G-Quadruplex

The fundamental structural unit is the G-tetrad, a planar square arrangement of four guanine bases. Each guanine acts as both a hydrogen bond donor and acceptor, forming eight Hoogsteen hydrogen bonds per tetrad. Two or more tetrads stack vertically via π-π interactions to create the quadruplex core, which is significantly more thermodynamically stable than canonical base pairs under physiological conditions.

G-quadruplex stability is strictly dependent on a central monovalent cation, typically K⁺ or Na⁺. The cation sits in the electronegative channel between stacked tetrads, coordinating with the O6 carbonyl groups of the guanines. Potassium ions are preferred due to their optimal ionic radius, which allows precise coordination between two tetrad planes and provides superior stabilization energy compared to sodium or lithium.

G-quadruplexes exhibit extreme topological diversity based on strand orientation and connectivity. They can be formed from one, two, or four separate RNA strands, classified as intramolecular or intermolecular. The four backbone strands can run in parallel, antiparallel, or hybrid orientations. The connecting loops—propeller, lateral, or diagonal—dictate the final topology and influence the width and depth of the surrounding grooves.

In the transcriptome, RNA G-quadruplexes are enriched in functionally critical regions. They are prevalent in 5' untranslated regions (UTRs) where they act as translation repressors by blocking ribosome scanning. They also appear in telomeric repeat-containing RNA (TERRA) and 3' UTRs, influencing mRNA localization and alternative polyadenylation. Their formation is dynamically regulated by RNA helicases in vivo.

G-quadruplex formation is experimentally validated using circular dichroism (CD) spectroscopy. Parallel topologies exhibit a characteristic positive peak at ~260 nm and a negative peak at ~240 nm, while antiparallel forms show a positive peak at ~295 nm. Nuclear magnetic resonance (NMR) spectroscopy resolves imino proton peaks in the 10-12 ppm region, confirming Hoogsteen hydrogen bonding distinct from Watson-Crick base pairs.

Standard secondary structure prediction tools like ViennaRNA and mfold fail to predict G-quadruplexes because their dynamic programming algorithms are restricted to canonical Watson-Crick and wobble base pairs. Specialized tools such as QGRS Mapper and G4RNA screener use sequence-based scoring of G-rich motifs, but they cannot model 3D topology or cation coordination. Full structural prediction requires physics-based molecular dynamics or knowledge-based fragment assembly.

The non-canonical hydrogen bonding pattern that stabilizes G-quadruplexes. Unlike standard Watson-Crick pairs, guanines interact via their Hoogsteen edge, forming four hydrogen bonds per tetrad. This edge involves the N7 and O6 atoms of one guanine pairing with the N1 and N2 amino group of the adjacent guanine. Eight hydrogen bonds per G-tetrad create a planar, highly stable macrocycle. The Leontis-Westhof classification formally annotates these interactions as trans Hoogsteen/Sugar-edge pairs, distinguishing them from Watson-Crick geometry.

The central channel of a G-quadruplex requires a monovalent cation for stabilization. The cation coordinates with the electronegative O6 carbonyl groups of the guanines, neutralizing the electrostatic repulsion of the central oxygen lone pairs. The ionic radius dictates selectivity: K⁺ (1.38 Å) fits optimally between tetrads, while Na⁺ (1.02 Å) sits within the plane. K⁺ confers greater thermodynamic stability, making physiological conditions (high intracellular K⁺) highly permissive for G4 formation. Divalent cations like Mg²⁺ are generally excluded due to size and charge density.

G-quadruplexes exhibit extreme structural diversity based on strand orientation and loop geometry:

• Parallel: All four guanine tracts oriented in the same direction; all glycosidic bonds in anti conformation
• Antiparallel: Alternating strand directions; alternating syn and anti glycosidic bonds
• Hybrid (3+1): Three strands parallel, one antiparallel Loop types—propeller (parallel), lateral (antiparallel), and diagonal—further diversify topology. This polymorphism makes G4 prediction exceptionally challenging for standard secondary structure algorithms.

Experimental methods to identify G4 formation in vitro and in cellulo:

• DMS footprinting: Dimethyl sulfate methylates N7 of guanine; G4-involved guanines are protected from modification
• SHAPE: 2'-hydroxyl acylation reports on nucleotide flexibility; G4 regions show constrained reactivity
• G4-specific antibodies: BG4 antibody binds G4 structures with high affinity for immunofluorescence and ChIP-seq
• Ligand-induced stabilization: Small molecules like pyridostatin and PDS stabilize G4s, enabling functional studies These reactivity profiles serve as experimental constraints for prediction algorithms.

Computational approaches to identify putative G4-forming sequences:

• Quadparser: Regex-based identification of the motif G₃₋₅N₁₋₇G₃₋₅N₁₋₇G₃₋₅N₁₋₇G₃₋₅
• G4Hunter: Scoring algorithm incorporating G-richness and G-skew; returns a continuous score rather than binary classification
• pqsfinder: R/Bioconductor package for exhaustive G4 motif search with user-defined loop and bulge parameters
• G4RNA screener: Machine learning classifier trained on experimental G4 data using sequence and thermodynamic features These tools identify ~370,000 putative G4 sites in the human genome, but false positive rates remain high.

RNA G-quadruplexes regulate post-transcriptional gene expression:

• Translation repression: G4s in 5' UTRs block ribosome scanning; the DEAD-box helicase eIF4A resolves these structures
• mRNA localization: G4s in 3' UTRs serve as trafficking signals for neuronal mRNAs
• Alternative polyadenylation: G4s near poly(A) signals influence isoform selection
• Telomere biology: TERRA (telomeric repeat-containing RNA) forms G4s that regulate telomerase activity
• Viral genomes: G4s in HIV-1 and SARS-CoV-2 RNA genomes modulate replication