Effect of substituents on the excited-state dynamics of the modified DNA bases 2,4-diaminopyrimidine and 2,6-diaminopurine.

Zsolt Gengeliczki, Michael P Callahan, Nathan Svadlenak, Csaba István Pongor, Bálint Sztáray, Leo Meerts, Dana Nachtigallová, Pavel Hobza, Mario Barbatti, Hans Lischka, Mattanjah S de Vries

Index: Phys. Chem. Chem. Phys. 12(20) , 5375-88, (2010)

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Abstract

To explore the excited state dynamics of pyrimidine derivatives, we performed a combined experimental and theoretical study. We present resonant two-photon ionization (R2PI) and IR-UV double resonance spectra of 2,4-diaminopyrimidine and 2,6-diaminopurine seeded in a supersonic jet by laser desorption. For 2,4-diaminopyrimidine (S(0)-->S(1) 34,459 cm(-1)), we observed only the diamino tautomer with an excited state lifetime bracketed between experimental limits of 10 ps and 1 ns. For 2,6-diaminopurine, we observed two tautomers, the 9H- (S(0)-->S(1) 34,881 cm(-1)) and 7H- (S(0)-->S(1) 32,215 cm(-1)) diamino forms, with excited state lifetimes of 6.3±0.4 ns and 8.7±0.8 ns, respectively. We investigated the nature of the excited state of 2,4-diaminopyrimidine by means of multi-reference ab initio methods. The calculations of stationary points in the ground and excited states, minima on the S(0)/S(1) crossing seam and connecting reaction paths show that several paths with negligible barriers exist, allowing ultrafast radiationless deactivation if excited at energies slightly higher than the band origin. The sub-nanosecond lifetime found experimentally is in good agreement with this finding.This journal is © the Owner Societies 2010