Proton exchangeable

Isotopic enrichment is clearly not a prerequisite for the observation of hydroxyl protons in oligosaccharides. However, use of enriched material does simplify the attenuation of the strong H2O resonance and moreover permits spectral editing in a third dimension that can be very useful in overcoming resonance overlap that is typically observed in the hydroxyl region of the H spectrum [45]. 

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Studies of electrical interactions in proteins, polypeptides, and amino acids started over 60 years ago [1]. To a large extent, electrostatic properties of proteins are determined by the ability of certain amino acids to exchange protons with their environment and the dependence of these processes on pH. The proton occupies a special position as a promoter and iiuxliator in... 

Aprotic solvent (Section 8 12) A solvent that does not have easily exchangeable protons such as those bonded to oxy gen of hydroxyl groups... 

That is, terms of the form (1 — jr -I- jr< )/) appear in the denominator for all reactant sites having exchangeable protons and similarly in the numerator for all transition state sites. If there is no change in the fractionation factor for a site, its contribution cancels. If the solvent is a reactant, its term disappears because the solvent fractionation factor is unity by definition. 

All these applications involve equilibrium between species in solution, especially between species that can exchange protons. The common theme throughout this chapter is that all these equilibria can be treated in a similar way ... 

H spectra of samples were recorded on a Varian Unity Plus 600 MHz n.m.r. spectrometer by Dr. Feng Qiu (Department of Chemistry, CUNY College of Staten Island). Samples were lyophilized twice from D2O to eliminate exchangeable protons. 

Soft-pulse multiple irradiation In this method, pre-saturation is done using shaped pulses having a broader excitation profile. Therefore, it is a more suitable method for the suppression of multiplets. This technique is very effective, easy to apply and easy to implement within most NMR experiments. In aqueous solutions, however, slowly exchanging protons would be detectable due to the occurrence of transfer of saturation. In addition, the spins with resonances close to the solvent frequency will also be saturated. 

If an amphiprotic solvent contains an acid and base that are neither mutually conjugate nor are conjugated with the solvent, a protolytic reaction occurs between these dissolved components. Four possible situations can arise. If both the acid and base are strong, then neutralization occurs between the lyonium ions and the lyate ions. If the acid is weak and the base strong, the acid reacts with the lyate ions produced by the strong base. The opposite case is analogous. A weak acid and a weak base exchange protons ... 

Fig. 3. Rapid Mo(V) EPR signals obtained on reducing xanthine oxidase at pH 10 with 15 moles of xanthine for 1 min. at about 20 °. The upper four spectra are at 9.1 GHz and the lower four at 34.4 GHz. a, a, c, 8 refer to H2O as solvent and b, b, d, d to D2O. a, b, c, d are computer simulations of the experimental spectra, a, b, c, d, respectively. The interpretation is that two species, each having exchangeable protons which interact with Mo(V), are responsible for the signals. For one of these (dotted complex type II) there are two equivalent interacting protons and for the other (dashed complex type I), two non-equivalent protons. These species are believed to correspond to two different complexes of reduced xanthine oxidase with xanthine. (Reproduced from ref. 78 see also Table 2 for the parameters of the signals.)...

Differentiating these two compounds, particularly in isolation, would not be easy by proton NMR. The temptation to irradiate the —OH should be resisted (note the irradiation of exchangeable protons in NOE experiments is not generally recommended, even if they give rise to sharp peaks) as these compounds can undergo tautomerism and exist in the forms shown in Structure 8.4. 

This exchange process can also be a problem where the water in a solvent becomes unintentionally irradiated during an NOE experiment because the protons of the water are in constant chemical exchange with all exchangeable protons in the molecule being studied. Consider for example, the following hypothetical problem. You wish to distinguish between the two compounds shown in Structure 8.5. 

Q7. Propose a structure from Spectra 15.10-15.13. The compound is known to be a free base and is composed of only carbon, hydrogen, oxygen and nitrogen. It has a molecular weight of 267. To further complicate matters, the compound was extracted into CDCI3 solution from D20/sodium carbonate so that no exchangeable protons can be observed. 

An innovative approach to CEST agents was introduced by Aime et al. (173). The CEST effect is proportional to the number of exchangeable protons or water molecules. When a lanthanide... 

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