Sugar Puckering Drives G‐Quadruplex Refolding: Implications for V‐Shaped Loops

A DNA G‐quadruplex adopting a (3+1) hybrid structure was modified in two adjacent syn positions of the antiparallel strand with anti‐favoring 2′‐deoxy‐2′‐fluoro‐riboguanosine ((F)rG) analogues. The two substitutions promoted a structural rearrangement to a topology with the 5′‐terminal G residue located in the central tetrad and the two modified residues linked by a V‐shaped zero‐nucleotide loop. Strikingly, whereas a sugar pucker in the preferred north domain is found for both modified nucleotides, the (F)rG analogue preceding the V‐loop is forced to adopt the unfavored syn conformation in the new quadruplex fold. Apparently, a preferred C3′‐endo sugar pucker within the V‐loop architecture outweighs the propensity of the (F)rG analogue to adopt an anti glycosidic conformation. Refolding into a V‐loop topology is likewise observed for a sequence modified at corresponding positions with two riboguanosine substitutions. In contrast, 2′‐F‐arabinoguanosine analogues with their favored south‐east sugar conformation do not support formation of the V‐loop topology. Examination of known G‐quadruplexes with a V‐shaped loop highlights the critical role of the sugar conformation for this distinct structural motif.