rate of triplet
decay in 6-selenoguanine increases by 835-fold compared to that
in the 6-thioguanine prodrug. This appears to be an extreme mani-
festation of the classical heavy-atom effect in organic photochem-
istry, which challenges conventional wisdom.
Figure 1 shows the absorption spectra for 6SeGua and 6tGua
normalized to their lowest energy π → π* transition. 6SeGua ex-
hibits maxima at 357 and 209 nm, whereas 6tGua has maxima at
341, 254, and 204 nm. Selenium substitution broadens and red
shifts the UVA transition by 16 nm (1314.3 cm-1) relative to sulfur
substitution. The combined shifting and broadening of 6SeGua’s
absorption give rise to a tail absorbing in the visible region. The
oscillator strength of the UVA vertical transition for 6SeGua,
shown in Figure 1, has very similar magnitude than that for
6tGua,22 suggesting that the molar absorptivity in the UVA region
is similar for both molecules. We estimate that using 6SeGua as
photosensitizer could facilitate up to 90% deeper tissue treatment
by UVA radiation than 6tGua (see Figure S1 and the Supporting
Information (SI) for details).
Quantum-chemical computations have shown that the N9 tauto-
mer of 6SeGua is most stable in solution.23–26 We have performed
ground state optimizations and subsequent vertical excitation en-
ergy calculations for the N9 tautomer of 6SeGua to characterize the
excited states available to UVA excitation. A complete summary
of computational results is given in Tables S1 to S3 and Figure S2.
UVA excitation of 6SeGua in aqueous solution excites the S2(ππ*)
state. Four lower-lying states are also available for electronic relax-
ation: namely, a dark S1(nπ*) state and three triplet states. Both the
S2(ππ*) and T3(ππ*) states, as well as the S1(nπ*) and T2(nπ*)
states, are isoenergetic within computational uncertainty. The
S1(nπ*) and T1(ππ*) states are separated by an energy gap of 0.64
eV.
In addition to optimizing DNA for its role as the genetic blue-
print of life, evolution has tailored the structure of DNA nucleotides
such that they efficiently dissipate energy absorbed from ultraviolet
(UV) radiation.1,2 Numerous theoretical and experimental investi-
gations have demonstrated that the naturally occurring nucleo-
bases, nucleosides, and nucleotides recover their electronic ground
states within ~1 ps of excitation.1–4 Ultrafast ground state recovery
has made nucleic acids robust against the solar onslaught of one of
the most ubiquitous carcinogens−UV radiation.5,6 Conversely, sul-
fur substitution of carbonyl oxygen atoms on nucleobases leads to
the efficient population of long-lived, reactive triplet excited states
in high yields.7–11 As a result, the photochemistry of sulfur-substi-
tuted DNA/RNA nucleobases (a.k.a., thiobases) have drawn in-
creased attention because of their use in photodynamic therapy and
photocrosslinking applications.9-19
The high triplet yield makes the thiobases prime sensitizers for
targeting DNA/RNA damage,10 however, their absorption proper-
ties limit the applications to the treatment of skin cancer cells.12
Replacement of the sulfur atom with selenium or tellurium should
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R = Se, 6-Selenoguanine (6SeGua)
R = S, 6-Thioguanine (6tGua)
Figure 1. Ground state absorption spectra of 6SeGua and 6tGua normalized to their lowest-energy π → π* transitions. All spectra were
obtained in pH 7.4 phosphate buffer saline (PBS) at room temperature. Oscillator strengths obtained from TDDFT computations at the
PBE0/IEF-PCM/6-311++G(d,p) level of theory are included (see SI for details).
Femtosecond broadband transient absorption experiments were
performed to unveil the electronic decay pathways following UVA
excitation. The dynamics of selenium substitution at the C6 position
were compared to sulfur substitution by performing successive ex-
periments on both 6SeGua and 6tGua. Figure 2 shows the transient
absorption spectra (TAS) of 6SeGua and 6tGua following 352 nm
excitation. For 6SeGua, two bands appear concurrently on a sub-
picosecond time-scale: one with a maximum ca. 490 nm and an-
other with a tail apparent at 675 nm. A negative amplitude signal is
observed at short wavelengths immediately after excitation. The
signal overlaps with the ground state absorption spectrum in Figure
1, and is assigned to ground state depopulation. From a delay time
of 0.6 to 104 ps, the 490 nm band blue shifts 30 nm. Simultaneous
to this shift, the ground state bleach increases in amplitude (Figure
2b, inset). Following the blue shift, all spectral features decay uni-
formly through 3 ns.
