Triple helix-forming oligonucleotides which make imperfect Watson-Crick duplexes that compete with the creation of the triplex

Because of the repetitive nature of the sequence, when titrating a G,A-rich, triple helix-forming oligonucleotide (TFO) with increasing concentrations of target duplex in order to obtain the dissociation constant of the complex, a duplex is sometimes first generated at intermediate concentrations of the target. This duplex is based on an imperfect Watson-Crick pairing of the TFO to the pyrimidine-rich strand of the target. An explanation is proposed for this duplex being obtained only in a certain domain of the titration range, before the triple helix becomes predominant.