Tolerance of N²-heteroaryl modifications on guanine bases in a DNA G-quadruplex

To systematically determine the effect of N2-heteroaryl modification on the stability of G-quadruplex structures, six types of N2-heteroarylated deoxyguanosines were incorporated into oligonucleotides with intramolecular quadruplex-forming sequences obtained from the human telomere sequence. A UV melting experiment, electrophoretic mobility shift assay, and circular dichroism were performed to evaluate the influence of N2-heteroaryl modification. Among the N2-heteroaryl modifications used in this study, N2-(pyrimidin-2-yl) modification markedly destabilized the G-quadruplex structure. Interestingly, N2-heteroaryl modification in the middle guanine base of the fourth strand was well-tolerated and formed a G-quadruplex, suggesting that the fourth strand is the least important strand in sustaining the G-quadruplex structure.