Recently, the 1H-detected in-cell NMR spectroscopy has emerged as a unique tool allowing the characterization of interactions between nucleic acid-based targets and drug-like molecules in living human cells. Here, we assess the application potential of 1H and 19F-detected in-cell NMR spectroscopy to profile drugs/ligands targeting DNA G-quadruplexes, arguably the most studied class of anti-cancer drugs targeting nucleic acids. We show that the extension of the original in-cell NMR approach is not straightforward. The severe signal broadening and overlap of 1H in-cell NMR spectra of polymorphic G-quadruplexes and their complexes complicate their quantitative interpretation. Nevertheless, the 1H in-cell NMR can be used to identify drugs that, despite strong interaction in vitro, lose their ability to bind G-quadruplexes in the native environment. The in-cell NMR approach is adjusted to a recently developed 3,5-bis(trifluoromethyl)phenyl probe to monitor the intracellular interaction with ligands using 19F-detected in-cell NMR. The probe allows dissecting polymorphic mixture in terms of number and relative populations of individual G-quadruplex species, including ligand-bound and unbound forms in vitro and in cellulo. Despite the probe's discussed limitations, the 19F-detected in-cell NMR appears to be a promising strategy to profile G-quadruplex-ligand interactions in the complex environment of living cells.

Central European Institute of Technology Masaryk University Kamenice 753 5 625 00 Brno Czech Republic

Institute of Biophysics Czech Academy of Sciences Královopolská 135 612 65 Brno Czech Republic

National Centre for Biomolecular Research Masaryk University Kamenice 5 625 00 Brno Czech Republic

Zobrazit více v PubMed

Imming P., Sinning C., Meyer A. Drugs, their targets and the nature and number of drug targets. Nat. Rev. Drug Discov. 2006;5:821–834. doi: 10.1038/nrd2132. PubMed DOI

Chaires J.B., Randazzo A., Mergny J.L. Targeting DNA. Biochimie. 2011;93:v–vi. doi: 10.1016/S0300-9084(11)00240-9. PubMed DOI

Wildey M.J., Haunso A., Tudor M., Webb M., Connick J.H. Chapter Five High-Throughput Screening. In: Goodnow R.A., editor. Annual Reports in Medicine Chemistry. Volume 50. Academic Press; Cambridge, MA, USA: 2017. pp. 149–195. Platform Technologies in Drug Discovery and Validation.

Dailey M.M., Hait C., Holt P.A., Maguire J.M., Meier J.B., Miller M.C., Petraccone L., Trent J.O. Structure-based drug design: From nucleic acid to membrane protein targets. Exp. Mol. Pathol. 2009;86:141–150. doi: 10.1016/j.yexmp.2009.01.011. PubMed DOI PMC

Murat P., Singh Y., Defrancq E. Methods for investigating G-quadruplex DNA/ligand interactions. Chem. Soc. Rev. 2011;40:5293–5307. doi: 10.1039/c1cs15117g. PubMed DOI

Pagano B., Cosconati S., Gabelica V., Petraccone L., De Tito S., Marinelli L., La Pietra V., di Leva F.S., Lauri I., Trotta R., et al. State-of-the-art methodologies for the discovery and characterization of DNA G-quadruplex binders. Curr. Pharm. Des. 2012;18:1880–1899. doi: 10.2174/138161212799958332. PubMed DOI

Sheng J., Gan J., Huang Z. Structure-based DNA-targeting strategies with small molecule ligands for drug discovery. Med. Res. Rev. 2013;33:1119–1173. doi: 10.1002/med.21278. PubMed DOI PMC

Krafcikova M., Dzatko S., Caron C., Granzhan A., Fiala R., Loja T., Teulade-Fichou M.-P., Fessl T., Hänsel-Hertsch R., Mergny J.-L., et al. Monitoring DNA–Ligand Interactions in Living Human Cells Using NMR Spectroscopy. J. Am. Chem. Soc. 2019;141:13281–13285. doi: 10.1021/jacs.9b03031. PubMed DOI

Sink R., Gobec S., Pečar S., Zega A. False positives in the early stages of drug discovery. Curr. Med. Chem. 2010;17:4231–4255. doi: 10.2174/092986710793348545. PubMed DOI

Kotera N., Granzhan A., Teulade-Fichou M.-P. Comparative study of affinity and selectivity of ligands targeting abasic and mismatch sites in DNA using a fluorescence-melting assay. Biochimie. 2016;128:133–137. doi: 10.1016/j.biochi.2016.08.004. PubMed DOI

