Dissecting non-canonical interactions in frameshift-stimulating mRNA pseudoknots.

A variety of powerful NMR experiments have been introduced over the last few years that allow for the direct identification of different combinations of donor and acceptor atoms involved in hydrogen bonds in biomolecules. This ability to directly observe tertiary structural hydrogen bonds in solution tremendously facilitates structural studies of nucleic acids. We show here that an adiabatic HNN-COSY pulse scheme permits observation and measurement of J(N,N) couplings for nitrogen sites that are separated by up to 140 ppm in a single experiment at a proton resonance frequency of 500 MHz. Crucial hydrogen bond acceptor sites in nucleic acids, such as cytidine N3 nitrogens, can be unambiguously identified even in the absence of detectable H41 and H42 amino protons using a novel triple-resonance two-dimensional experiment, denoted H5(C5C4)N3. The unambiguous identification of amino nitrogen donor and aromatic nitrogen acceptor sites associated with both major groove as well as minor groove triple base pairs reveal the details of hydrogen bonding networks that stabilize the complex architecture of frameshift-stimulating mRNA pseudoknots. Another key tertiary interaction involving a 2'-OH hydroxyl proton that donates a hydrogen bond to an aromatic nitrogen acceptor in a cis Watson-Crick/sugar edge interaction can also be directly detected using a quantitative J(H,N) 1H,15N-HSQC experiment.