Figure 1. Single nucleotide 2′‐deoxyguanosine unit with phosphodiester functionalities at the 5′ and 3′ ends. The conformation of each DNA nucleotide unit is determined by the values of six backbone torsion angles and the glycosidic torsion angle defining the relative disposition of the nucleobase and the deoxyribose sugar. Hydrogen atoms are annotated.

Figure 2. Pentofuranosyl moiety of nucleosides and nucleotides preferentially adopts two distinct puckered North (around C3′‐endo–C2′‐exo) and South (around C3′‐exo–C2′‐endo) conformations. The two‐state N ↔ S pseudorotational equilibrium is controlled by the competing stereoelectronic effects including anomeric and gauche effects. The O4′–C1′–N9/1 anomeric effect drives the N ↔ S pseudorotational equilibrium towards N, where the nucleobase is in the pseudoaxial orientation. The gauche effect refers to the energetic preference for the gauche rather than trans orientation of the two vicinal C–O bonds. In DNA, the gauche effect of the [O4′–C4′–C3′–O3′] fragment drives the N ↔ S equilibrium towards S.

Figure 3. Imino regions of NMR spectra of canonical Watson–Crick double helical structures in relation to multistranded i‐motifs and G‐quadruplexes. A simple one‐dimensional spectrum allows for assessment of the quality of the sample as well as a preliminary estimate of the expected topology of the adopted structure.

Figure 4. Structure and pairing scheme of A·T and G·C base pairs in Watson–Crick geometry.

Figure 5. Schematic presentation of a trinucleotide dGGG DNA sequence. Aromatic and sugar protons are related with a set of NOE interactions, which can be intramolecular (shown as full lines) or intermolecular (shown as dashed lines). H8 of a residue n exhibits NOE connectivities with H1′ _n and H1′ _n−1 , while H1′ of the same residue exhibits connectivities with H8 _n and H8 _n+1 . In this way, a sequential walk H8 _n−1 –H1′ _n−1 –H8 _n–H1′ _n–H8 _n+1 –H1′ _n+1 shown in red is made possible. Alternative sequential walks can be made by utilizing NOE contacts between H8 and H2′ (shown in green) or between H8 and H3′ (shown in blue).

Figure 6. Dinucleotide G–G step. Typical intranucleotide and sequential NOE connectivities involving non‐ and exchangeable protons in double‐helical structures are at the heart of resonance assignments.

Figure 7. Expanded NOESY (250 ms mixing time) spectrum. Correlations between the nucleobase protons (δ7.2–8.2 ppm) and the anomeric, H1′, sugar protons (δ5.5–6.5 ppm) of the 12δmer DNA oligonucleotide indicate a sequential walk.

Figure 8. Expanded TOCSY (80 ms mixing time) spectrum. Correlations between the H1′ protons (δ5.5–6.8 ppm) and the rest of the sugar protons (δ2.0–5.8 ppm) of the 19mer DNA oligonucleotide enable (partial) assignments of spin systems of individual deoxyribose rings.

Figure 9. Expanded region of DQF‐COSY spectrum. Clear correlations between the H2′, H2″ protons (δ1.7–3.1 ppm) and the H1′ sugar protons (δ5.4–6.6 ppm) of the 11mer DNA oligonucleotide adopting a dimeric structure indicate a preference for S‐type sugar conformations. Some cross‐peaks can be analyzed to the level of extracting (estimates of) _H1′H2′ coupling constants.