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: [email protected] 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.
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