PubMed · 6743999
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W S Moore. 1984. Basic physics and relaxation mechanisms.. https://doi.org/10.1093/oxfordjournals.bmb.a071957
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Accurate rules have been established to build binomial pulses up to fifth order, for selectively saturating protons at any given T2 with minimum power deposition. Pulse performance and sensitivity to experimental defects have been evaluated; the third order is generally found to be best suited. It is shown, by combining theory and experiment performed at 0.1 T, that matching the saturation pulse to T2 of the motionally restricted pool is essential to reveal exchange with free water protons. It is emphasized that, to date, lack of magnetization transfer contrast with binomial pulses is due mainly to insufficient RF level available with most MR imaging systems, especially at high magnetic field.
Current concepts in heteronuclear multidimensional NMR spectroscopy are reviewed. Methods to improve the sensitivity and the efficiency of data collection include constant time, compression through the overlap of chemical shift evolution and dephasing and rephasing periods, and dual or time-shared evolution. Two classes of three-dimensional and four-dimensional triple-resonance experiments applied to proteins are considered. The first class correlates 1H, 15N, and 13C signals of the protein backbone. The second class correlates both backbone and side-chain signals. Application of triple resonance to RNA is also discussed. Heteronuclear cross polarization (HCP) is considered as an alternative to INEPT transfer, and its application to nucleic acids is presented. Finally, two methods of employing pulsed field gradients (PFGs) are reviewed.
Peptide nucleic acids (PNA) incorporating nucleic acid bases into an achiral polyamide backbone bind to DNA in a sequence-dependent manner. The structure of a PNA-ribonucleic acid (RNA) complex was determined with nuclear magnetic resonance methods. A hexameric PNA formed a 1:1 complex with a complementary RNA that is an antiparallel, right-handed double helix with Watson-Crick base pairing similar to the "A" form structure of RNA duplexes. The achiral PNA backbone assumed a distinct conformation upon binding that differed from previously proposed models and provides a basis for further structure-based design of antisense agents.