Fiala R., Špačková N., Foldynová-Trantírková S., Šponer J., Sklenář V., Trantírek L. NMR Cross-Correlated Relaxation Rates Reveal Ion Coordination Sites in DNA. J. Am. Chem. Soc. 2011;133:13790–13793. doi: 10.1021/ja202397p. PubMed DOI

Nakano S., Miyoshi D., Sugimoto N. Effects of Molecular Crowding on the Structures, Interactions, and Functions of Nucleic Acids. Chem. Rev. 2014;114:2733–2758. doi: 10.1021/cr400113m. PubMed DOI

Nakano S., Sugimoto N. Model studies of the effects of intracellular crowding on nucleic acid interactions. Mol. BioSyst. 2016;13:32–41. doi: 10.1039/C6MB00654J. PubMed DOI

Nakano S.-I., Sugimoto N. The structural stability and catalytic activity of DNA and RNA oligonucleotides in the presence of organic solvents. Biophys. Rev. 2016;8:11–23. doi: 10.1007/s12551-015-0188-0. PubMed DOI PMC

Hancock R. Crowding, Entropic Forces, and Confinement: Crucial Factors for Structures and Functions in the Cell Nucleus. Biochemistry. 2018;83:326–337. doi: 10.1134/S0006297918040041. PubMed DOI

Azarkh M., Okle O., Singh V., Seemann I.T., Hartig J.S., Dietrich D.R., Drescher M. Long-range distance determination in a DNA model system inside Xenopus laevis oocytes by in-cell spin-label EPR. Chembiochem. 2011;12:1992–1995. doi: 10.1002/cbic.201100281. PubMed DOI

Krstić I., Hänsel R., Romainczyk O., Engels J.W., Dötsch V., Prisner T.F. Long-range distance measurements on nucleic acids in cells by pulsed EPR spectroscopy. Angew. Chem. Int. Ed. Engl. 2011;50:5070–5074. doi: 10.1002/anie.201100886. PubMed DOI

Fessl T., Adamec F., Polívka T., Foldynová-Trantírková S., Vácha F., Trantírek L. Towards characterization of DNA structure under physiological conditions in vivo at the single-molecule level using single-pair FRET. Nucleic Acids Res. 2012;40:e121. doi: 10.1093/nar/gks333. PubMed DOI PMC

Holder I.T., Drescher M., Hartig J.S. Structural characterization of quadruplex DNA with in-cell EPR approaches. Bioorg. Med. Chem. 2013;21:6156–6161. doi: 10.1016/j.bmc.2013.04.014. PubMed DOI

Azarkh M., Singh V., Okle O., Seemann I.T., Dietrich D.R., Hartig J.S., Drescher M. Site-directed spin-labeling of nucleotides and the use of in-cell EPR to determine long-range distances in a biologically relevant environment. Nat. Protoc. 2013;8:131–147. doi: 10.1038/nprot.2012.136. PubMed DOI

Hänsel R., Luh L.M., Corbeski I., Trantirek L., Dötsch V. In-cell NMR and EPR spectroscopy of biomacromolecules. Angew. Chem. Int. Ed. Engl. 2014;53:10300–10314. doi: 10.1002/anie.201311320. PubMed DOI

Salgado G.F., Cazenave C., Kerkour A., Mergny J.-L. G-quadruplex DNA and ligand interaction in living cells using NMR spectroscopy. Chem. Sci. 2015;6:3314–3320. doi: 10.1039/C4SC03853C. PubMed DOI PMC

Giassa I.-C., Rynes J., Fessl T., Foldynova-Trantirkova S., Trantirek L. Advances in the cellular structural biology of nucleic acids. FEBS Lett. 2018;592:1997–2011. doi: 10.1002/1873-3468.13054. PubMed DOI

Manna S., Srivatsan S.G. Fluorescence-based tools to probe G-quadruplexes in cell-free and cellular environments. RSC Adv. 2018;8:25673–25694. doi: 10.1039/C8RA03708F. PubMed DOI PMC

Yamaoki Y., Kiyoishi A., Miyake M., Kano F., Murata M., Nagata T., Katahira M. The first successful observation of in-cell NMR signals of DNA and RNA in living human cells. Phys. Chem. Chem. Phys. 2018;20:2982–2985. doi: 10.1039/C7CP05188C. PubMed DOI