6SeGua’s ISC lifetime (τ1), which is three times faster than that in
6tGua, may increase the triplet yield (assuming that the lifetime of
any other competitive relaxation pathway is similar in both mole-
cules), arguably rendering 6SeGua a more effective photosensitizer
when incorporated in DNA/RNA.
Table 1. Global lifetimes for 6SeGua and 6tGua following
excitation at 352 nm under N2-saturated conditions
Lifetimes
6SeGua
6tGua
τ1 / ps
τ2 / ps
31 ± 2
18 ± 1
τ3 / ns
1.7 ± 0.1a
1420 ± 180c
0.13 ± 0.05
0.35 ± 0.04b
a Estimated by extrapolating fit past 1 ns; b Same lifetime
value was obtained within experimental uncertainties excit-
ing at 342 nm; c Reproduced from ref. 22, where an excita-
tion wavelength of 345 nm was used.
The TAS for 6tGua (Figure 2d-f) are similar to those of 6SeGua
and consistent with those reported by Ashwood et al.22 A UV band
ca. 375 nm, a visible band ca. 500 nm, and ground state depopula-
tion signal are observed on a sub-picosecond timescale. The UV
band increases in amplitude from a time delay of 0.77 to 68 ps,
while the red edge of the visible band narrows. Like 6SeGua, a
slight increase in negative amplitude signal occurs during this pro-
cess. No apparent spectral changes occur from 68 ps through 3 ns.
El-Sayed’s propensity rules suggest that ultrafast ISC should oc-
cur between singlet and triplet states of different character.27,28
Pathways that obey this rule for 6SeGua include S2(ππ*) → T2(nπ*)
and S1(nπ*) → T1(ππ*) (Figure S2). Calculations for 6-selenogua-
nosine and 6-thioguanosine predict that ISC predominately occurs
via the S1(nπ*) → T1(ππ*) pathway.21 However, non-adiabatic dy-
namics simulations indicate that ISC in 6tGua predominately oc-
curs from the S1(nπ*) minimum to the T2(nπ*) state.29 Given the
similarities between the theoretical and experimental results for
6SeGua and 6tGua herein, the S1(nπ*) → T2(nπ*) ISC pathway
cannot be excluded. Collectively, the theoretical results suggest
that after excitation of 6SeGua to the S2(ππ*) state, electron popu-
lation internally converts to the S1(nπ*) state before ISC to T2(nπ*)
and/or T1(ππ*) state occurs: S2 → S1 → T2 → T1.
Kinetics traces corresponding to the TAS in Figure 2 were ana-
lyzed globally. Figure 3 shows representative traces with fits ob-
tained from three- and two-lifetime sum of exponential functions
for 6SeGua and 6tGua, respectively. Lifetimes obtained from these
fits are reported in Table 1. For both 6SeGua and 6tGua, τ1 reflects
ultrafast ISC responsible for growth of the T1(ππ*) absorption
bands, τ2 corresponds to spectral evolution shown in Figure 2b, and
τ3 corresponds to the uniform triplet decay.
