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 over lapped imino proton resonances in the downfield NMR spectra of nucleic acids according to their N-15 chemical shifts, and that these resonan ces can be assigned by base type on that basis, independent of conform ation. 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 versi on of an N-15-H-1 HMQC experiment (GE-JRSE HMQC) that is sensitive eno ugh to work on unlabeled nucleic acid samples at normal NMR concentrat ions. This experiment has led to the assignment of imino proton resona nces with non-Watson-Crick chemical shifts in the spectrum of a 29 res idue oligoribonucleotide that models the sarcin/ricin loop from 28S rR NA.