Consequences of methylation on the amino group of adenine. A proton two-dimensional NMR study of d(GGATATCC) and d(GGm6ATATCC).
A two-dimensional 500-MHz 1H-NMR study of two oligonucleotides, d(GGATATCC) and d(GGm6ATATCC), is presented in which we have investigated the effects of adenine methylation. The two-dimensional nuclear Overhauser spectra (NOESY) show that both oligonucleotides adopt a normal right-handed B-type helix and one-dimensional nuclear Overhauser enhancement (NOE) studies demonstrate that any difference in conformation must be small. However methylation drastically slows down the helix in equilibrium coil exchange which becomes slow on a proton NMR time scale. While d(GGATATCC) fits a two-site exchange model, d(GGm6ATATCC) does not and we invoke the presence of a third species which may be an intermediate in helix formation. NMR and ultraviolet spectroscopy show that methylation destabilizes the helix, measured by the melting temperature and enthalpy of dissociation.
D000225:Adenine / Q000031:analogs & derivatives*; D004247:DNA; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008745:Methylation; D009690:Nucleic Acid Conformation; D009691:Nucleic Acid Denaturation; D009838:Oligodeoxyribonucleotides; D013329:Structure-Activity Relationship
D000225:Adenine; D004247:DNA; D009838:Oligodeoxyribonucleotides; C000730567:6-methyladenine