The influence of urea on G-quadruplex and i-motif structures in complementary DNA sequences

In the present study, the methods of circular dichroism and UV/Vis spectrophotometry were used to study the influence of urea on the structural transitions i-motif $\leftrightarrows$ unfolded single strand in cytosine-rich d[3$^{\prime}$-(CCCAAT)$_{3}$CCC-5)$^{\prime}$] region of telomeric DNA (Tel22C) and G-quadruplex $\leftrightarrows$ unfolded single strand in complementary guanine-rich strand d[5$^{\prime}$-A(GGGTTA)$_{3}$GGG-3$^{\prime}$] (Tel22G) at pH 5.5 and 400 mM Na$^+$. Under these conditions, Tel22C and Tel22G were found to form stable i-motif and G-quadruplex structures. It has been shown that urea (0–8 M) destabilizes the i-motif and G-quadruplex structures, but unlike thermal denaturation, it does not destroy the structures completely. The melting processes of G-quadruplex and i-motif are separated in the temperature scale (at any concentration of urea, the melting of the G-quadruplex starts at temperatures where the melting of the i-motif has already been completed.