Intercalated Motif (i-Motif) DNA

An intercalated motif (i-motif) is a four-stranded (tetraplex) non-B DNA secondary structure that forms within cytosine-rich nucleic acid sequences through the mutual, antiparallel intercalation of two parallel-stranded duplexes held together by hemiprotonated cytosine–protonated cytosine (C:C⁺) base pairs. First solved at atomic resolution via solution nuclear magnetic resonance (NMR) spectroscopy by Kalle Gehring, Jean-Louis Leroy, and Maurice Guéron in 1993, the i-motif occurs on the C-rich strand directly opposite guanine-rich tracts capable of folding into a G-quadruplex (G4). Whereas canonical Watson-Crick B-DNA relies on antiparallel sugar-phosphate backbones paired across the helix axis via G:C and A:T hydrogen bonding, an intramolecular i-motif folds a single C-rich strand containing four contiguous cytosine tracts—consensus motif (C_3–5N_1–7)₃C_3–5—so that two parallel C:C⁺ duplexes interdigitate their base pairs in an alternating antiparallel orientation.

Because forming each C:C⁺ base pair requires protonating the ring nitrogen N3 of one cytosine moiety (intrinsic monomeric p*K*_a ≈ 4.2 to 4.5 in dilute aqueous solution), i-motifs were initially regarded as in vitro structural curiosities restricted to mildly acidic pH (pH 4.5–6.0). However, subsequent biophysical and cellular investigations—culminating in the direct visualization of endogenous i-motif foci inside living human cell nuclei using the structure-specific single-chain antibody fragment iMab by Mahdi Zeraati et al. (2018)—demonstrated that negative torsional supercoiling during transcription, molecular crowding inside the nucleoplasm, epigenetic 5-methylcytosine dna-methylation, and longer cytosine tracts shift the apparent folding p*K*_a upward to physiological neutrality (pH 6.8–7.4). Within the human genome, i-motif-forming sequences cluster inside key oncogene promoters (MYC, BCL2, KRAS, VEGF), insulin (INS) regulatory elements, centromeric satellite repeats, and C-rich telomeres.

Hemiprotonated C:C+ Hydrogen Bonding and p*K*a Modulation

At the core of every i-motif is the symmetric hemiprotonated C:C⁺ base pair, in which one neutral cytosine and one N3-protonated cytosine (C⁺) form three planar hydrogen bonds: a central shared proton between the two ring nitrogens (N3–H⁺···N3) flanked by two outer exocyclic amino-to-carbonyl hydrogen bonds (N4–H···O=C2 and C2=O···H–N4). In this trans Watson-Crick/Watson-Crick geometry, the glycosidic bonds of the two paired cytosines are oriented in a parallel (rather than antiparallel) direction, and the N3–H⁺···N3 hydrogen bond exhibits a double-well or low-barrier hydrogen-bond potential with rapid proton hopping (~10⁸ s⁻¹) between the two equivalent N3 atoms.

Each C:C⁺ base pair contributes an enthalpy of stabilization Δ*H*° ≈ −24 to −30 kJ/mol, exceeding the stability of canonical neutral Watson-Crick base pairs. Because the shared proton bridges two N3 sites simultaneously, the cooperative folding of n stacked C:C⁺ pairs elevates the effective transition pH (pH_T, where 50% of strands are folded at 25 °C) well above the monomeric p*K*_a of 4.2. Three physiological parameters further drive i-motif folding at neutral pH (pH 7.0–7.4):

1. Negative Torsional Supercoiling: Behind advancing RNA Polymerase II complexes, underwound duplex DNA experiences negative superhelical density (σ ≈ −0.05 to −0.07), lowering the free-energy barrier for duplex melting and stabilizing folded i-motifs and z-dna at pH 7.2.
2. Macromolecular Crowding and Dehydration: High concentrations of nuclear proteins and chromatin (300–400 g/L, modeled in vitro with 20–40% polyethylene glycol) decrease water activity and raise the dielectric p*K*_a of N3 protonation from ~4.5 to >7.0.
3. Epigenetic Cytosine Modifications: Replacing cytosine with 5-methylcytosine (5mC) via dna-methylation increases base hydrophobicity and electron density at N3, raising the thermal melting temperature (T_m) and pH_T of the i-motif, whereas oxidative 5-hydroxymethylcytosine (5hmC) introduces steric clash in the narrow groove and destabilizes the tetraplex.

Antiparallel Duplex Intercalation and 3′E vs. 5′E Topologies

An i-motif does not stack contiguous C:C⁺ pairs from the same parallel duplex directly on top of one another; instead, two parallel-stranded C:C⁺ duplexes orient antiparallel to one another and intercalate step-by-step like the teeth of a zipper. As a result, adjacent stacked C:C⁺ base pairs belong to opposite duplexes, with an axial rise of only 3.1 Å (compared to 3.4 Å in B-DNA) and a right-handed helical twist of ~12° to 16° between contiguous cytosines of the same strand. Because the two intercalated duplexes cross at an asymmetric angle, the i-motif cross-section displays two wide grooves (~15.6 Å across) and two narrow grooves (~6.2 Å across, where sugar-phosphate backbones approach within van der Waals contact and are stabilized by C1′–H···O4′ sugar-sugar interactions).

Depending on which terminal C:C⁺ base pair sits at the outermost face of the intercalated core, intramolecular i-motifs adopt one of two distinct topological isomers:

• 3′-End (3′E) Topology: The outermost intercalated C:C⁺ base pair includes the 3′-most cytosine of the folding tract (observed in the human telomeric C-strand repeat 5′-(CCCTAA)₃CCCT-3′).
• 5′-End (5′E) Topology: The outermost intercalated C:C⁺ base pair includes the 5′-most cytosine of the folding tract, differing by a 1-nucleotide shift in core register.

