Structural polymorphism of the four-repeat Oxytricha nova telomeric DNA sequences

G-quadruplexes are four-stranded nucleic acid complexes that exhibit a great deal of polymorphism. Recently a group described the polymorphism exhibited by the four-repeat of the Oxytricha nova telomeric sequences (Lee, J.Y., Yoon, J., Kihm, H.W., Kim, D.S., Biochemistry 2008, 47, 3389-3396). In this study we evaluated the effects of G-tract and loop lengths on this behaviour using circular dichroism (CD) and gel electrophoresis. The largest changes were detected for oligonucleotides with different numbers of consecutive G residues. Furthermore, decreasing the number of residues between the G runs, the loops, from four to three only results in minor alteration in the polymorphism. However, the shortening of the G-tract from four to three guanine residues led to characteristically anti-parallel G-quadruplex CD spectra. Finally, we show that adenine bases in the loop sequences are less likely to form G-quadruplexes in the presence of Na+ cations than those comprised of thymine residues. The results presented here are an addition to the modest information available for predicting the type of G-quadruplex to be formed from G-rich sequences in aqueous solutions containing sodium or potassium ions.