Biophysical characterization of the human telomeric (TTAGGG)4 repeat in a potassium solution

Quadruplex structures arise from four coplanar G bases arranged in a Hoogsteen base pairing motif to create a central pore that can coordinate cations. The termini of eukaryotic chromosomes contain structures, known as telomeres, which are capable of forming quadruplex structures. Quadruplexes have been implicated in a variety of disease states, including cancer. The literature seems to agree that the human telomeric repeat containing four stretches of three guanines displays conformational states that are different in the presence of Na+ and K+ and an unknown number of species involved in the quadruplex to single strand transition. Using circular dichroism spectroscopy, differential scanning calorimetry, and singular-value decomposition, the number of species present in the dissociation process is assessed. The results indicate that three species exist in equilibria during the melting process. We present a model for the heat-induced denaturation from the folded to the unfolded state, whereby the hybrid parallel-antiparallel quadruplex undergoes a transition to an unknown intramolecular intermediate followed by a transition to a single strand.