Structural polymorphism and thermal stability of telomere DNAs (T2G4)n and (T4G4)n.

: The ends of eukaryotic chromosomes, termed telomeres, contain a single-stranded 3' overhang composed of tandemly repeated guanine-rich sequences, such as (T2G4)n and (T4G4)n, along one strand. The sequences can form defined folded tetraplex structures. Here we have systematically examined structural polymorphism and thermal stability of a series of oligonucleotide sequences, Tet n: (T2G4)n and Oxy n: (T4G4)n (n = 1, 2, 3, 4), using circular dichroism (CD) spectroscopy. The CD spectra of Tet 1 and Oxy 1 are consistent with those observed in tetraplex structures consisting of four parallel strands (type I conformation); whereas the spectra of Tet 2, Oxy 2, Oxy 3, and Oxy 4 correspond with those observed for tetraplex conformations where the strands are antiparallel (type II conformation). The spectra of Tet 3 and Tet 4 suggests that Tet 3 and Tet 4 can adopt both type I and type II conformations and they are structurally polymorphic. The melting temperatures of Tet n and Oxy n (n = 1, 2, 3, 4) measured by CD melting are consistent with the previously reported values obtained from differential scanning calorimetry (DSC). Furthermore, the CD melting of Tet 4 suggests that the type II conformation of Tet 4 changes into type I conformation between 55 degrees C and 70 degrees C.