The TAS for 6SeGua and 6tGua are qualitatively similar through
the second lifetime. Concurrent to ground state depopulation, the
490 and 675 nm bands rise during τ1. We assign the rise of these
two bands to T1(ππ*) absorption. This assignment is supported by
the fact that 6tGua undergoes an ultrafast ISC event following
UVA excitation, and similarly shows T1(ππ*) absorption with max-
ima at 375 and 500 nm.22 Heavy atom substitution enhances ISC
dynamics in 6tGua relative to guanine,22 and selenium substitution
should further enhance ISC relative to 6tGua. Indeed, the spin orbit
coupling between the S1(nπ*) and T1(ππ*) states increases nearly
five-fold for 6-selenoguanosine compared to 6-thioguanosine.21
The spectral dynamics that occur during τ2 are also similar for
both 6SeGua and 6tGua. The blue shift of the 495 nm band in 6Se-
Gua to 460 nm and the narrowing of the visible band in 6tGua both
suggest vibrational relaxation during τ2. Thus, the blue shift is as-
signed to vibrational cooling of the hot T1(ππ*) state following pop-
ulation via ultrafast ISC (see Figures S2 and S3). This assignment
is supported by the observation that the shifting process is com-
pleted prior to the uniform decay of both triplet bands, and excess
vibrational energy is dissipated prior to the monotonic decay
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Figure 2. (a-c) TAS for 6SeGua following 352 nm excitation through a time delay of 3 ns. (2b, Inset) Ground state recovery concurrent with
blue shift of 490 nm band. (d-f) TAS for 6tGua obtained back-to-back with 6SeGua. The stimulated Raman emission band of water is
observed around 400 nm within the cross correlation of the pump and probe beams. TAS were obtained in pH 7.4 PBS at room temperature
under N2-purged conditions (See SI for details).
of the T1(ππ*) state.30,31
nm band (ca. 0.2 m∆ꢀ). This event is therefore concurrent, yet un-
related to the vibrational cooling dynamics observed in the T1(ππ*)
state. Ground state recovery during τ2 may be achieved via internal
conversion of a residual S1(nπ*) state population to the ground
state. Triplet self-quenching11,32,33 is unlikely because preliminary
experiments in which the relative absorbance of the 6SeGua solu-
tion or the excitation intensity were varied by up to 68 and 60%,
respectively, do not support a triplet quenching pathway.
The negative amplitude signal at ca. 370 nm increases simulta-
neously to vibrational cooling of the T1(ππ*) state. This phenome-
non is also observed for 6tGua,22 for which it was unclear whether
the increase was due to ground state recovery or overlap with a
growing UV band (Figure 2e). For 6SeGua, this event corresponds
to ground state recovery, as the absorption change at 360 nm (ca. 2
m∆ꢀ) is an order of magnitude larger than the decline of the 495
Figure 3. Kinetics decay traces for (a) 6SeGua and (b) 6tGua, featuring global fits yielded by a sum of exponentials model.
The most remarkable difference between 6SeGua and 6tGua is
manifested in τ3. Kinetic traces for 6tGua reach an offset following
τ2, indicating that further relaxation events occur on a nanosecond
timescale. Previous work has shown that triplet decay occurs over
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1420 ns under N2-saturated conditions.22 Due to its long-lived tri-
REFERENCES
plet state, photoexcited 6tGua sensitizes singlet oxygen with a 21%
yield.22 6SeGua instead exhibits immediate, uniform decay of both
triplet bands following τ2. Simultaneous recovery of ground state
bleaching signal confirms that this event represents triplet decay
with a lifetime of 1.7 ns (Figure 3). In addition to the stronger T1/S0
spin-orbit coupling in 6SeGua than in 6tGua, we argue that there is
a smaller energy barrier to access the region of the T1 potential en-
ergy surface where the ISC event occurs, explaining the 835-fold
faster triplet decay in 6SeGua.34 Given the sub-2 ns triplet decay
lifetime, the yield of singlet oxygen generation for 6SeGua should
be smaller than for 6tGua because diffusional encounter to sensitize
molecular oxygen becomes less competitive as the rate of triplet
decay increases.
1
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3
4
5
6
7
8
(1) Crespo-Hernández, C. E.; Cohen, B.; Hare, P. M.; Kohler, B. Ultrafast
Excited-State Dynamics in Nucleic Acids. Chem. Rev. 2004, 104, 2077–
2019.
(2) Middleton, C. T.; de La Harpe, K.; Su, C.; Law, Y. K.; Crespo-
Hernández, C. E.; Kohler, B. DNA Excited-State Dynamics: From Single
Bases to the Double Helix. Annu. Rev. Phys. Chem. 2009, 60, 217–239.
