NMR Scalar Couplings across Intermolecular Hydrogen Bonds between Zinc-Finger Histidine Side Chains and DNA Phosphate Groups.

NMR scalar couplings across hydrogen bonds represent direct evidence for the partial covalent nature of hydrogen bonds and provide structural and dynamic information on hydrogen bonding. In this article, we report heteronuclear N-P and H-P scalar couplings across the intermolecular hydrogen bonds between protein histidine (His) imidazole and DNA phosphate groups. These hydrogen-bond scalar couplings were observed for the Egr-1 zinc-finger-DNA complex. Although His side-chain NH protons are typically undetectable in heteronuclear H-N correlation spectra due to rapid hydrogen exchange, this complex exhibited two His side-chain NH signals around H 14.3 ppm and N 178 ppm at 35 °C. Through various heteronuclear multidimensional NMR experiments, these signals were assigned to two zinc-coordinating His side chains in contact with DNA phosphate groups. The data show that the N atoms of these His side chains are protonated and exhibit the H-N cross-peaks. Using heteronuclear H, N, and P NMR experiments, we observed the hydrogen-bond scalar couplings between the His N/H and DNA phosphate P nuclei. These results demonstrate the direct involvement of the zinc-coordinating His side chains in the recognition of DNA by the CysHis-class zinc fingers in solution.