Sequential NMR assignments of labile protons in DNA using two‐dimensional nuclear‐Overhauser‐enhancemnt spectroscopy with three jump‐and‐return pulse sequences

Two‐dimensional nuclear Overhauser enhancement (NOESY) spectra of labile protons were recorded in H₂O solutions of a protein and of a DNA duplex, using a modification of the standard NOESY experiment with all three 90° pulses replaced by jump‐and‐return sequences. For the protein as well as the DNA fragment the strategically important spectral regions could be recorded with good sensitivity and free of artifacts. Using this procedure, sequence‐specific assignments were obtained for the imino protons, C2H of adenine, and C4NH₂ of cytosine in a 23‐base‐pair DNA duplex which includes the 17‐base‐pair O_R3 repressor binding site of bacteriophage λ. Based on comparison with previously published results on the isolated O_R3 binding site, these data were used for a study of chain termination effects on the chemical shifts of imino proton resonances of DNA duplexes.