(3) Improta, R.; Santoro, F.; Organo, C.; Blancafort, L. Quantum
Mechanical Studies on the Photophysics and the Photochemistry of Nucleic
Acids and Nucleobases. Chem. Rev. 2016, 116, 3540–3593.
(4) Beckstead, A. A.; Zhang, Y.; de Vries, S.; Kohler, B. Life in the Light:
Nucleic Acid Photoproperties as a Legacy of Chemical Evolution. Phys.
Chem. Chem. Phys. 2016, 18, 24228–24238.
(5) The International Agency for Research on Cancer, in Monographs on
the Evaluation of Carcinogenic Risks to Humans; 2012; 110D, 35–101.
(6) World Health Organization. (2018). The Known Health Sffects of UV.
[online] Available at: http://www.who.int/uv/faq/uvhealtfac/en/ [Accessed
15 Jul. 2018].
(7) Reichardt, C.; Guo, C.; Crespo-Hernández, C. E. Excited-State
Dynamics in 6-Thioguanosine from the Femtosecond to Microsecond Time
Scale. J. Phys. Chem. B. 2011, 115, 3263–3270.
(8) Taras-Goślińska, K.; Burdziński, G.; Wenska, G. Relaxation of the T1
Excited State of 2-Thiothymine, its Riboside and Deoxyribose-Enhanced
Nonradiative Decay Rate Induced by Sugar Substituent. J. Photochem.
Photobiol. A 2014, 275, 89–95.
(9) Pollum, M.; Jockusch, S.; Crespo-Hernández, C. E. 2,4-Dithiothymidine
as a Potent UVA Chemotherapeutic Agent. J. Am. Chem. Soc. 2014, 136,
17930–17933.
(10) Ashwood, B.; Pollum, M.; Crespo-Hernández, C. E. Photochemical
and Photodynamical Properties of Sulfur-Substituted Nucleic Acid Bases.
Photochem. Photobiol. 2018, DOI: 10.1111/php.12975.
(11) Pollum, M.; Martínez-Fernández, L.; Crespo-Hernández, C. In
Photoinduced Phenomena in Nucleic Acids I, Topics in Current Chemistry;
Springer, Cham, 2015, vol. 355, pp 245–327.
(12) Pollum, M.; Lam, M.; Jockusch, S.; Crespo-Hernández, C. E.
Dithionated Nucleobases as Effective Photodynamic Agents against
Human Epidermoid Carcinoma Cells. Chem. Med. Chem. 2018, 13, 1044–
1050.
(13) Pollum, M.; Jockusch, S.; Crespo-Hernández, C. E. Increase in
Photoreactivity of Uracil Derivatives by Doubling Thionation. Phys. Chem.
Chem. Phys. 2015, 17, 27851–27861.
(14) Reelfs, O.; Karran, P.; Young, A. R. 4-Thiothymidine Sensitization of
DNA to UVA Offers Potential for a Novel Photochemotherapy. Photochem.
Photobiol. Sci. 2012, 11, 148–154.
(15) Pridgeon, S. W.; Heer, R.; Taylor, G. A.; Newell, D. R.; O’Toole, K.;
Robinson, M.; Xu, Y.-Z.; Karran, P.; Boddy, A. V. Thiothymidine Com-
bined with UVA as a Potential Novel Therapy for Bladder Cancer. Brit. J.
Cancer 2011, 104, 1869–1876.
(16) Brem, R.; Daehn, I.; Karran, P. Efficient DNA Interstrand Crosslinking
by 6-Thioguanine and UVA Radiation. DNA Repair. 2011, 10, 869–876.
(17) Favre, A.; Saintomé, C.; Fourrey, J.; Clivio, P.; Laugâa, P. New Trends
in Photobiology Thionucleobases as Intrinsic Photoaffinity Probes of Nu-
cleic Acid Structure and Nucleic Acid-Protein Interactions. J. Photochem.
Photobiol. B 1998, 42, 109–124.
