Proton exchange rate

It is clear from the above equations that numerous parameters (proton exchange rate, kcx = l/rm rotational correlation time, tr electronic relaxation times, 1 /rlj2e Gd proton distance, rGdH hydration number, q) all influence the inner-sphere proton relaxivity. Simulated proton relaxivity curves, like that in Figure 3, are often used to visualize better the effect of the... 

Protonation of cryptands. Novel protonation behaviour has been observed for these (diaza) cryptands (Cheney Lehn, 1972 Kresge, 1975). Internal protonation of ligands such as 1.1.1 and 2.1.1 leads to species exhibiting very slow proton exchange rates. In contrast, for simple systems, proton exchange rates are invariably very fast. For 1.1.1, the... 

The dioxo complexes of W(IV) and Mo(IV), having high pKa values (Table II), are formed via hydrolysis as the rate-determining step (Scheme 4) and the observed rate constants for the inversion along the O-M-O axis for the W(IV) and the Mo(IV) complexes are therefore defined by Eq. (18). These were calculated as a function of pH, using the proton exchange rate constants (Table IV) and the acid dissociation constants (Table II)... 

Proton exchange rates in aqueous solutions are enhanced by small amounts (0.5% V/V) of hydrophobic substances (e.g., methanol, dioxane) because of a consequent increase in H-bonded water structure in the hydration shells through which the proton transfer is mediated (9). 

Significant improvements in the appearance of the NMR spectrum of molten globule states are, however, observed upon addition of small amounts of trifluoroethanol (TFE), less than 10 vol%. Amide proton exchange rates are reduced and resonances are sharpened although the chemical shift dispersion is also decreased [51]. The net result is an increased resolution that simphfies considerably the spectral assignment. The increased spectral resolution is in fact due to the fact that the peptides become more denatured rather than more structured and that, in order to obtain more structural information about the folded state, the peptide is partially unfolded ... 

The light absorption act is fast compared with proton exchange rates and therefore the ultraviolet spectrum of a system in protonation equilibrium (1) is the superposition of the spectra of the base and the conjugate acid, each contributing according to its concentration. So the observed molar extinction coefficient is ven by (4), where a is the fraction of the base converted into the... 

In gramicidin SA four protons were shown to have very slow deuterium-proton exchange rates which could be assigned to specific protons. 

Indirect Exchange Rates. In this case, the line shape is indirectly related to the acid-base equilibrium. Besides measuring intermolecular processes like the proton exchange rates, DNMR often has been used to measure intramolecular processes like conformational changes that occur on the same time scale. When the activation energy of such a process is very different in the acidic and basic forms for an indicator, DNMR can be used to measure the ionization ratio. 

Fig. 1. A Gd(III) complex with one inner sphere water molecule, surrounded by bulk water. Inner sphere proton relaxivity is due to interactions between the Gd electron spin and the water protons on the inner sphere water. Outer sphere relaxivity arises from interactions between the Gd electron spin and bulk water protons. rR stands for the rotational correlation time of the molecule, kex for the water/proton exchange rate and 1/T, 2c for the electron spin relaxation rates of the Gd(III)...

The four examples given above illustrate the effect of water exchange or more exactly proton exchange rate and of the number of water molecules in the first coordination sphere of the paramagnetic metal ion on the water proton relaxation rate. Nonetheless, the relaxivity of a contrast agent can also be strongly... 

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