Nuclear magnetic resonance proton exchange

Physical Chemical Characterization. Thiamine, its derivatives, and its degradation products have been fully characterized by spectroscopic methods (9,10). The ultraviolet spectmm of thiamine shows pH-dependent maxima (11). H, and nuclear magnetic resonance spectra show protonation occurs at the 1-nitrogen, and not the 4-amino position (12—14). The H spectmm in D2O shows no resonance for the thiazole 2-hydrogen, as this is acidic and readily exchanged via formation of the thiazole yUd (13) an important intermediate in the biochemical functions of thiamine. Recent work has revised the piC values for the two ionization reactions to 4.8 and 18 respectively (9,10,15). The mass spectmm of thiamine hydrochloride shows no molecular ion under standard electron impact ionization conditions, but fast atom bombardment and chemical ionization allow observation of both an intense peak for the patent cation and its major fragmentation ion, the pyrimidinylmethyl cation (16). 

An X-ray crystallographic study of 2-hydroxy-4,6-dimethylpyrimi-dine led to no conclusions regarding its structure. Because of the rapid exchange of the NH protons of pyrimidin-2-one both in dimethyl sulfoxide and in water, nuclear magnetic resonance spectroscopy does not afford positive evidence for either the oxo or the hydroxy formulation. The statement that 4,6-dimethylpyrimidin-2-one had been isolated in two modifications, 94 and 95, was soon disproved. ... 

Richardson, S.J. 1989. Contribution of proton exchange to the oxygen-17 nuclear magnetic resonance transverse relaxation rate in water and starch-water systems. Cereal Chem. 66, 244-246. Richardson, M.J. and Saville, N.G. 1975. Derivation of accurate glass transition temperatures by differential scanning calorimetry. Polymer 16, 753-757. 

The same nucleus (say methyl protons) in different chemical environments A and B will generally have nuclear magnetic resonances at different frequencies. If the exchange of pro-... 

This technique has been taken to new heights through the use of 2-D nuclear magnetic resonance " to assign many, if not all, exchangeable protons in terms of the residues to which they are attached. As a result, one can assess individual rate constants defining the kinetics of exchange. 

Tables 3-2 and 3-3 summarize the infrared and proton-NMR (nuclear magnetic resonance) spectroscopic properties of alcohols and ethers. In the proton NMR, the oxygen atom is deshielding. Phenols and alcohols rapidly exchange protons so their NMR spectra are solvent dependant. The alcohol and ether groups don t have any characteristics absorptions in UV-vis spectra.

Fig. 14. Plot of line shape change versus rate of exchange of protons between environments A and B. The intensities of the various line functions are not comparable. Reproduced by permission from "High-Resolution Nuclear Magnetic Resonance, by Pople, Schneider, and Bernstein. McGraw-Hill, New York, 1959.

The mechanism for the bond cleavages indicated in figure 10.1b was clarified by Ronald Breslow. In one of the earliest applications of nuclear magnetic resonance to biochemical mechanisms, he demonstrated that the proton bonded to C-2 in the thiazolium ring is readily exchangeable with the protons of H20 and deuterons of D20 in a base-catalyzed reaction... 

In protein solutions the water protons may be considered to reside in two different environments, i.e. the bulk water, and the hydration spheres of the protein molecules. If there is fast exchange of protons between these environments a single proton nuclear magnetic resonance will be observed, which corresponds to the average of the resonances in the different environments. Following McConnell (74) the observed longitudinal relaxation time is to a good approximation... 

Seitz, L. M., and Brown, T. L., Organometallic exchange reactions. I. Lithium-7 and proton nuclear magnetic resonance spectra of methyllithium and ethyllithium in ether, J. Am. Chem. Soc., 88, 2174 (1966). 

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