Solute resonances, broadening

Guanine cytosine, G.C, (11 to 13 ppm) and adenine uracil, A.U, (13 to 15 ppm) NH resonances are shifted by the ring currents of nearby bases, and calculations [42] indicate that the solid-state clover leaf structure is appropriate in solution. NH Resonances disappear when the helices melt and Mg helix stabilisation is readily observed by NMR [43] but optical melting points are higher than the temperature range over which NH resonances broaden and disappear [44]. for the double helix of d(A-A-C-A-A) with d(T-T-G-T-T), for example, is 28°C but resonances have disappeared by 9°C T is thymine. 

Viscosity-induced resonance broadening. Syn. viscosity broadening. The increase in the line width of peaks in a spectrum caused by the decrease in theTj relaxation time that results from a slowing of the molecular tumbling rate. Saturated solutions and solutions at a temperature Just above their freezing point often show this broadening behavior. 

Single-channel NMR probes were initially built to allow the observation of the NMR signal, and these probes produced fine spectra. A single-channel NMR probe for observing cannot suppress the resonance broadenings caused by J-coupling to nearby H s. The solution is addition of a second coil to allow application of a continuous stream of RF tuned to the H NMR frequency to rapidly scramble (decouple) the H s. 

The selective reduction of a solvent water resonance can also be achieved in a similar way if the transverse relaxation time of the water protons can be reduced (i.e. the resonance broadened) such that this becomes very much shorter than that of the solutes under investigation. This can be achieved by the addition of suitable paramagnetic relaxation agents (about which the water molecules form a hydration sphere) or by reagents that promote chemical exchange. Ammonium chloride and hydroxylamine have been used to great effect in this way [4,5], as illustrated for the proton spectrum of the reduced arginine... 

This peak is broadened and contact shifted down field by the unpaired electron (Fig. 24). A spectrum of a mixture of methylcobinamide and free nitroxide shows broadening of the methyl resonance but no shift in resonance position. Thus the nitroxide must remain attached to the cobalt atom in solution. 

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