Yamaoki Y., Nagata T., Sakamoto T., Katahira M. Recent progress of in-cell NMR of nucleic acids in living human cells. Biophys. Rev. 2020;12:411–417. doi: 10.1007/s12551-020-00664-x. PubMed DOI PMC

Yamaoki Y., Nagata T., Sakamoto T., Katahira M. Observation of nucleic acids inside living human cells by in-cell NMR spectroscopy. Biophys. Physicobiol. 2020;17:36–41. doi: 10.2142/biophysico.BSJ-2020006. PubMed DOI PMC

Viskova P., Krafcik D., Trantirek L., Foldynova-Trantirkova S. In-Cell NMR Spectroscopy of Nucleic Acids in Human Cells. Curr. Protoc. Nucleic Acid Chem. 2019;76:e71. doi: 10.1002/cpnc.71. PubMed DOI

Dzatko S., Fiala R., Hänsel-Hertsch R., Foldynova-Trantirkova S., Trantirek L. Chapter 16: In-cell NMR Spectroscopy of Nucleic Acids. Royal Society of Chemistry; Cambridge, UK: 2019. pp. 272–297.

Bao H.-L., Xu Y. Telomeric DNA–RNA-hybrid G-quadruplex exists in environmental conditions of HeLa cells. Chem. Commun. 2020;56:6547–6550. doi: 10.1039/D0CC02053B. PubMed DOI

Collauto A., von Bülow S., Gophane D.B., Saha S., Stelzl L.S., Hummer G., Sigurdsson S.T., Prisner T.F. Compaction of RNA Duplexes in the Cell*. Angew. Chem. Int. Ed. Engl. 2020;59:23025–23029. doi: 10.1002/anie.202009800. PubMed DOI PMC

Juliusson H.Y., Sigurdsson S.T. Reduction Resistant and Rigid Nitroxide Spin-Labels for DNA and RNA. J. Org. Chem. 2020;85:4036–4046. doi: 10.1021/acs.joc.9b02988. PubMed DOI

Broft P., Dzatko S., Krafcikova M., Wacker A., Hänsel-Hertsch R., Dötsch V., Trantirek L., Schwalbe H. In-Cell NMR Spectroscopy of Functional Riboswitch Aptamers in Eukaryotic Cells. Angew. Chem. Int. Ed. Engl. 2021;60:865–872. doi: 10.1002/anie.202007184. PubMed DOI PMC

Hänsel R., Foldynová-Trantírková S., Löhr F., Buck J., Bongartz E., Bamberg E., Schwalbe H., Dötsch V., Trantírek L. Evaluation of Parameters Critical for Observing Nucleic Acids Inside Living Xenopus laevis Oocytes by In-Cell NMR Spectroscopy. J. Am. Chem. Soc. 2009;131:15761–15768. doi: 10.1021/ja9052027. PubMed DOI

Hänsel R., Foldynová-Trantírková S., Dötsch V., Trantírek L. Investigation of quadruplex structure under physiological conditions using in-cell NMR. Top. Curr. Chem. 2013;330:47–65. doi: 10.1007/128_2012_332. PubMed DOI

Hänsel R., Löhr F., Trantirek L., Dötsch V. High-resolution insight into G-overhang architecture. J. Am. Chem. Soc. 2013;135:2816–2824. doi: 10.1021/ja312403b. PubMed DOI

Bao H.-L., Ishizuka T., Sakamoto T., Fujimoto K., Uechi T., Kenmochi N., Xu Y. Characterization of human telomere RNA G-quadruplex structures in vitro and in living cells using 19F NMR spectroscopy. Nucleic Acids Res. 2017;45:5501–5511. doi: 10.1093/nar/gkx109. PubMed DOI PMC

Bao H.-L., Xu Y. Investigation of higher-order RNA G-quadruplex structures in vitro and in living cells by 19F NMR spectroscopy. Nat. Protoc. 2018;13:652–665. doi: 10.1038/nprot.2017.156. PubMed DOI

Manna S., Sarkar D., Srivatsan S.G. A Dual-App Nucleoside Probe Provides Structural Insights into the Human Telomeric Overhang in Live Cells. J. Am. Chem. Soc. 2018;140:12622–12633. doi: 10.1021/jacs.8b08436. PubMed DOI PMC

Hänsel R., Löhr F., Foldynová-Trantírková S., Bamberg E., Trantírek L., Dötsch V. The parallel G-quadruplex structure of vertebrate telomeric repeat sequences is not the preferred folding topology under physiological conditions. Nucleic Acids Res. 2011;39:5768–5775. doi: 10.1093/nar/gkr174. PubMed DOI PMC

