Thermal Stability Changes in Telomeric G-Quadruplex Structures Due to N 6 -Methyladenine Modification.

N 6 -methyladenine modification (m 6 dA) has recently been identified in eukaryote genomic DNA. The methylation destabilizes the duplex structure when the adenine forms a Watson-Crick base pair, whereas the methylation on a terminal unpaired adenine stabilizes the duplex structure by increasing the stacking interaction. In this study, the effects of m 6 dA modification on the thermal stability of four distinct telomeric G-quadruplex (G4) structures were investigated. The m 6 dA-modified telomeric oligonucleotide d[AGGG(TTAGGG) 3 ] that forms a basket-type G4 in Na + , d[(TTAGGG) 4 TT] that forms a hybrid-type G4 in K + (Form-2), d[AAAGGG(TTAGGG) 3 AA] that forms a hybrid-type G4 in K + (Form-1), and d[GGG(TTAGGG) 3 T] that forms a basket-type G4 with two G-tetrads in K + (Form-3) were analyzed. Circular dichroism melting analysis demonstrated that (1) A7- and A19-methylation destabilized the basket-type G4 structure that formed in Na + , whereas A13-methylation stabilized the structure; (2) A15-methylation stabilized the Form-2 G4 structure; (3) A15- and A21-methylations stabilized the Form-1 G4 structure; and (4) A12-methylation stabilized the Form-3 G4 structure. These results suggest that m 6 dA modifications may affect the thermal stability of human telomeric G4 structures in regulating the biological functions.