Fast exchanges of water molecules

For some ions, the water molecules which are bound to the ion exchange rapidly with the water molecules outside the primary solvation shell. If this is the case only one proton signal will be detected. This will represent an average of the environments of the proton in the primary solvation shell and in the rest of the water, i.e. in the secondary shell of the cation, the primary and secondary shells of the anion and the bulk water. Situations where fast exchange is found and one signal observed show that  

Special techniques are required to analyse this situation, and unfortunately only total hydration numbers are obtained when only one resonance signal is observed. Methods for splitting up these into individual hydration numbers are necessary (see below). 

The chemical shift observed for a given electrolyte depends on the nature of both the cation and the anion. Unlike the slow exchange where the chemical shift is the same whatever the concentration of the solution, the position of the resonance signal if the exchange of water is rapid depends on the concentration. This dependency is utilised in the determination of the overall hydration number for the electrolyte in question. 

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Hiller, W., Messinger, J., and Wydrzynski, T. (1988) Kinetic determination of die fast exchanging substrate water molecule in the S3 state of Photosystem II, Biochemistry 37, 16908-16914. 

Figure 7 shows a typical NMR spectrum, which was obtained for a 20 mM Br NB-ds solution containing 115 mM H2O. As seen in the figure, the water protons always appear as a singlet, because the exchange of water molecules associated and unassociated with the anion is very fast. Under these conditions, the chemical shift of water protons should change with the distribution of water molecules between some different states. 

This suggests enhanced clustering of water with stabilization of WAW. Only one signal of mobile water (other protons are not observed in the spectra) can be caused by fast (in the NMR timescale) exchange of water molecules from different structures (Emsley et al. 1965) because the amounts of water are enough to form multilayer coverage of the HGA particles, and the HGA powder is characterized by the textural porosity. 

There is other evidence that the exchange of water molecules between the site in A W B and bulk solvent is slow compared to both proton transfer within the complex and the separation of A from B. Regarding the former, the fact that so many proton-transfer reactions in which AG° is negative are diffusion-controlled proves that the proton-transfer step is fast compared to the dissociation of the complex. Regarding the latter, if the departure of a water molecule from A W B were fast compared to the dissociation of the complex, one would expect to find more examples of rapid direct bimolecular proton transfer without solvent participation. 

Because distance and time can be coupled by motion, we could also view the timescales available to be probed with NMR and would find the same staggering range (Belton, 1995). Time constants for molecular processes can be quantified by magnetic resonance techniques ranging from extremely fast (picoseconds, such as for the tumbling of water molecules) to extremely slow (tens of seconds, such as for selected chemical reactions or exchange). 

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... 

Aqueous micelles are thermodynamically stable and kinetically labile spherical assemblies. Their association-dissociation process is very fast and occurs within milliseconds. The actual order is less than shown in Figure 1. Driving forces for the formation of aqueous micelles or vesicles are the solvation of the headgroup and the desolvation of the alkyl chain ( hydrophobic effect ). Because of the rapid exchange of surfactants, the core of the micelle contains a small percentage of water molecules. Aqueous assemblies are preferentially stabilized by entropy, and reverse micelles by enthalpy [4]. The actual formation of micelles begins above a certain temperature (Krafffs point) and above a characteristic concentration (critical micelle concentration, CMC). Table 1 shows a selection of typical micelle-forming surfactants and their CMCs. 

A. Theories of Fast and Slow Exchange. In 1957i Zimmerman and Britten published a theoretical treatment of the relaxation of water protons absorbed on silica gel (l6). In this system, both uni- and multiphasic relaxation decays were observed. The authors were able to account for the change in the number of observed phases by taking into account the relaxation times of the water protons in bound (absorbed) or free (non-absorbed) states, and the lifetimes of water molecules in each state. Two asymptotic expressions were derived, which have frequently been used in subsequent studies of water relaxation. 

Interesting results have been obtained by a combination of NMR and quasi-elastic incoherent neutron scattering. The presence of one single line in an NMR spectrum, for all the water concentrations, can be interpreted in two ways either we have only one kind of water molecule with a very well defined environment or we have different kinds subject to a fast chemical exchange (t < 10 3 sec.). Two regimes of absorption have been demonstrated and two different motions have been characterized both by NMR spin lattice relation time measurements and quasi-elastic incoherent neutron scattering. From these results and from results obtained on the Nafion salts W a structural model will be proposed (9). 

Electron and proton transfer reactions between natural products are, with the exception of C-H bond cleavage, very fast. Essentially each single collision between a donor and acceptor leads to a reaction ((diffusion controlled (reaction) and velocity constants are in the order of k = 10 ° mol s. Proton exchange between water molecules is an example of such a reaction. It has no mechanism and obeys the thermodynamic laws of reversible processes. 

In macroporous systems, there is normally fast exchange of the water molecules between the surface and bulk environments owing to rapid diffusion. In that case, the water molecules within the pore will experience an average environment with a single relaxation time, T, that obeys... 

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