C5′ Methylene Proton Signal Assignment of DNA/RNA Oligomers Labeled with C5′‐Monodeuterated Nucleosides by 1H–31P HSQC Spectroscopy

A new strategy for assigning the methylene protons attached to the C5′ of nucleic acids is described for a DNA dodecamer duplex, d(CGCGAATTCGCG)2. It employs 2D 1H–31P HSQC spectroscopy for DNA composed of deoxyribonucleotides, having one deuteron at their C5′ positions. Due to the lack of scalar as well as dipolar coupling between the C5′ methylene proton pair, the cross peaks for the residual H5′/H5″ protons and the 31P nuclei (at the 5′‐terminal side) appear as much narrower signals, as compared to those of the intact proton pairs in the unlabeled dodecamer. The overall resolution and sensitivity of the 1H–31P HSQC cross peaks for the residual signals of the deuterated C5′‐methylenes have thus been drastically improved over those for the unlabeled oligomer. All of the H5′/H5″ signals of the dodecamer are clearly observed, except for the unphosphorylated 5′‐terminus, and each has been assigned. Most of the diastereotopic proton pairs at C5′ showed different relative peak intensities, which may be related to the fact that the present C5′ monodeuteration procedures proceed slightly stereoselectively, namely 60–70% 2H at H5′ (pro‐S) and 40–30% 2H at H5″ (pro‐R), respectively. The possibility of correlating these peak intensity difference with the stereospecific assignment is discussed.