THE EFFECT OF THE POSITION AND DEGREE

MPM Conference in Nantes, April 21 – 24, 2015
THE EFFECT OF THE POSITION AND DEGREE OF
THIOSUBSTITUTION ON THE POPULATION OF
THE TRIPLET STATES OF THIOURACILS
ARSLANCAN SerraA; CHEN Kathy J.B; MARTÍNEZ-FERNANDEZ LaraA; BOGGIO-PASQUA
MartialC; CORRAL InésA
A) Departamento de Química, Universidad Autónoma de Madrid, Cantoblanco, 28049,
Madrid, Spain; B) Laboratoire CEISAM (UMR 6230), Université de Nantes, 2 rue de la
Houssinière, BP92208, 44322 Nantes Cedex 3, France; C) Laboratoire de Chimie et Physique
Quantiques (IRSAMC), Université de Toulouse, 118 route de Narbonne 31062 Toulouse Cedex 09,
France
Contact: arslancan.serra@uam.es
Canonical DNA and RNA nucleobases are known as highly photostable species due to their ability
to undergo ultrafast radiationless decay from excited electronic states back to the ground state.1
Modified canonical nucleobases can present, however, markedly different deactivation pathways
when exposed to UV light.2 For instance, thiosubstituted derivatives of nucleobases lack ultrafast
radiationless relaxation mechanisms and instead populate their triplet states with high efficiency.3
From the triplet manifold, these modified nucleobases are able to generate DNA-damaging reactive
oxygen species, which is the basis for the phototherapeutic applications of these chromophores4
but also for the increase of skin cancer incidence in patients prescribed with these drugs.5
Therefore, it is crucial to rationalize the impact of those structural changes on nucleobases’ excited
state behavior for designing new
photoinitiated functional drugs and
for preventing DNA lesions.
In this pursuit, we have selected
three thiosubstituted derivatives of
the RNA nucleobase uracil with
different patterns of sulphur
substitution, namely 4-thiouracil, 2thiouracil and 2,4-dithiouracil. We
studied the effect of thiosubstitution on the gas phase photophysics by calculating the singlet and
triplet vertical spectra and exploring the main internal conversion and intersystem crossing
channels connected to their spectroscopic states using the CASPT2/CASSCF protocol. Finally,
this gas phase picture will be compared with that obtained after explicitly including a reduced
number of solvent molecules coordinated to the nucleobase. This allows us to evaluate how
environmental effects influence the intrinsic photophysics and photochemistry of these systems.
KEYWORDS: Nucleobase derivatives; photostability; triplet manifold population; solvent effects.
1
(a) A. Giussani, J. Segarra-Martí, D. Roca-Sanjuán, and M. Merchán, Top Curr Chem 355 (2015) 57-98. (b) C. T.
Middleton, K. de La Harpe, C. Su, Y. K. Law, C. E. Crespo-Hernández and B. Kohler, Annu. Rev. Phys. Chem. 60 (2009)
217-239.
2
L. Serrano-Andrés and M.Merchán, J. Photochem. Photobiol. C: Photochem. Rev. 10 (2009) 21–32.
3
(a) M. Pollum, L. Martínez-Fernandez and C. E. Crespo-Hernandez, Top Curr Chem 355 (2015) 245–328. (b) C. Reichardt
and C. E. Crespo-Hernández, J. Phys. Chem. Lett. 1 (2010) 2239–2243 (c) C. Reichardt, C. Guo and C. E. CrespoHernández, J. Phys. Chem. B 115 (2011) 3263–3270. (d) L. Martínez-Fernandez, L. Gonzalez, I. Corral, Chem. Commun.
48 (2012) 2134-2136. (e) L. Martínez-Fernandez, I. Corral, G. Granucci, M. Persico, Chem. Sci. 5 (2014) 1336-1347. (f) M.
Pollum and C. E. Crespo-Hernández, J. Chem. Phys. 140, (2014) 071101-7.
4
G. B. Elion Science 244 (1989) 41-47.
5
S. Euvrard, J. Kanitakis and A. Claudy, N. Engl. J. Med. 348 (2003) 1681-1691.