Krafcikova M., Hänsel-Hertsch R., Trantirek L., Foldynova-Trantirkova S. In Cell NMR Spectroscopy: Investigation of G-Quadruplex Structures Inside Living Xenopus laevis Oocytes. In: Yang D., Lin C., editors. G-Quadruplex Nucleic Acids: Methods and Protocols. Springer; New York, NY, USA: 2019. pp. 397–405. Methods in Molecular Biology. PubMed

Dzatko S., Krafcikova M., Hänsel-Hertsch R., Fessl T., Fiala R., Loja T., Krafcik D., Mergny J.-L., Foldynova-Trantirkova S., Trantirek L. Evaluation of the Stability of DNA i-Motifs in the Nuclei of Living Mammalian Cells. Angew. Chem. Int. Ed. Engl. 2018;57:2165–2169. doi: 10.1002/anie.201712284. PubMed DOI PMC

Bao H.-L., Liu H.-S., Xu Y. Hybrid-type and two-tetrad antiparallel telomere DNA G-quadruplex structures in living human cells. Nucleic Acids Res. 2019;47:4940–4947. doi: 10.1093/nar/gkz276. PubMed DOI PMC

Cheng M., Qiu D., Tamon L., Ištvánková E., Víšková P., Amrane S., Guédin A., Chen J., Lacroix L., Ju H., et al. Thermal and pH Stabilities of i-DNA: Confronting in vitro Experiments with Models and In-Cell NMR Data. Angew. Chem. Int. Ed. Engl. 2021 doi: 10.1002/anie.202016801. PubMed DOI

Huppert J.L., Balasubramanian S. Prevalence of quadruplexes in the human genome. Nucleic Acids Res. 2005;33:2908–2916. doi: 10.1093/nar/gki609. PubMed DOI PMC

Huppert J.L., Balasubramanian S. G-quadruplexes in promoters throughout the human genome. Nucleic Acids Res. 2007;35:406–413. doi: 10.1093/nar/gkl1057. PubMed DOI PMC

Spiegel J., Adhikari S., Balasubramanian S. The Structure and Function of DNA G-Quadruplexes. Trends Chem. 2020;2:123–136. doi: 10.1016/j.trechm.2019.07.002. PubMed DOI PMC

Varshney D., Spiegel J., Zyner K., Tannahill D., Balasubramanian S. The regulation and functions of DNA and RNA G-quadruplexes. Nat. Rev. Mol. Cell. Biol. 2020;21:459–474. doi: 10.1038/s41580-020-0236-x. PubMed DOI PMC

Dai J., Carver M., Yang D. Polymorphism of human telomeric quadruplex structures. Biochimie. 2008;90:1172–1183. doi: 10.1016/j.biochi.2008.02.026. PubMed DOI PMC

Vorlíčková M., Kejnovská I., Sagi J., Renčiuk D., Bednářová K., Motlová J., Kypr J. Circular dichroism and guanine quadruplexes. Methods. 2012;57:64–75. doi: 10.1016/j.ymeth.2012.03.011. PubMed DOI

Lim K.W., Ng V.C.M., Martín-Pintado N., Heddi B., Phan A.T. Structure of the human telomere in Na+ solution: An antiparallel (2+2) G-quadruplex scaffold reveals additional diversity. Nucleic Acids Res. 2013;41:10556–10562. doi: 10.1093/nar/gkt771. PubMed DOI PMC

You H., Zeng X., Xu Y., Lim C.J., Efremov A.K., Phan A.T., Yan J. Dynamics and stability of polymorphic human telomeric G-quadruplex under tension. Nucleic Acids Res. 2014;42:8789–8795. doi: 10.1093/nar/gku581. PubMed DOI PMC

Dolinnaya N.G., Ogloblina A.M., Yakubovskaya M.G. Structure, Properties, and Biological Relevance of the DNA and RNA G-Quadruplexes: Overview 50 Years after Their Discovery. Biochemistry. 2016;81:1602–1649. doi: 10.1134/S0006297916130034. PubMed DOI PMC

Winnerdy F.R., Bakalar B., Maity A., Vandana J.J., Mechulam Y., Schmitt E., Phan A.T. NMR solution and X-ray crystal structures of a DNA molecule containing both right- and left-handed parallel-stranded G-quadruplexes. Nucleic Acids Res. 2019;47:8272–8281. doi: 10.1093/nar/gkz349. PubMed DOI PMC

