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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

GMP geometries and main geometrical parameters of the characterized conical intersections between the lowest-lying excited states computed at the CASSCF/6-31G* level of theory.
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Figure 4: GMP geometries and main geometrical parameters of the characterized conical intersections between the lowest-lying excited states computed at the CASSCF/6-31G* level of theory.

Mentions: The lowest-lying excited state is the ππ* La state, which is expected to be the main spectroscopic state due to its lowest-lying position and relatively large oscillator strength. This state is generally assumed to present an easily accessible CI along its relaxation pathway toward the ground-state characterized by a ring-puckering motion (see Figure 4) widely featured in the DNA/RNA nucleobases (Giussani et al., 2013a) in vacuo. This profile is slightly altered in the presence of polar environments as it has been previously reported for other purine nucleobases (Conti et al., 2010), where a shallow stationary point (La)sp arises along the MEP close to the CI with the ground state being placed at 1.15 eV vertically and adiabatically at 3.30 eV from the ground state (see Figure 5). Two different CIs have been characterized in the vicinity of this stationary point, one optimized directly in solvent corresponding to the minimum energy conical intersection (MECI) also reported by Serrano-Andrés et al. (2008), and another relating to the CI found in the same study along the MEP in vacuo, which we have tentatively named (La/GS)CI-1 and (La/GS)CI-2 in the present study, respectively. Both CIs as well as the (La)sp present ring-puckering structures in the A6 cycle yielding slight bond-length alterations (C2-N3, N1-C2, N3-C4, and C4-C5) compared to the FC structure and featuring prominent N1-N-C2-N3 dihedral angle distortions at 133°, 132°, and 140° for the (La/GS)CI-1, (La/GS)CI-2, and (La)sp structures, respectively. These distortions are quantitatively different to those reported in vacuo, stressing out the importance of the solvent where the (La/GS)CI-1 presents a ~143° dihedral angle as compared to its 133° solvated counterpart, together with slightly pronounced bond-length differences as can be seen in Table 1. Both CIs here characterized present very similar structures (see Figure 4) and charge distributions very close to those found in the gas phase, presenting an inversion in their energetic order in solution being (La/GS)CI-1 the energetically lowest-lying adiabatically at 3.31 eV, (La/GS)CI-2 being placed 0.3 eV higher in energy and 3.60 eV adiabatically from 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)

GMP geometries and main geometrical parameters of the characterized conical intersections between the lowest-lying excited states computed at the CASSCF/6-31G* level of theory.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 4: GMP geometries and main geometrical parameters of the characterized conical intersections between the lowest-lying excited states computed at the CASSCF/6-31G* level of theory.
Mentions: The lowest-lying excited state is the ππ* La state, which is expected to be the main spectroscopic state due to its lowest-lying position and relatively large oscillator strength. This state is generally assumed to present an easily accessible CI along its relaxation pathway toward the ground-state characterized by a ring-puckering motion (see Figure 4) widely featured in the DNA/RNA nucleobases (Giussani et al., 2013a) in vacuo. This profile is slightly altered in the presence of polar environments as it has been previously reported for other purine nucleobases (Conti et al., 2010), where a shallow stationary point (La)sp arises along the MEP close to the CI with the ground state being placed at 1.15 eV vertically and adiabatically at 3.30 eV from the ground state (see Figure 5). Two different CIs have been characterized in the vicinity of this stationary point, one optimized directly in solvent corresponding to the minimum energy conical intersection (MECI) also reported by Serrano-Andrés et al. (2008), and another relating to the CI found in the same study along the MEP in vacuo, which we have tentatively named (La/GS)CI-1 and (La/GS)CI-2 in the present study, respectively. Both CIs as well as the (La)sp present ring-puckering structures in the A6 cycle yielding slight bond-length alterations (C2-N3, N1-C2, N3-C4, and C4-C5) compared to the FC structure and featuring prominent N1-N-C2-N3 dihedral angle distortions at 133°, 132°, and 140° for the (La/GS)CI-1, (La/GS)CI-2, and (La)sp structures, respectively. These distortions are quantitatively different to those reported in vacuo, stressing out the importance of the solvent where the (La/GS)CI-1 presents a ~143° dihedral angle as compared to its 133° solvated counterpart, together with slightly pronounced bond-length differences as can be seen in Table 1. Both CIs here characterized present very similar structures (see Figure 4) and charge distributions very close to those found in the gas phase, presenting an inversion in their energetic order in solution being (La/GS)CI-1 the energetically lowest-lying adiabatically at 3.31 eV, (La/GS)CI-2 being placed 0.3 eV higher in energy and 3.60 eV adiabatically from 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