It is well known that 2D 15N-1H correlation spectra can resolve overlapped imino proton resonances in the downfield NMR spectra of nucleic acids according to their 15N chemical shifts, and that these resonances can be assigned by base type on that basis, independent of conformation. This information can be extremely important in determining the solution structure of a nucleic acid by NMR, but previously could only be obtained using 15N-labeled, or very concentrated samples. Here we report the design of a gradient-enhanced, jump-return spin echo version of an 15N-1H HMQC experiment (GE-JRSE HMQC) that is sensitive enough to work on unlabeled nucleic acid samples at normal NMR concentrations. This experiment has led to the assignment of imino proton resonances with non-Watson Crick chemical shifts in the spectrum of a 29 residue oligoribonucleotide that models the sarcin/ricin loop from 28S rRNA.