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

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.

The photodynamics of 2,4-diaminopyrimidine in comparison with 4-aminopyrimidine: The effect of amino-substitution

“…The spectrum is characterized by a single broad band with origin at 4.46 eV and maximum at 5.10 eV. In our previous study on 2,4-DAPy and related compounds we have shown that these values obtained using the CC2/SVP method [23] are uniformly shifted by 0.31 eV to lower energy with respect to present CASSCF results. The vertical excitation energies and oscillator strengths of the 2 1 A 1 (pp * ) and 3 1 A 1 (np * ) states calculated at the CASSCF level are indicated in the graph.…”

“…Calculated linear interpolation curves of 2,4-DAPy between the S 1 minima towards various MXS reported in our previous study [23] show that the additional NH 2 group causes a sizeable barrier of about 1 eV on the reaction paths towards the 1 S 2 structure thereby immobilizing the MXS located at the C2 atom. There are however still other paths connecting the S1min_C6 with other MXSs ( 1 S 6 and B 1,4 and 3 H 4 ) with negligible barriers allowing for a fast relaxation process.…”

“…Recently, we have performed a combined experimental and theoretical study on the modified DNA base 2,4-diaminopyrimidine (2,4-DAPy; see Scheme 1) [23].…”

The effect of hydration on the photo-deactivation pathways of 4-aminopyrimidine

“…[36,37] represents Ade in several interesting aspects (vertical excitations, MXS structures and reaction paths) very well. Moreover, 4-APy is also of interest in connection with investigations on the photophysics of diaminopyrimidine [38,39], which forms an unnatural base pair with xanthine [40]. Water was chosen as solvent because of its outstanding importance for biological systems.…”

Relaxation mechanisms of UV-photoexcited DNA and RNA nucleobases

“…Will it be long-lived? The answer can be given from previous experience with the dynamics of 6-APy (40) and recent static (41) and photodynamical (42) calculations for 2,6-DAPy, which show that either keeping or not the blocking C 2 substitution on the pyrimidine ring, the removal of the imidazole ring will allow fast deactivation by puckering at the C 4 position.…”