Distance measurements between paramagnetic ligands bound to parallel stranded guanine quadruplexes

Aside from a double helix, deoxyribonucleic acid (DNA) folds into non-canonical structures, one of which is the guanine quadruplex. Cationic porphyrins bind guanine quadruplexes, but the effects of ligand binding on the structure of guanine quadruplexes with more than four contiguous guanine quartets remains to be fully elucidated. Double electron-electron resonance (DEER) spectroscopy conducted at 9.5 GHz (X-band) using broadband, shaped inversion pulses was used to measure the distances between cationic copper porphyrins bound to model parallel-stranded guanine quadruplexes with increasing numbers of guanine quartets. A single Gaussian component was found to best model the time domain datasets, characteristic of a 2 : 1 binding stoichiometry between the porphyrins and each quadruplex. The measured Cu(2+)-Cu(2+) distances were found to be linearly proportional with the number of guanines. Rather unexpectedly, the ligand end-stacking distance was found to monotonically decreases the overall quadruplex length was extended, suggesting a conformational change in the quadruplex secondary structure dependent upon the number of successive guanine quartets.