Exciton Absorption and Luminescence in i-Motif DNA

Zakhar V Reveguk,Evgeny V Khoroshilov,Andrey A Buglak,Alexei I Kononov,Vladimir A Pomogaev,Andrey V Sharkov,Alexander N Tarnovsky

doi:10.1038/s41598-019-52242-1

Abstract

We have studied the excited-state dynamics for the i-motif form of cytosine chains (dC)10, using the ultrafast fluorescence up-conversion technique. We have also calculated vertical electronic transition energies and determined the nature of the corresponding excited states in a model tetramer i-motif structure. Quantum chemical calculations of the excitation spectrum of a tetramer i-motif structure predict a significant (0.3 eV) red shift of the lowest-energy transition in the i-motif form relative to its absorption maximum, which agrees with the experimental absorption spectrum. The lowest excitonic state in i-(dC)10 is responsible for a 2 ps red-shifted emission at 370 nm observed in the decay-associated spectra obtained on the femtosecond time-scale. This delocalized (excitonic) excited state is likely a precursor to a long-lived excimer state observed in previous studies. Another fast 310 fs component at 330 nm is assigned to a monomer-like locally excited state. Both emissive states form within less than the available time resolution of the instrument (100 fs). This work contributes to the understanding of excited-state dynamics of DNA within the first few picoseconds, which is the most interesting time range with respect to unraveling the photodamage mechanism, including the formation of the most dangerous DNA lesions such as cyclobutane pyrimidine dimers.

Highlights

It is well known that sun light is a mutagenic agent that causes various DNA damage[1,2,3,4]
Based on quantum chemical calculations, Improta and Barone showed that the excited state dynamics in stacked adenines is complex, including localization of the excitation on a single base, “neutral” and “charge-transfer”excimers formation, involving a decrease of the stacking distance[26]
In this work, using the ultrafast fluorescence up-conversion technique, we study the excited state dynamics in cytosine DNA tracts

Summary

Introduction

It is well known that sun light is a mutagenic agent that causes various DNA damage[1,2,3,4]. Answering the questions like what is the character of excited states from which the photochemical reactions start and what is the nature of the photochemical reaction pathway is of vital importance for the understanding of the fundamental principles of DNA photochemistry These primary photoprocesses occur on a femtosecond time scale and greatly affect the subsequent photochemistry. Kohler et al believed that long-lived charge-transfer states (excimers) are formed directly from the Franck-Condon state of the stacked bases[22,23]. We found that the hemi-protonated i-motif structure of (dC)[10] exhibits two spectrally different components due to the local (monomer-like) and delocalized (exciton) emissive states both formed from the Franck-Condon excited state in less than 100 fs

Methods

Results

Conclusion