LAUSR

Infrared spectroscopy detects the formation of G‐quadruplexes in guanine‐rich nucleic acid sequences through shifts in the guanine C=O stretch mode. Here, we use ultrafast 2D infrared (IR) spectroscopy and isotope substitution to show that these shifts arise from vibrational delocalization among stacked G‐quartets. This provides a direct measure of the sizes of locally ordered motifs in heterogeneous samples with substantial disordered regions. We find that parallel‐stranded, potassium‐bound DNA G‐quadruplexes are limited to five consecutive G‐quartets and 3–4 consecutive layers are preferred for longer polyguanine tracts. The resulting potassium‐dependent G‐quadruplex assembly landscape reflects the polyguanine tract lengths found in genomes, the ionic conditions prevalent in healthy mammalian cells, and the onset of structural disorder in disease states. Our study describes spectral markers that can be used to probe other G‐quadruplex structures and provides insight into the fundamental limits of their formation in biological and artificial systems.