Cell-Cycle Dynamics and Mutual Exclusivity with G-Quadruplexes

Immunofluorescence imaging with the iMab antibody across synchronized human MCF7, U2OS, and HeLa cells reveals that nuclear i-motif formation is regulated across the cell cycle in opposition to G-quadruplex dynamics. Endogenous i-motif puncta peak at the late G1 / G1–S boundary when active transcription of growth-regulatory genes peaks, and diminish during S phase as dna-replication forks unwind secondary structures. Conversely, G-quadruplex staining (via the BG4 antibody) reaches its maximum during S phase when replication forks expose single-stranded G-rich templates.

Single-molecule laser-tweezers and FRET experiments on double-stranded promoter fragments demonstrate that at a single locus, the G-rich strand (forming a G4) and the complementary C-rich strand (forming an i-motif) are often sterically mutually exclusive when zero-nucleotide flanking offsets exist, because the two folded structures collide across the duplex junction; however, when separated by ≥4–5 bp or assisted by single-stranded binding proteins, both structures can coexist as an extruded cruciform-like G4/i-motif pair.

Oncogene Promoter Switching (BCL2, MYC, KRAS) and Telomere Capping

At the P1 promoter of the anti-apoptotic proto-oncogene BCL2, a 39-nucleotide C-rich element (5′-C₃GC₄TTC₂TC₄GCGC₄TC₄-3′) adopts a dynamic equilibrium between a rigid folded i-motif and a flexible hairpin recognized by the heterogeneous nuclear ribonucleoprotein hnRNP LL. Binding of hnRNP LL via its RRM1 and RRM2 domains unfolds the BCL2 i-motif into a single-stranded lateral loop that recruits transcriptional machinery and upregulates BCL2 expression; conversely, small-molecule ligands (such as IMC-48) that trap the folded i-motif repress transcription, whereas hairpin-stabilizing steroids (IMC-76) shift the equilibrium in the opposite direction. Similar transcriptional switches mediated by hnRNP K and NM23-H2 govern i-motifs in the nuclease-hypersensitive element III₁ (NHE III₁) of MYC and the promoter of KRAS. At human telomeres, folding of the C-rich strand (5′-CCCTAA-3′)_n into an i-motif or the complementary G-strand into a G4 inhibits extension by telomerase, regulating telomere homeostasis.

How can i-motifs fold inside the cell nucleus if physiological pH is around 7.2 to 7.4?

Although isolated short C-rich oligonucleotides in dilute aqueous buffer require mildly acidic conditions (pH 5.0–6.0) to protonate cytosine N3, the intracellular nucleus is a crowded environment (~300–400 mg/mL macromolecules) subjected to negative superhelical torque generated by RNA Polymerase II. Molecular crowding lowers water activity and suppresses electrostatic repulsion, while longer tracts of ≥4–5 cytosines per run and epigenetic 5-methylcytosine modifications cooperatively raise the transition pH_T to 7.0–7.4.

Why do i-motifs and G-quadruplexes peak at different phases of the human cell cycle?

Live-cell immunostaining with the structure-specific antibodies iMab (for i-motifs) and BG4 (for G-quadruplexes) shows that i-motif formation peaks in late G1 phase, driven by transcription-induced negative supercoiling at active gene promoters and recognition by transcription factors such as hnRNP K and hnRNP LL. By contrast, G-quadruplexes peak during S phase, when replicative helicases unwind the double helix and transiently expose single-stranded G-rich lagging-strand templates during dna-replication.

How does epigenetic 5-methylcytosine (5mC) vs. 5-hydroxymethylcytosine (5hmC) affect i-motif stability?

Methylation of cytosine at the C5 carbon (5mC) by DNA methyltransferases donates electron density into the pyrimidine ring, increasing the basicity (p*K*_a) of the N3 nitrogen and strengthening hydrophobic base-stacking interactions, which stabilizes the i-motif and raises its thermal melting temperature (T_m) by 3–6 °C. Conversely, TET-mediated oxidation of 5mC to 5-hydroxymethylcytosine (5hmC) introduces a bulky polar hydroxyl group that disrupts hydration and destabilizes the intercalated core.

1. Gehring, K., Leroy, J.-L., & Guéron, M. (1993). "A tetrameric DNA structure with protonated cytosine-cytosine base pairs." Nature, 363(6429), 561–565. https://doi.org/10.1038/363561a0
2. Zeraati, M., Langley, D. B., Schofield, P., Moye, A. L., Rouet, R., Hughes, W. E., Bryan, T. M., Dinger, M. E., & Christ, D. (2018). "I-motif DNA structures are formed in the nuclei of human cells." Nature Chemistry, 10(6), 631–637. https://doi.org/10.1038/s41557-018-0046-3
3. Kang, H.-J., Kendrick, S., Hecht, S. M., & Hurley, L. H. (2014). "The transcriptional complex between the BCL2 i-motif and hnRNP LL is a molecular switch for control of gene expression that can be modulated by small molecules." Journal of the American Chemical Society, 136(11), 4172–4185. https://doi.org/10.1021/ja4109352
4. Abou Assi, H., Garavis, M., González, C., & Damha, M. J. (2018). "i-Motif DNA: structural features, stabilization, and biological significance." Nucleic Acids Research, 46(16), 8038–8056. https://doi.org/10.1093/nar/gky735