Relative Stability of the La and Lb Excited States in Adenine and Guanine: Direct Evidence from TD-DFT Calculations of MCD Spectra.
The relative position of La and Lb ππ* electronic states in purine nucleobases is a much debated topic, since it can strongly affect our understanding of their photoexcited dynamics. To assess this point, we calculated the absorption and magnetic circular dichroism (MCD) spectra of adenine, guanine, and their nucleosides in gas-phase and aqueous solution, exploiting recent developments in MCD computational technology within time-dependent density functional theory. MCD spectroscopy allows us to resolve the intense S0→ La transition from the weak S0→ Lb transition. The spectra obtained in water solution, by using B3LYP and CAM-B3LYP functionals and describing solvent effect by cluster models and by the polarizable continuum model (PCM), are in very good agreement with the experimental counterparts, thus providing direct and unambiguous evidence that the energy ordering predicted by TD-DFT, La < Lb, is the correct one.
†Istituto di Chimica dei Composti Organometallici (ICCOM-CNR), Area della Ricerca del CNR, via Moruzzi 1, I-56124 Pisa, Italy.
Fabrizio
F
Santoro
‡Istituto di Biostrutture e Bioimmagini-CNR, Via Mezzocannone 6, I-80134 Napoli, Italy.
Roberto
R
Improta
¶Department of Physics, Chemistry and Biology, Linköping University, SE-581 83 Linköping, Sweden.
Tobias
T
Fahleson
¶Department of Physics, Chemistry and Biology, Linköping University, SE-581 83 Linköping, Sweden.
Joanna
J
Kauczor
¶Department of Physics, Chemistry and Biology, Linköping University, SE-581 83 Linköping, Sweden.
Patrick
P
Norman
§Dipartimento di Scienze Chimiche e Farmaceutiche, Università degli Studi di Trieste, via L. Giorgieri 1, I-34127 Trieste, Italy.
Sonia
S
Coriani
DNA bases; complex polarization propagator; hidden transitions; magnetic circular dichroism; photostability; solvent effect