Folding dynamics and conformational heterogeneity of human telomeric G-quadruplex structures in Na(+) solutions by single molecule FRET microscopy

G-quadruplex structures can occur throughout the genome, including at telomeres. They are involved in cellular regulation and are potential drug targets. Human telomeric G-quadruplex structures can fold into a number of different conformations and show large conformational diversity. To elucidate the different G-quadruplex conformations and their dynamics, we investigated telomeric G-quadruplex folding using single molecule FRET microscopy in conditions where it was previously believed to yield low structural heterogeneity. We observed four FRET states in Na(+) buffers: an unfolded state and three G-quadruplex related states that can interconvert between each other. Several of these states were almost equally populated at low to medium salt concentrations. These observations appear surprising as previous studies reported primarily one G-quadruplex conformation in Na(+) buffers. Our results permit, through the analysis of the dynamics of the different observed states, the identification of a more stable G-quadruplex conformation and two transient G-quadruplex states. Importantly these results offer a unique view into G-quadruplex topological heterogeneity and conformational dynamics.