Role of Excitonic Coupling and Charge-Transfer States in the Absorption and CD Spectra of Adenine-Based Oligonucleotides Investigated through QM/MM Simulations

In this work, we study the photophysical properties of an adenine-based oligonucleotide using an ensemble of about 200 configurations obtained from molecular dynamics simulations. Specifically, a QM/MM approach is used to obtain the excited-state energies and properties of (dA)(20)(dT)(20) with a dimer of p-stacked adenine bases included in the quantum region. The absorption and circular dichroism spectra are computed and analyzed using the algebraic diagrammatic construction through second order level of theory method (ADC(2)) combined with classical mechanics. We find that the experimentally observed red-shifted shoulder in the absorption spectrum is due to excitonic interactions, while charge-transfer states are present within the absorption band at the higher-energy end of the spectrum. More importantly, low-energy states with charge-transfer mixing exist, which could lead to excimers and bonded excimers. These observations suggest that mixing between charge-transfer and excitonic states plays an important role in the photophysics of oligonucleotides. They also highlight the importance of taking into account the conformational flexibility of the oligonucleotide when investigating photophysical properties.