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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 and the three different QM/MM partitions defined in the present study. The high layer treating the guanine moiety at the QM level (ball and stick representation), the medium layer encompassing the movable MM water molecules and the phosphate group enclosed within a 10 Å radius distance from GMP, and the low layer containing the remainder of the MM region that is kept frozen throughout the calculations (see text).
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Figure 2: GMP and the three different QM/MM partitions defined in the present study. The high layer treating the guanine moiety at the QM level (ball and stick representation), the medium layer encompassing the movable MM water molecules and the phosphate group enclosed within a 10 Å radius distance from GMP, and the low layer containing the remainder of the MM region that is kept frozen throughout the calculations (see text).

Mentions: QM/MM calculations were performed using the COBRAMM interface developed in our group (Altoé et al., 2007a,b). The cut between the QM and MM regions has been done so that it includes all guanine atoms in the QM region, placing the link atom between the N9 of guanine and the carbon of the sugar ring directly attached to the nucleobase (see Figure 2). The choice of the cut between the QM and MM regions is justified in terms of charge redistribution on the nucleobase and its π-system in order to better reflect the covalent link between guanine and the monophosphate group. A three-layer approach (high, medium, and low) was used throughout (Altoé et al., 2007a): guanine was included in the QM region (high layer, ball, and stick representation in Figure 2), whereas the medium layer comprises the movable MM atoms within a 10 Å radius surrounding the GMP moiety, the remainder of the MM system being kept frozen during all optimization procedures in the low layer. Equilibrium geometries and photoreaction paths (Garavelli, 2006) were determined by using fully unconstrained optimizations and minimum energy path (MEP) computations on the relevant potential energy hypersurfaces by employing the intrinsic reaction coordinate (IRC) and optimization algorithms as implemented in the Gaussian 09 program package (Frisch et al., 2009) interfaced with COBRAMM. CI optimizations were performed with the gradient projection algorithm of Bearpark et al. (1994) as implemented in COBRAMM at the QM/MM level. Further details can be found in Conti et al. (2015).


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 and the three different QM/MM partitions defined in the present study. The high layer treating the guanine moiety at the QM level (ball and stick representation), the medium layer encompassing the movable MM water molecules and the phosphate group enclosed within a 10 Å radius distance from GMP, and the low layer containing the remainder of the MM region that is kept frozen throughout the calculations (see text).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 2: GMP and the three different QM/MM partitions defined in the present study. The high layer treating the guanine moiety at the QM level (ball and stick representation), the medium layer encompassing the movable MM water molecules and the phosphate group enclosed within a 10 Å radius distance from GMP, and the low layer containing the remainder of the MM region that is kept frozen throughout the calculations (see text).
Mentions: QM/MM calculations were performed using the COBRAMM interface developed in our group (Altoé et al., 2007a,b). The cut between the QM and MM regions has been done so that it includes all guanine atoms in the QM region, placing the link atom between the N9 of guanine and the carbon of the sugar ring directly attached to the nucleobase (see Figure 2). The choice of the cut between the QM and MM regions is justified in terms of charge redistribution on the nucleobase and its π-system in order to better reflect the covalent link between guanine and the monophosphate group. A three-layer approach (high, medium, and low) was used throughout (Altoé et al., 2007a): guanine was included in the QM region (high layer, ball, and stick representation in Figure 2), whereas the medium layer comprises the movable MM atoms within a 10 Å radius surrounding the GMP moiety, the remainder of the MM system being kept frozen during all optimization procedures in the low layer. Equilibrium geometries and photoreaction paths (Garavelli, 2006) were determined by using fully unconstrained optimizations and minimum energy path (MEP) computations on the relevant potential energy hypersurfaces by employing the intrinsic reaction coordinate (IRC) and optimization algorithms as implemented in the Gaussian 09 program package (Frisch et al., 2009) interfaced with COBRAMM. CI optimizations were performed with the gradient projection algorithm of Bearpark et al. (1994) as implemented in COBRAMM at the QM/MM level. Further details can be found in Conti et al. (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