Bound water lifetime

Figjure 3. Plot of the Jhactional decrease in bulk water signal intensity expected for a PARACEST agent having a bound water lifetime of3ps... 

Because of its carcinogenic potential, the EPA-recommended concentration for trichloroethylene in ambient water is zero. However, because attainment of this level may not be possible, levels that correspond to upper-bound incremental lifetime cancer risks of 10, lO , and 10 are estimated. 

A strong decrease in relaxivity (from 12.8mM-1s-1 to 2mM-1s-1) between pH 6 and 11 has been reported for a positively charged macrocyclic Gdm complex (Scheme 10), which was explained by the successive deprotonation of the coordinated water molecules.167 Luminescence lifetime measurements of a Yb111 analogue proved that the complex possesses three bound waters at pH 5.5. Above pH 11, a di-oxo-bridged dimer is formed that has no more bound water or OH groups. 

The lifetime of the hydrogen bonds that the bound water molecules form with the polar headgroups of the surfactant are characterized in terms of two time correlation functions, ShbW and CnB(t), defined as [10],... 

Fig. 3 Hydrogen bond lifetime correlation function for bound water molecules in the CsPFO micellar solution. Inset The same for bulk water. Note the much slower decay of the bound water species.

First of all, what was considered were bare hydronium H3+0 ions with three equivalent protons, a hydrated hydronium ion with three strongly bound water molecules (i.e., Eigen cluster H904+), and the symmetric H502+ complex in which a proton is shared between two water molecules (i.e., the Zundel ion). Many intermediates or more-complex states of the hydrated proton, H+(H20) , may also exist. All clusters have a finite lifetime and transform between each other during charge transport. Due to the variation of the relative abundance of these three basic states, proton transfer may occur via different pathways. 

The formation of these ternary luminescent lanthanide complexes was the result of displacement of the two labile metal-bound water molecules, which was necessary because the energy transfer process between the antenna and the Ln(III) metal centre is distance-dependent. This ternary complex formation was confirmed by analysis of the emission lifetimes in the presence of DMABA and showed the water molecules were displaced by a change in the hydration state q from 2 to 0, with binding constants of log fCa = 5.0. The Eu(III) complexes were not modulated in either water or buffered solutions at pH 7.4. Lifetime analysis of these complexes showed that the metal-bound water molecules had not been displaced and that the ternary complex was not formed. Of greater significance, both Tb -27 and Tb -28 could selectively detect salicylic acid while aspirin was not detected in buffered solutions at pH 7.4, using the principle as discussed for DMABA where excitation of the binding antenna resulted in a luminescent emission upon coordination of salicylic acid to the complex. 

Spectrum shifted, broader, less intense Lifetime of e decreased aq Bounded water only, slower motion... 

The hydration munber, or the nimiber of bound water molecules in the lanthanide coordination sphere, can be calculated using a method introduced by Horrocks and Sudnick for terbimn and europium complexes (50). The relationship between Tb or Eu excited state lifetimes (t), which are experimentally determined in H2O and D2O, and the hydration number (q) is given in Eq. (1)... 

The unit risk is defined as the upper bound additional lifetime cancer risk associated with exposure to either 1 pg 1 in water or 1 pg m in air. The dose or exposure concentration associated with a given risk can also be calculated by rearranging terms in the slope factor equation shown above to solve for the dose term. The result is termed the risk specific dose (or concentration). The risk specific dose is often used as the basis for the exposure criteria for carcinogens. 

We first consider the A contribution. Equation 1, and an explanation in terms of a two-site model i.e., a model in which a water molecule exchanges between solution and sites (or class of sites) on or near a protein molecule such that at least one direction fixed in the water molecule is constrained to move rigidly with the protein molecule. In the simplest case, a water molecule attaches rigidly to the protein, moves with it for a while, and then leaves. In a somewhat more complex case, the attachment may be less rigid so that the water molecule is free to rotate about an axis fixed with respect to the protein. Additionally, a situation in which water molecules partially orient in the electric fields near the protein surface because of their electric dipole moments would also be a two-site model. Characteristic of a two site-model is that a time Tj, or a distribution of such times, can be defined that measures the mean lifetime of a water molecule in the protein-associated state. Moreover, such a time is in principle a measurable quantity, and its value must satisfy two criteria it must be at least comparable to if not longer than Tj, otherwise the nuclei of the bound water molecules could not sense the rotational motion of the protein molecules and it must be comparable to or shorter than the nuclear relaxation time of a bound water molecule, else it could not communi-... 

Photoisomerization of retinal from the all-trans to the 13-ct.v form leads to a cyclic photoreaction with intermediates, bR, K, L, M, N, and O, which are subsequently formed before recovery of the initial state, as schematically illustrated, with the absorption wavelength in the suffix (Fig. 5), together with their individual lifetimes.50 51 The first proton transfer in the photocycle of the all-trans, 15-anti isomer is from the retinal Schiff base to Asp 85 at the central part of the protein. Then, a proton is released from the proton release complex (PRC) consisting of Glu 204, 194 and a bound water molecule at the extracellular surface, as illustrated by arrow A. This is followed by reprotonation of... 

For [Eu(a-2-P2Wi706i)2]. x (H2O) = 3.02 msec and x (D2O) = 4.79 msec, whereas for the 1 1 complex, x (H2O) = 0.23 msec and t (D2O) = 2.8 msec. Solving the equation, q = 0.12 for [Eu(a-2-P2Wi7061)2] , consistent with the crystal structure, where no water molecules are coordinated to the Ln. [Luo, 2000 292] For the K7[Eu(a-2-P2W 706i)] complex, q= 4.19, consistent with four water molecules coordinated to the Eu for the 1 1 complex. Previously, the excitation spectrum and luminescence lifetime ofthe [Eu(a-l-P2Wi706i)] species have been reported, leading to q=4 for that species, consistent with the number of bound water molecules for the 1 1 [Eu(a-2-P2W 706i)] complex reported here. 

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