ASSIGNMENT OF NH RESONANCES IN NUCLEIC-ACIDS USING NATURAL-ABUNDANCE N-15-H-1 CORRELATION SPECTROSCOPY WITH SPIN-ECHO AND GRADIENT PULSES

It is well known that 2D N-15-H-1 correlation spectra can resolve overlapped imino proton resonances in the downfield NMR spectra of nucleic acids according to their N-15 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 N-15-labeled, or very concentrated samples. Here we report the design of a gradient-enhanced, jump-return spin echo version of an N-15-H-1 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.