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Deciphering the photochemical mechanisms describing the UV-induced processes occurring in solvated guanine monophosphate.

Altavilla SF, Segarra-Martí J, Nenov A, Conti I, Rivalta I, Garavelli M - Front Chem (2015)

Bottom Line: Solvated electronic excitation energies of solvated GMP at the Franck-Condon (FC) region show a red shift for the ππ(*) La and Lb states, whereas the energy of the oxygen lone-pair nπ(*) state is blue-shifted.The role of the nπ(*) state in the solvated system is severely diminished as the crossings with the initially populated La state and also with the Lb state are placed too high energetically to partake prominently in the deactivation photo-process.The proposed mechanism present in solvated and in vacuo DNA/RNA chromophores validates the intrinsic photostability mechanism through CI-mediated non-radiative processes accompanying the bright excited-state population toward the ground state and subsequent relaxation back to the FC region.

View Article: PubMed Central - PubMed

Affiliation: Dipartimento di Chimica "G. Ciamician," Università di Bologna Bologna, Italy.

ABSTRACT
The photophysics and photochemistry of water-solvated guanine monophosphate (GMP) are here characterized by means of a multireference quantum-chemical/molecular mechanics theoretical approach (CASPT2//CASSCF/AMBER) in order to elucidate the main photo-processes occurring upon UV-light irradiation. The effect of the solvent and of the phosphate group on the energetics and structural features of this system are evaluated for the first time employing high-level ab initio methods and thoroughly compared to those in vacuo previously reported in the literature and to the experimental evidence to assess to which extent they influence the photoinduced mechanisms. Solvated electronic excitation energies of solvated GMP at the Franck-Condon (FC) region show a red shift for the ππ(*) La and Lb states, whereas the energy of the oxygen lone-pair nπ(*) state is blue-shifted. The main photoinduced decay route is promoted through a ring-puckering motion along the bright lowest-lying La state toward a conical intersection (CI) with the ground state, involving a very shallow stationary point along the minimum energy pathway in contrast to the barrierless profile found in gas-phase, the point being placed at the end of the minimum energy path (MEP) thus endorsing its ultrafast deactivation in accordance with time-resolved transient and photoelectron spectroscopy experiments. The role of the nπ(*) state in the solvated system is severely diminished as the crossings with the initially populated La state and also with the Lb state are placed too high energetically to partake prominently in the deactivation photo-process. The proposed mechanism present in solvated and in vacuo DNA/RNA chromophores validates the intrinsic photostability mechanism through CI-mediated non-radiative processes accompanying the bright excited-state population toward the ground state and subsequent relaxation back to the FC region.

No MeSH data available.


Related in: MedlinePlus

CASPT2 energies of the ground (GS) and lowest-lying singlet excited states (ππ* La, π π* Lb and nOπ*) of the GMP from the FC geometry and along the nOπ* CASSCF MEP.
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Figure 6: CASPT2 energies of the ground (GS) and lowest-lying singlet excited states (ππ* La, π π* Lb and nOπ*) of the GMP from the FC geometry and along the nOπ* CASSCF MEP.

Mentions: As previously stated, nOπ* represents the third energetically wise excited state for the GMP in solvent. The optimized nOπ* minimum found in solvent is prominently characterized by an elongation of the C6-O bond distance with respect to the ground state. Table 1 shows the main differences in bond lengths, where the C6-O bond is elongated in this minimum from 1.22 Å at the FC region to 1.41 Å. The minimum of this excited state in solvent is very similar to the one reported in vacuo (Serrano-Andrés et al., 2008), as would be expected given the small effects provided by the solvent on nπ* excited states. Energetically it is placed 3.97 eV adiabatically and 2.67 eV vertically with respect to the ground state (see Figure 6), which constitutes a stabilization of 1.8 eV from its initial value at the FC region.


Deciphering the photochemical mechanisms describing the UV-induced processes occurring in solvated guanine monophosphate.

Altavilla SF, Segarra-Martí J, Nenov A, Conti I, Rivalta I, Garavelli M - Front Chem (2015)

CASPT2 energies of the ground (GS) and lowest-lying singlet excited states (ππ* La, π π* Lb and nOπ*) of the GMP from the FC geometry and along the nOπ* CASSCF MEP.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4403598&req=5

Figure 6: CASPT2 energies of the ground (GS) and lowest-lying singlet excited states (ππ* La, π π* Lb and nOπ*) of the GMP from the FC geometry and along the nOπ* CASSCF MEP.
Mentions: As previously stated, nOπ* represents the third energetically wise excited state for the GMP in solvent. The optimized nOπ* minimum found in solvent is prominently characterized by an elongation of the C6-O bond distance with respect to the ground state. Table 1 shows the main differences in bond lengths, where the C6-O bond is elongated in this minimum from 1.22 Å at the FC region to 1.41 Å. The minimum of this excited state in solvent is very similar to the one reported in vacuo (Serrano-Andrés et al., 2008), as would be expected given the small effects provided by the solvent on nπ* excited states. Energetically it is placed 3.97 eV adiabatically and 2.67 eV vertically with respect to the ground state (see Figure 6), which constitutes a stabilization of 1.8 eV from its initial value at the FC region.

Bottom Line: Solvated electronic excitation energies of solvated GMP at the Franck-Condon (FC) region show a red shift for the ππ(*) La and Lb states, whereas the energy of the oxygen lone-pair nπ(*) state is blue-shifted.The role of the nπ(*) state in the solvated system is severely diminished as the crossings with the initially populated La state and also with the Lb state are placed too high energetically to partake prominently in the deactivation photo-process.The proposed mechanism present in solvated and in vacuo DNA/RNA chromophores validates the intrinsic photostability mechanism through CI-mediated non-radiative processes accompanying the bright excited-state population toward the ground state and subsequent relaxation back to the FC region.

View Article: PubMed Central - PubMed

Affiliation: Dipartimento di Chimica "G. Ciamician," Università di Bologna Bologna, Italy.

ABSTRACT
The photophysics and photochemistry of water-solvated guanine monophosphate (GMP) are here characterized by means of a multireference quantum-chemical/molecular mechanics theoretical approach (CASPT2//CASSCF/AMBER) in order to elucidate the main photo-processes occurring upon UV-light irradiation. The effect of the solvent and of the phosphate group on the energetics and structural features of this system are evaluated for the first time employing high-level ab initio methods and thoroughly compared to those in vacuo previously reported in the literature and to the experimental evidence to assess to which extent they influence the photoinduced mechanisms. Solvated electronic excitation energies of solvated GMP at the Franck-Condon (FC) region show a red shift for the ππ(*) La and Lb states, whereas the energy of the oxygen lone-pair nπ(*) state is blue-shifted. The main photoinduced decay route is promoted through a ring-puckering motion along the bright lowest-lying La state toward a conical intersection (CI) with the ground state, involving a very shallow stationary point along the minimum energy pathway in contrast to the barrierless profile found in gas-phase, the point being placed at the end of the minimum energy path (MEP) thus endorsing its ultrafast deactivation in accordance with time-resolved transient and photoelectron spectroscopy experiments. The role of the nπ(*) state in the solvated system is severely diminished as the crossings with the initially populated La state and also with the Lb state are placed too high energetically to partake prominently in the deactivation photo-process. The proposed mechanism present in solvated and in vacuo DNA/RNA chromophores validates the intrinsic photostability mechanism through CI-mediated non-radiative processes accompanying the bright excited-state population toward the ground state and subsequent relaxation back to the FC region.

No MeSH data available.


Related in: MedlinePlus