Guanine-rich sequences in nucleic acids can fold into $G$ quadruplexes, in which four guanines on a single strand combine to form $G$-tetrad planes stabilized by metallic ions. Sequence motifs which are predicted to form a $G$ quadruplex are found throughout the genome and are believed to regulate a variety of biological processes. Detailed knowledge of the kinetics of $G$-quadruplex folding and unfolding would provide critical insight into these processes. To probe its structural stability, we used optical tweezers to disrupt single molecules of a single-stranded DNA $G4$ quadruplex. Dynamic force spectroscopy was employed, in which the distribution of rupture forces was measured for different loading rates and used to infer the nature of the transition state barrier for unfolding of the structure. The distance and height of the energy barriers were extracted for two observed conformations. The energy barrier was found to be close to the folded conformation, resulting in a high disruption force despite the relatively low energy barrier height.

• Received 15 February 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.058101