Ma Y., Iida K., Nagasawa K. Topologies of G-quadruplex: Biological functions and regulation by ligands. Biochem. Biophys. Res. Commun. 2020;531:3–17. doi: 10.1016/j.bbrc.2019.12.103. PubMed DOI

Luchinat E., Barbieri L., Campbell T.F., Banci L. Real-Time Quantitative In-Cell NMR: Ligand Binding and Protein Oxidation Monitored in Human Cells Using Multivariate Curve Resolution. Anal. Chem. 2020;92:9997–10006. doi: 10.1021/acs.analchem.0c01677. PubMed DOI PMC

Luchinat E., Barbieri L., Cremonini M., Nocentini A., Supuran C.T., Banci L. Drug Screening in Human Cells by NMR Spectroscopy Allows the Early Assessment of Drug Potency. Angew. Chem. Int. Ed. 2020;59:6535–6539. doi: 10.1002/anie.201913436. PubMed DOI PMC

Luchinat E., Barbieri L., Cremonini M., Nocentini A., Supuran C.T., Banci L. Intracellular Binding/Unbinding Kinetics of Approved Drugs to Carbonic Anhydrase II Observed by in-Cell NMR. ACS Chem. Biol. 2020;15:2792–2800. doi: 10.1021/acschembio.0c00590. PubMed DOI PMC

Barbieri L., Luchinat E. Monitoring Protein-Ligand Interactions in Human Cells by Real-Time Quantitative In-Cell NMR using a High Cell Density Bioreactor. J. Vis. Exp. 2021 doi: 10.3791/62323. PubMed DOI

Ishizuka T., Bao H.-L., Xu Y. 19F NMR Spectroscopy for the Analysis of DNA G-Quadruplex Structures Using 19F-Labeled Nucleobase. Methods Mol. Biol. 2019;2035:407–433. doi: 10.1007/978-1-4939-9666-7_26. PubMed DOI

Nambiar M., Goldsmith G., Moorthy B.T., Lieber M.R., Joshi M.V., Choudhary B., Hosur R.V., Raghavan S.C. Formation of a G-quadruplex at the BCL2 major breakpoint region of the t(14;18) translocation in follicular lymphoma. Nucleic Acids Res. 2011;39:936–948. doi: 10.1093/nar/gkq824. PubMed DOI PMC

Read M., Harrison R.J., Romagnoli B., Tanious F.A., Gowan S.H., Reszka A.P., Wilson W.D., Kelland L.R., Neidle S. Structure-based design of selective and potent G quadruplex-mediated telomerase inhibitors. Proc. Natl. Acad. Sci. USA. 2001;98:4844–4849. doi: 10.1073/pnas.081560598. PubMed DOI PMC

Harrison R.J., Cuesta J., Chessari G., Read M.A., Basra S.K., Reszka A.P., Morrell J., Gowan S.M., Incles C.M., Tanious F.A., et al. Trisubstituted acridine derivatives as potent and selective telomerase inhibitors. J. Med. Chem. 2003;46:4463–4476. doi: 10.1021/jm0308693. PubMed DOI

Gowan S.M., Harrison J.R., Patterson L., Valenti M., Read M.A., Neidle S., Kelland L.R. A G-quadruplex-interactive potent small-molecule inhibitor of telomerase exhibiting in vitro and in vivo antitumor activity. Mol. Pharmacol. 2002;61:1154–1162. doi: 10.1124/mol.61.5.1154. PubMed DOI

Zhou G., Liu X., Li Y., Xu S., Ma C., Wu X., Cheng Y., Yu Z., Zhao G., Chen Y. Telomere targeting with a novel G-quadruplex-interactive ligand BRACO-19 induces T-loop disassembly and telomerase displacement in human glioblastoma cells. Oncotarget. 2016;7:14925–14939. doi: 10.18632/oncotarget.7483. PubMed DOI PMC

Liu C., Zhou B., Geng Y., Tam D.Y., Feng R., Miao H., Xu N., Shi X., You Y., Hong Y., et al. A chair-type G-quadruplex structure formed by a human telomeric variant DNA in K+ solution. Chem. Sci. 2018;10:218–226. doi: 10.1039/C8SC03813A. PubMed DOI PMC

