Consequences of methylation on the amino group of adenine: A proton two‐dimensional NMR study of d(GGATATCC) and d(GGm6ATATCC)

Abstract

A two‐dimensional 500‐MHz 1H‐NMR study of two oligonucleotides, d(GGATATCC) and d(GGm6‐ATATCC), is presented in which we have investigated the effects of adenine methylation. The two‐dimentional 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 ↔ 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. Copyright © 1985, Wiley Blackwell. All rights reserved