Paramagnetic relaxation agent

Addition of a paramagnetic relaxation agent enables quantitative determination of carbons by C NMR. This technique has been applied to a series of phenylindoles using chromium(lll) acetylacetonate (Cr(acac)3) as the relaxation reagent <1996M111>. 

Metal acetylacetonates can be used as NMR shift reagents. Thus, Cr(acac)3 and Fe-(acac)3 can be used as paramagnetic relaxation agents to decrease the spin-lattice relaxation times of all C signals in poly(Me methacrylate), poly(Bu methacrylate) and poly(hydroxyethyl methacrylate) . ... 

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... 

Additional substances are sometimes added to the sample solution in order to facilitate measurements or to provide an internal reference for chemical shift or quantitation. Certain experiments benefit from the use of a small amount of a paramagnetic relaxation agent such as chromium(III) acetylacetonate (Secs. II.D.4 and II.G). A chemical-shift reference standard may be added if exact shifts are required. Some quantitative analyses employ an internal spin-counting intensity reference such a standard must not interfere with the sample spectrum. If the reference material is inert, it can be added directly to the sample. If it might react with the sample, the... 

Fig. 8. Continuous-flow c NMR spectrum of ethylbenzene using paramagnetic relaxation agents.

A considerable limitation of NMR spectroscopy is its inherent low sensitivity. Most of the measuring time is used by the spin system to return to its Boltzmann equilibrium after excitation, which is determined by Ti in CP NMR experiments. Paramagnetic relaxation agents such as Cu -EDTA and Gd -complexes accelerate this process of relaxation considerably resulting in enhanced sensitivity. Their effect on has been assessed on the membrane protein proteorhodopsin. 

Cortese, ).D., Voglino, A.L., and Hackenbrock, C.R. (1998) Membrane location of spin-labeled cytochrome c determined by paramagnetic relaxation agents. Biochemistry, 37, 6402-6409. 

---