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The incorporation of 6SeGua into DNA and RNA has been
demonstrated,35-37 and molecular dynamics simulations have
shown that 6SeGua-substituted DNA duplexes are stable.38,39
Hence, regardless of the sub-2 ns triplet decay lifetime, the incor-
poration of 6SeGua into DNA/RNA duplexes should increase the
probability that a reaction between the triplet state of 6SeGua and
an adjacent nucleobase may occur. The close contact of 6SeGua to
an adjacent π-stacked canonical nucleobase in a DNA/RNA duplex
relaxes the requirement of diffusional encounter for a reaction to
occur, enabling 6SeGua to photosensitize damage to DNA before
triplet state decay. 6SeGua is a known chemotherapeutic agent.40
The results presented in this study further show that it is prudent to
investigate the photosensitization mechanism of 6SeGua-substi-
tuted DNA/RNA duplexes prior to studying tellurium-substituted
nucleobases. The substitution with tellurium atoms is expected to
further increase the rate of triplet decay to the ground state, which
may further decrease the probability of a photosensitized reaction
with an adjacent nucleobase compared to a selenium-substituted
nucleobase.
ASSOCIATED CONTENT
Supporting Information
Materials and computational and experimental methods; data anal-
ysis and supporting results; single-electron transitions and charac-
ter composition of computed excited states; Kohn-Sham orbitals
corresponding to vertical electronic transitions; Jablonski dia-
grams; plot of depth of tissue penetration; decay associated spectra
obtained from sum of exponential model; procedure for synthesis
of 6-selenoguanine; NMR spectra.
AUTHOR INFORMATION
Corresponding Author
* carlos.crespo@case.edu
(18) Favre, A. In Bioorganic Photochemistry, Volume 1: Photochemistry
and the Nucleic Acids; 1990; pp 379–425.
ORCID: 0000-0002-3594-0890
§ Participated as an undergraduate research assistant. Present Address: Department of Chemistry,
(19) Gemenetzidis, E.; Shavorskaya, O.; Xu, Y.-Z.; Trigiante, G. Topical 4-
Thiothymidine is a Viable Photosensitizer for the Photodynamic Therapy
of Skin Malignancies. J. Dermatol. Treat. 2013, 24, 209–214.
(20) Pirillo, J.; de Simon, B. C.; Russo, N. Photophysical Properties Predic-
tion of Selenium and Tellurium Substituted Thymidine as Potential UVA
Chemotherapeutic Agents. Theor. Chem. Acc 2016, 135, 1–5.
(21) Pirillo, J.; Mazzone, G.; Russo, N.; Bertini, L. Photophysical Properties
of S, Se and Te-Substituted Deoxyguanosines: Insight into Their Ability to
Act as Chemotherapeutic Agents. J .Chem. Inf .Mod. 2017, 57, 234-242.
(22) Ashwood, B.; Jockusch, S.; Crespo-Hernández, C. E. Excited-State
Dynamics of the Thiopurine Prodrug 6-Thioguanine: Can N9-Glycosyla-
tion Affect Its Phototoxic Activity? Molecules 2017, 22, 379.
(23) Venkateswarlu, D.; Leszczyński, J. Tautomerism and Proton Transfer
in 6-Selenoguanine: A Post Hartree-Fock Level ab Initio SCF-MO Investi-
gation. J. Phys. Chem. A 1998, 102, 6161–6166.
University of Wisconsin-Madison, Madison, Wisconsin 53706
¶ Present Address: Lawrence Berkeley National laboratory, AMO Experimental Group, Chemical
Sciences Division, Berkeley, California 94720
ACKNOWLEDGMENT
The authors acknowledge funding from the National Science Foun-
dation (Grant No. CHE-1800052). The Faculty Early Career De-
velopment Program from NSF (Grants No. CHE-1255084 and
CHE-1539808) is also acknowledge for initial support of this work.
The authors also thank Mr. Sean Hoehn and Mr. Luis Rodríguez-
Ortiz for performing preliminary TAS experiments at different ex-
citation intensities and concentrations for 6SeGua.
(24) Leszczyński, J. Guanine, 6-Thioguanine and 6-Selenoguanine: Ab In-
itio HF/DZP and MP2/DZP Comparative Studies. J. Mol. Struct. 1994, 311,
37–44.
