Water molecule, residence times

Nuclear Overhauser effect (NOE) is another techiuque used to study the dynamics of water near a heterogeneous surface. NOE intensities are modulated by dipole-dipole interactions between protons of protein and water in the hydration layer. This interaction varies as where R is the separation between the two protons. Measurements of magnetization transfer using NOE have been used to obtain the residence time of the hydration water. The residence time of water molecules in the hydration layer immediate to the protein is not easily available by other techniques and is valuable information in quantifying the rigidity of the layer. 

Hydration numbers, hi, are the time-average numbers of water molecules residing in the first (and second, if formed) hydration shell of ions. If directional coordinate bonds are formed with the water molecules in the first hydration shell, hi equals the coordination number. 

NMR has been used to determine the exchange rates and lifetimes of phospholipids in reverse micelles formed in organic solvents in the presence of water. The residence time of the unsaturated dilinoleylphos-phatidylcholine molecule in a micelle was found to be close to that of a monomer in bulk solution in benzene, around 30 ms. For the saturated dipalm-itoylphosphatidylcholine, the residence time in a micelle was about twice as long. 

The NMR study by Wiithrich and coworkers has shown that there is a cavity between the protein and the DNA in the major groove of the Antennapedia complex. There are several water molecules in this cavity with a residence time with respect to exchange with bulk water in the millisecond to nanosecond range. These observations indicate that at least some of the specific protein-DNA interactions are short-lived and mediated by water molecules. In particular, the interactions between DNA and the highly conserved Gin 50 and the invariant Asn 51 are best rationalized as a fluctuating network of weak-bonding interactions involving interfacial hydration water molecules. 

Among the dynamical properties the ones most frequently studied are the lateral diffusion coefficient for water motion parallel to the interface, re-orientational motion near the interface, and the residence time of water molecules near the interface. Occasionally the single particle dynamics is further analyzed on the basis of the spectral densities of motion. Benjamin studied the dynamics of ion transfer across liquid/liquid interfaces and calculated the parameters of a kinetic model for these processes [10]. Reaction rate constants for electron transfer reactions were also derived for electron transfer reactions [11-19]. More recently, systematic studies were performed concerning water and ion transport through cylindrical pores [20-24] and water mobility in disordered polymers [25,26]. 

The shape of the probability density function, depends on the system. Some examples are shown in Fig. 4-4. This figure also contains probability density of age (see Section 4.2.3). Figure 4-4a might correspond to a lake with inlet and outlet on opposite sides of the lake. Most water molecules will then have a residence time in the lake roughly equal to the time it takes for the mean current to carry the water from the... 

This behavior suggests that the Na and crions keep their solvation shells intact upon adsorption, with only small changes due to the restriction put on the water structure hy the metal lattice. For example, when an external electric field is applied, stronger ion-metal interactions are able to strip a small part of the coordination shell, but only for large fields. The electric field is also found to decrease the residence time of water molecules in the ion s coordination shell. 

Bo is the measurement frequency. Rapid exchange between the different fractions is assumed the bulk, water at the protein surface (s) and interior water molecules, buried in the protein and responsible for dispersion (i). In fact, protons from the protein surface exchanging with water lead to dispersion as well and should fall into this category Bulk and s are relevant to extreme narrowing conditions and cannot be separated unless additional data or estimations are available (for instance, an estimation of fg from some knowledge of the protein surface). As far as quadrupolar nuclei are concerned (i.e., and O), dispersion of Rj is relevant of Eqs. (62) and (63) (this evolves according to a Lorentzian function as in Fig. 9) and yield information about the number of water molecules inside the protein and about the protein dynamics (sensed by the buried water molecules). Two important points must be noted about Eqs. (62) and (63). First, the effective correlation time Tc is composed of the protein rotational correlation time and of the residence time iw at the hydration site so that... 

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