Abstract
Thymine photochemistry is important for understanding DNA photodamage. In the gas phase, thymine undergoes a fast non-radiative decay from S to S. In the S state, it gets trapped for several picoseconds until returning to the ground-state S. Here, we explore the electrostatic effects of nanomeric droplets of methanol and water on the excited states of thymine. For this purpose, we develop and implement an electrostatic embedding TD-DFT/MM method based on a QM/MM coupling defined through electrostatic potential fitting charges. We show that both in methanol and water, the mechanism is similar to the gas phase. The solvent molecules participate in defining the branching plane of S/S intersection and have a negligible effect on the S/S intersection. Despite the wrong topology of the ground/excited state intersections, electrostatic embedding TD-DFT/MM allows for a fast exploration of the potential energy surfaces and a qualitative picture of the photophysics of thymine in solvent droplets.
Highlights
Photodamage of deoxyribonucleic acid (DNA) originating from ultraviolet (UV) sunlight radiation is one of the main sources of skin cancer [1]
Despite the wrong topology of the ground/excited state intersections, electrostatic embedding time-dependent density-functional theory (TDDFT)/MM allows for a fast exploration of the potential energy surfaces and a qualitative picture of the photophysics of thymine in solvent droplets
We develop and implement a new quantum mechanics/molecular mechanics (QM/MM) hybrid electrostatic embedding method combining time-dependent density-functional theory (TDDFT) [23], and classical force fields via an electrostatic potential fitting (ESPF) interaction
Summary
Photodamage of deoxyribonucleic acid (DNA) originating from ultraviolet (UV) sunlight radiation is one of the main sources of skin cancer [1]. UV light can induce irreparable damage to the genetic code, despite the many natural protection and correction mechanisms that exist [2]. The most basic protection against UV radiation is found at the nucleobase level. It has been shown that isolated guanine, adenine, thymine and cytosine undergo ultrafast non-radiative decays back to the ground state [3,4,5,6,7]. UV light brings the nucleobase in a singlet excited state of ππ ∗ character, which relaxes to lower-energy dark states of nπ ∗ character (n is a lone-pair electron of a heteroatom in the system) and back to the ground state. The nucleobases decay mechanisms depend strongly on the embedding environment [8,9,10,11,12]
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