Kerkour A., Marquevielle J., Ivashchenko S., Yatsunyk L.A., Mergny J.-L., Salgado G.F. High-resolution three-dimensional NMR structure of the KRAS proto-oncogene promoter reveals key features of a G-quadruplex involved in transcriptional regulation. J. Biol. Chem. 2017;292:8082–8091. doi: 10.1074/jbc.M117.781906. PubMed DOI PMC

Nicoludis J.M., Barrett S.P., Mergny J.-L., Yatsunyk L.A. Interaction of human telomeric DNA with N-methyl mesoporphyrin IX. Nucleic Acids Res. 2012;40:5432–5447. doi: 10.1093/nar/gks152. PubMed DOI PMC

Tippana R., Xiao W., Myong S. G-quadruplex conformation and dynamics are determined by loop length and sequence. Nucleic Acids Res. 2014;42:8106–8114. doi: 10.1093/nar/gku464. PubMed DOI PMC

Sasaki S., Ma Y., Ishizuka T., Bao H.-L., Hirokawa T., Xu Y., Tera M., Nagasawa K. Linear consecutive hexaoxazoles as G4 ligands inducing chair-type anti-parallel topology of a telomeric G-quadruplex. RSC Adv. 2020;10:43319–43323. doi: 10.1039/D0RA09413G. PubMed DOI PMC

De Cian A., Delemos E., Mergny J.-L., Teulade-Fichou M.-P., Monchaud D. Highly efficient G-quadruplex recognition by bisquinolinium compounds. J. Am. Chem. Soc. 2007;129:1856–1857. doi: 10.1021/ja067352b. PubMed DOI

De Cian A., Cristofari G., Reichenbach P., De Lemos E., Monchaud D., Teulade-Fichou M.-P., Shin-Ya K., Lacroix L., Lingner J., Mergny J.-L. Reevaluation of telomerase inhibition by quadruplex ligands and their mechanisms of action. Proc. Natl. Acad. Sci. USA. 2007;104:17347–17352. doi: 10.1073/pnas.0707365104. PubMed DOI PMC

De Cian A., Grellier P., Mouray E., Depoix D., Bertrand H., Monchaud D., Teulade-Fichou M.-P., Mergny J.-L., Alberti P. Plasmodium telomeric sequences: Structure, stability and quadruplex targeting by small compounds. Chembiochem. 2008;9:2730–2739. doi: 10.1002/cbic.200800330. PubMed DOI

Piazza A., Boulé J.-B., Lopes J., Mingo K., Largy E., Teulade-Fichou M.-P., Nicolas A. Genetic instability triggered by G-quadruplex interacting Phen-DC compounds in Saccharomyces cerevisiae. Nucleic Acids Res. 2010;38:4337–4348. doi: 10.1093/nar/gkq136. PubMed DOI PMC

Chung W.J., Heddi B., Hamon F., Teulade-Fichou M.-P., Phan A.T. Solution Structure of a G-quadruplex Bound to the Bisquinolinium Compound Phen-DC3. Angew. Chem. Int. Ed. 2014;53:999–1002. doi: 10.1002/anie.201308063. PubMed DOI

Sharaf N.G., Barnes C.O., Charlton L.M., Young G.B., Pielak G.J. A bioreactor for in-cell protein NMR. J. Magn. Reson. 2010;202:140–146. doi: 10.1016/j.jmr.2009.10.008. PubMed DOI PMC

Kubo S., Nishida N., Udagawa Y., Takarada O., Ogino S., Shimada I. A Gel-Encapsulated Bioreactor System for NMR Studies of Protein–Protein Interactions in Living Mammalian Cells. Angew. Chem. Int. Ed. 2013;52:1208–1211. doi: 10.1002/anie.201207243. PubMed DOI

Inomata K., Kamoshida H., Ikari M., Ito Y., Kigawa T. Impact of cellular health conditions on the protein folding state in mammalian cells. Chem. Commun. 2017;53:11245–11248. doi: 10.1039/C7CC06004A. PubMed DOI

Burz D.S., Breindel L., Shekhtman A. Improved sensitivity and resolution of in-cell NMR spectra. Methods Enzymol. 2019;621:305–328. doi: 10.1016/bs.mie.2019.02.029. PubMed DOI PMC

Sklenář V., Bax A. Spin-echo water suppression for the generation of pure-phase two-dimensional NMR spectra. J. Magn. Reson. (1969) 1987;74:469–479. doi: 10.1016/0022-2364(87)90269-1. DOI

Polymorphic and Higher-Order G-Quadruplexes as Possible Transcription Regulators: Novel Perspectives for Future Anticancer Therapeutic Applications