Water exchange with bound

In the case of Gd3+, there is a rapid water exchange with respect to the relaxation 7im [28]. For water exchange reaction of Gd3+ aquo ion the water tumbling time is 7 x 10 11 s. When Gd3+ is bound to a macromolecule, part of the hydration sphere is substituted by a protein molecule, the effective correlation changes (i.e.) effectively it becomes the electron relaxation time [29] which is about 10-9 s. By virtue of binding to a macromolecule, a net enhancement in proton relaxation rate, Eq is observed which is characteristic of the Gd3+ complex and depends on the resonance frequency and temperature. Some data on the enhancements obtained for Gd3+ protein complexes are given in Table 11.5. 

Ra is continually produced in sediments by the decay of insoluble Th parents (see Fig. 7). While in fresh water Ra is bound tightly to particle surfaces, in seawater Ra readily undergoes cation exchange with other dissolved constituents. This provides an additional source of Ra to the water column, which controls the frequently observed non-... 

If the spectrum involves only one resonance (or if linewidths do not allow for the separation of several resonances), a single experiment can be run with acquisition of the amplitude of each echo along the pulse train (for sensitivity enhancement, accumulations can be carried out). This experiment is especially valuable for determining the relative proportions of two species which differ by their transverse relaxation time, for instance the two types of water (free and bound), if exchange between these two states is sufficiently slow. For this type of measurement, a low resolution spectrometer (without any shim system) proves to be quite sufficient. 

If the aqua complex is very inert, as for example in the case of [Rh(H20)6] " or [Ir(H20)6], the experiment is best performed by dissolving the complex enriched in 0-water, [M(H2 0)6] ", in non-enriched water (2,26). Preparing the initial condition in this way leads to a relatively intense signal for bound water. The exchange rate constant can then be measured by observing the decrease in the bound water signal with time. The decrease in the mole fraction of labeled water coordinated to the metal, x, is described by Eq. (7) ... 

In a classical paper. Swift and Connick (34,35) derived solutions of the equation for transverse relaxation, I/T2, and chemical shift, Am, in the case of dilute solutions of paramagnetic ions. Equation (9) gives the increase in transverse relaxation of the bulk water signal, l/72r, due to exchange with water bound to a paramagnetic ion and normalized by the mole fraction of bound water. Pm. 

The component typically analyzed in plants is cellulose, which is the major structural carbohydrate in plants (Epstein et al. 1976, 1977). Cellulose contains 70% carbon-bound hydrogen, which is isotopically non-exchangeable and 30% of exchangeable hydrogen in the form of hydroxyl groups (Epstein et al. 1976 Yapp and Epstein 1982). The hydroxyl-hydrogen readily exchanges with the enviromnen-tal water and its D/H ratio is not a useful indicator of the D/H ratio of the water used by the plants. 

A number of important structural aspects of zinc complexes as found in enzymes are introduced in this section to serve as background information for the subsequent sections. Aquated Zn(II) ions exist as octahedral [Zn(H20)6] + complexes in aqueous solution. The coordinated water molecules are loosely bound to the Zn + metal center and exchange rapidly with water molecules in the second coordination sphere (see Figure 1) with a rate constant of ca 10 s at 25 °C extrapolated from complex-formation rate constants of Zn + ions with a series of nucleophiles. The mechanism of the water exchange reaction on Zn(II) was studied theoretically, from which it was concluded that the reaction follows a dissociative mechanism as outlined in Figure 2. ... 

Tihe presence of exchangeable cations, framework oxygens, cavities of A different sizes, and previously adsorbed water molecules makes the interaction of water with zeolites complicated (1). As a result, in zeolites, water molecules with different physical properties exist, from tightly bound to liquid-like water. This was shown by NMR measurements (2,3 4) ... 

Because hydrogen atoms contain only one electron, and therefore scatter X-rays very weakly, they are usually not seen at all in X-ray structures of proteins. However, neutrons are scattered strongly by hydrogen atoms and neutron diffraction is a useful tool in protein structure determination.414 415 It has been used to locate tightly bonded protons that do not exchange with 2H20 as well as bound water (2H20). 

The water is tightly held in the network by hydrogen-bonding. Many soluble metals will also ion-exchange with the sodium along the polymer backbone and be bound. 

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