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(25) Karthika, L.; Senthilkumar, R.; Kanakaraju, M. Hydrogen-Bond Inter-
actions in Hydrated 6-Selenoguanine Tautomers: A Theoretical Study.
Struct. Chem. 2014, 25, 197–213.
the observed dynamics. Additional experiments in which the pH of the
solution is sytematically varied are necessary in order to examine the
putative extent of participation of deprotonated species in the dynamics
under physiological conditions. Such an investigation is currently underway
in our group, but outside of the scope of the present communication.
(35) Salon, J.; Gan, J.; Abdur, R.; Liu, H.; Huang, Z. Synthesis of 6-Se-
Guanosine RNAs for Structural Study. Org. Lett. 2013, 15, 3934–3937.
(36) Salon, J.; Jiang, J.; Sheng, J.; Gerlits, O. O.; Huang, Z. Derivatization
of DNAs with Selenium at 6-position of Guanine for Function and Crystal
Structure Studies. Nucleic Acids Res. 2008, 36, 7009–7018.
(37) Kuar, M.; Rob, A.; Caton-Williams, J.; Huang, Z. In Biochalcogen
Chemistry: The Biological Chemistry of Sulfur, Selenium, and Tellurium;
2013; pp 89–126.
(38) Salon, J.; Sheng, J.; Jiang, J.; Chen, G.; Caton-Williams, J.; Huang, Z.
Oxygen Replacement with Selenium at the Thymidine 4-Position for the Se
Base Pairing and Crystal Structure Studies. J. Am. Chem. Soc. 2007, 129,
4862–4863.
(39) Faustino, I.; Curutchet, C.; Luque, F. J.; Orozco, M. The DNA-
Forming Properties of 6-Selenoguanine. Phys. Chem. Chem. Phys. 2014,
16, 1101–1110.
1
2
3
4
5
6
7
8
(26) Kim, Y.; Jang, Y. H.; Cho, H.; Hwang, S. Tautomers and Acid Disso-
ciation Constants of 6-Selenoguanine from Density Functional Theoretical
Calculations. Bull. Korean Chem. Soc. 2010, 31, 3013–3016.
(27) El-Sayed, M. A. Spin—Orbit Coupling and the Radiationless
Processes in Nitrogen Heterocyclics. J. Chem. Phys. 1963, 38, 2834–2838.
(28) El-Sayed, M. A. The Radiationless Processes Involving Change of
Multiplicity in the Diazenes. J. Chem. Phys. 1962, 573, 573–574.
(29) Martínez-Fernández, L.; Corral, I.; Granucci, G.; Persico, M. Compet-
ing Ultrafast Intersystem Crossing and Internal Conversion: A Time Re-
solved Picture for the Deactivation of 6-Thioguanine. Chem. Sci. 2014, 5,
1336–1347.
(30) Kasha, M. Characterization of Electronic Transitions in Complex Mol-
ecules. Discuss. Faraday Soc. 1950, 9, 14–19.
(31) Kasha, M.; McGlynn, S. P. Molecular Electronic Spectroscopy. Annu.
Rev. Phys. Chem. 1956, 7, 403–424.
(32) Reichardt, C.; Crespo-Hernández, C. E. Ultrafast Spin Crossover in 4-
Thiothymidine in an Ionic Liquid. Chem. Commun. 2010, 46, 5963–5965.
(33) Reichardt, C.; Crespo-Hernández, C. E. Room-Temperature
Phosphorescence of the DNA Monomer Analogue 4-Thiothymidine in
Aqueous Solutions after UVA Excitation. J. Phys. Chem. Lett. 2010, 1,
2239–2243.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(40) Ross, A. F.; Agarwal, K. C.; Chu, S. H.; Parks, R. E. Studies on the
Biochemical Actions of 6-Selenoguanine and 6-Selenoguanosine. Biochem.
Pharmacol. 1973, 22, 141–154.
(34) It is possible that the presence of a deprotonated anionic species of
6SeGua in equilibrium with the neutral form26 at pH 7.4 may contribute to
TOC
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