Excited‐State Dynamics of Thienoguanosine, an Isomorphic Highly Fluorescent Analogue of Guanosine. Issue 30 (7th May 2019)

• Record Type: Journal Article
• Authors: Martinez‐Fernandez, Lara; Gavvala, Krishna; Sharma, Rajans; Didier, Pascal; Richert, Ludovic; Segarra Martì, Javier; Mori, Mattia; Mely, Yves; Improta, Roberto
• Is Part Of: Chemistry. Volume 25:Issue 30(2019)
• Page Start: 7375
• Page End: 7386
• Publication Date: 2019-05-07
• Subjects: density functional calculations -- DNA -- fluorescence spectroscopy -- nucleobases -- solvent effects -- TD-DFT calculations; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201900677
• Languages: English
• ISSNs: 0947-6539

Abstract

Abstract: Thienoguanosine ( th G) is an isomorphic analogue of guanosine with promising potentialities as fluorescent DNA label. As a free probe in protic solvents, th G exists in two tautomeric forms, identified as the H1, being the only one observed in nonprotic solvents, and H3 keto–amino tautomers. We herein investigate the photophysics of th G in solvents of different polarity, from water to dioxane, by combining time‐resolved fluorescence with PCM/TD‐DFT and CASSCF calculations. Fluorescence lifetimes of 14.5–20.5 and 7–13 ns were observed for the H1 and H3 tautomers, respectively, in the tested solvents. In methanol and ethanol, an additional fluorescent decay lifetime (≈3 ns) at the blue emission side ( λ ≈430 nm) as well as a 0.5 ns component with negative amplitude at the red edge of the spectrum, typical of an excited‐state reaction, were observed. Our computational analysis explains the solvent effects observed on the tautomeric equilibrium. The main radiative and nonradiative deactivation routes have been mapped by PCM/TD‐DFT calculations in solution and CASSCF in the gas phase. The most easily accessible conical intersection, involving an out‐of plane motion of the sulfur atom in the five‐membered ring of th G, is separated by a sizeable energy barrier (≥0.4 eV) from the minimum of the spectroscopic state, which explains the large experimental fluorescence quantum yield. Abstract : Ligthening up : By combining experiments and calculations, the complex photoactivated dynamics of one of the most promising fluorescent base analogues is unravelled. The reasons of the high fluorescence yield and laying the ground for new DNA‐labelling studies (see figure) are explained.