Evolution of the Excitonic State of DNA Stacked Thymines: Intrabase ππ* → S₀ Decay Paths Account for Ultrafast (Subpicosecond) and Longer (>100 ps) Deactivations

Abstract

Monomer-like ring puckering decay paths for two stacked quantum mechanical thymines inside a solvated DNA duplex described at the molecular mechanics level are mapped using a hybrid CASPT2//CASSCF/MM protocol that accounts for steric, electronic and electrostatic interactions within the nucleobases native environment. Asymmetric stacking between nucleobases open ups different intrabase ππ* decay paths accounting for distinctive excited state lifetimes, spanning the subps to subns time window.

A Unified Experimental/Theoretical Description of the Ultrafast Photophysics of Single and Double Thionated Uracils

“…The TA spectra and the decay paths were modeled by COBRAMM, our hybrid QM/MM scheme, that couples high level ab initio multireference dynamically correlated methods (CASPT2//CASSCF) of the photoactive thiouracil system with an explicit classical atomistic model (Amber force field) of the solvent (see Supporting Information for more details).…”

Linear absorption spectra of solvated thiouracils resolved at the hybrid RASPT2/MM level

“…3. QM/MM calculations: geometry refinement and electronic structure computations QM/MM calculations were performed with the COBRAMM suite developed in our group interfacing various QM codes with AMBER (presented in detail elsewhere [10,[74][75][76]). Prior to High/Medium/Low layer partitioning a spherical droplet centered at thiouracil with a radius of 12 Å (containing 260 waters in 2-thiouracil, 245 waters in 4thiouracil and 264 waters in 2,4-thiouracil) was cut from the cubic box.…”

Excitation energy transport in DNA modelled by multi-chromophoric field-induced surface hopping

“…While the excited state lifetime of the monomeric nucleobases is remarkably short (<1 ps), the deactivation in nucleobase multimers, despite the additional degrees of freedom, takes place on the timescale from 10 to several 100 ps [8][9][10][11][12] . Excitonic 5,[8][9][10][13][14][15][16][17][18] and delocalized charge transfer (CT) 5,6,10,12,19 states have been made responsible for this difference and the role of these states has been discussed extensively on the basis of experimental and theoretical 14,[20][21][22][23][24][25][26][27][28][29] work. The processes taking place in the UV excited DNA are until now not fully understood and this topic is at the very frontier of the current research 15 .…”

“…A recent study of the potential energy surface at QM(CASPT2//CASSCF)/MM level, having a stacked thymine dimer in the QM region, suggests that an ultrafast intrabase ππ * →S 0 transition reached by three different pathways is responsible for the short component observed in time-resolved experiments 20 . A fourth decay channel was found as well, which is however blocked by a 0.5 eV barrier arising due to a stabilizing inter-base excitonic state.…”