Thienoguanosine (th G) is an isomorphic nucleoside analogue acting as a faithful fluorescent substitute of G, with respectable quantum yield in oligonucleotides. Photophysical analysis of th G reveals the existence of two ground-state tautomers with significantly shifted absorption and emission wavelengths, and high quantum yield in buffer. Using (TD)-DFT calculations, the tautomers were identified as the H1 and H3 keto-amino tautomers. When incorporated into the loop of (-)PBS, the (-)DNA copy of the HIV-1 primer binding site, both tautomers are observed and show differential sensitivity to protein binding. The red-shifted H1 tautomer is strongly favored in matched (-)/(+)PBS duplexes, while the relative emission of the H3 tautomer can be used to detect single nucleotide polymorphisms. These tautomers and their distinct environmental sensitivity provide unprecedented information channels for analyzing G residues in oligonucleotides and their complexes.

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Laboratoire de Biophotonique et Pharmacologie, Faculte de Pharmacie, UMR 7213 CNRS, Universite de Strasbourg, 74 route du Rhin, 67401, Illkirch, France., Department of Chemistry, Kyiv National Taras Shevchenko University, 60 Volodymyrska street, 01033, Kyiv, Ukraine., Consiglio Nazionale delle Ricerche, Istituto di Biostrutture e Bioimmagini, Via Mezzocannone 16, 80134, Napoli, Italy., Dipartimento di Biotecnologie, Chimica e Farmacia, Universita degli Studi di Siena, Via Aldo Moro 2, 53100, Siena, Italy., Center for Life Nano Science@Sapienza, Istituto Italiano di Tecnologia, viale Regina Elena 291, 00161, Roma, Italy., Laboratoire O'Optique et Applications, Centre de Physique Atomique Moleculaire et Optique Quantique, Universite de Douala, BP 8580, Douala, Cameroon., Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Dr, La Jolla, CA, 92093-0358, USA., Department of Integrative Structural Biology, Institut de Genetique et de Biologie Moleculaire et Cellulaire, INSERM U964 UMR 7104 CNRS, Universite de Strasbourg, 67400, Illkirch, France., Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Dr, La Jolla, CA, 92093-0358, USA. ytor@ucsd.edu., Laboratoire de Biophotonique et Pharmacologie, Faculte de Pharmacie, UMR 7213 CNRS, Universite de Strasbourg, 74 route du Rhin, 67401, Illkirch, France. yves.mely@unistra.fr.,

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