Chemical exchange spectroscopy,

Oil and 0)2, and (b) 2D shift-correlation spectra, involving either coherent transfer of magnetization [e.g., COSY (Aue et al, 1976), hetero-COSY (Maudsley and Ernst, 1977), relayed COSY (Eich et al, 1982), TOCSY (Braunschweiler and Ernst, 1983), 2D multiple-quantum spectra (Braun-schweiler et al, 1983), etc.] or incoherent transfer of magnedzation (Kumar et al, 1980 Machura and Ernst, 1980 Bothner-By et al, 1984) [e.g., 2D crossrelaxation experiments, such as NOESY, ROESY, 2D chemical-exchange spectroscopy (EXSY) (Jeener et al, 1979 Meier and Ernst, 1979), and 2D spin-diffusion spectroscopy (Caravatti et al, 1985) ]. 

Methylpseudouridine (m vj/) has been synthesised and incorporated into RNA where it was found to be slightly destabilising compared to pseudouridine.Psuedouridine can be selectively cyanoethylated with acrylonitrile, aiding its detection in tRNA by MALDI mass spectrometry. The presence of pseudouridine in RNA has also been detected by NMR using chemical exchange spectroscopy as pseudouridine has an additional Another... 

Hwang, T.L., Mori, S., Shaka, A.J., van Zijl, P.C.M. (1997) Application of phase-modulated CLEAN chemical EXchange spectroscopy (CLEANEX-PM) to detect water-protein proton exchange and intermo-lecular NOEs. Journal of American Chemical Society, 119 (26), 6203-6204. 

Figure 6 Pulse sequences for nuclear Overhauser and chemical exchange spectroscopy (a) NOESY ...

Macura S, Wurthrich K, Ernst R R 1982 Separation and suppression of coherent transfer effects in two-dimensional NOE and chemical exchange spectroscopy. J Magn Res 46 269-282... 

In a coupled spin system, the number of observed lines in a spectrum does not match the number of independent z magnetizations and, fiirthennore, the spectra depend on the flip angle of the pulse used to observe them. Because of the complicated spectroscopy of homonuclear coupled spins, it is only recently that selective inversions in simple coupled spin systems [23] have been studied. This means that slow chemical exchange can be studied using proton spectra without the requirement of single characteristic peaks, such as methyl groups. 

Basic principles of modem NMR spectroscopy are the subject of many textbooks [167,188-196], including pulse techniques [197] for NMR of polymers, see Bodor [198]. A guide to multinuclear magnetic resonance is also available [199]. Several texts deal specifically with multidimensional NMR spectroscopy [169,197,200-202]. Ernst et al. [169] have reviewed the study of dynamic processes, such as chemical exchange... 

Multidimensional and heteronuclear NMR techniques have revolutionised the use of NMR spectroscopy for the structure determination of organic molecules from small to complex. Multidimensional NMR also allows observation of forbidden multiple-quantum transitions and probing of slow dynamic processes, such as chemical exchange, cross-relaxation, transient Over-hauser effects, and spin-diffusion in solids. 

The conformation of the mixed p agonist/5 antagonist H-Tyr-c[-D-Orn-2-Nal-D-Pro-Gly-] in comparison to that of H-Tyr-c[-D-Orn-Phe-D-Pro-Gly-] was studied in DMSO-d6 by NMR spectroscopy and by molecular mechanics calculations [62,64]. Neither peptide showed nuclear Overhauser effects between C H protons or chemical exchange cross peaks in spectra obtained by total correlation and rotating frame Overhauser enhance-... 

Molecular Motions and Dynamic Structures. Molecular motions are of quite general occurrence in the solid state for molecules of high symmetry (22,23). If the motion does not introduce disorder into the crystal lattice (as, for example, the in-plane reorientation of benzene which occurs by 60° jumps between equivalent sites) it is not detected by diffraction measurements which will find a seemingly static lattice. Such molecular motions may be detected by wide-line proton NMR spectroscopy and quantified by relaxation-time measurements which yield activation barriers for the reorientation process. In addition, in some cases, the molecular reorientation may be coupled with a chemical exchange process as, for example, in the case of many fluxional organometallic molecules. ... 

J.I. Kaplan and G. Fraenkel, NMR of Chemically Exchanging Systems (Academic Press, New York, 1980). L.M. Jackman and F.A. Cotton, Dynamic Nuclear Magnetic Resonance Spectroscopy (Academic Press, New York, 1975). 

The chemical exchange, in NMR sense, reflects all processes of intermolec-ular and intramolecular rearrangements that occur while the observed spins change their magnetic environments [12, 13]. However, for 2D exchange spectroscopy, only the slow processes in which the observed spins change their resonance frequencies are observable. Here, slow refers to an exchange rate kij between sites i and j that is smaller than the difference between... 

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