Salt-Jump

The self-assembly of this fundamental building block of chromatin is a topic of enduring interest. DNase I digestion experiments as well as spectroscopic studies indicate that nucleosome core particles can be reconstituted by salt-jump (i.e., diluting NaCl concentration from 2.0 to 0.2 M) or by direct mixing of histones and DNA at the lower salt concentration. Daban and Cantor used the increase in eximer fluorescence to investigate the reassembly process in terms of a two-state model ... 

It is then of interest to investigate how the final salt concentration influences the kinetics of vesicle formation. The increase in salt concentration provides the driving force for the reaction, and it is apparent that the larger the salt jump, the greater the tendency will be to form vesicles, which we would expect to be reflected in the observed rates. It should be... 

Porschke D and Obst A 1991 An electric field jump apparatus with ns time resolution for electro-optical measurements at physiological salt concentrations Rev. Sc/. Instnim. 62 818-20... 

Variamine blue (C.I. 37255). The end point in an EDTA titration may sometimes be detected by changes in redox potential, and hence by the use of appropriate redox indicators. An excellent example is variamine blue (4-methoxy-4 -aminodiphenylamine), which may be employed in the complexometric titration of iron(III). When a mixture of iron(II) and (III) is titrated with EDTA the latter disappears first. As soon as an amount of the complexing agent equivalent to the concentration of iron(III) has been added, pFe(III) increases abruptly and consequently there is a sudden decrease in the redox potential (compare Section 2.33) the end point can therefore be detected either potentiometrically or with a redox indicator (10.91). The stability constant of the iron(III) complex FeY- (EDTA = Na2H2Y) is about 1025 and that of the iron(II) complex FeY2 - is 1014 approximate calculations show that the change of redox potential is about 600 millivolts at pH = 2 and that this will be almost independent of the concentration of iron(II) present. The jump in redox potential will also be obtained if no iron(II) salt is actually added, since the extremely minute amount of iron(II) necessary is always present in any pure iron(III) salt. 

The mechanism of ion transport in the MEEP/metal salt complexes has been modelled on the PEO transport mechanism, that is to say in terms of jumps of the metal ion between the ether oxygen nuclei of the side groups, the nitrogen atoms of the backbone being not involved in the coordination [599]. 

The possibility of measuring the Volta potential in the system metal-solid-state electrolyte and using the data obtained to determine ionic components of the free lattice energy has been shown in our papers. Earlier, Copeland and Seifert measured the Volta potential between Ag and solid AgNOj in the temperature range between 190 and 280 °C. They investigated the potential jump during the phase transition from solid to liquid salt. 

The conductivities of melts, in contrast to those of aqueous solutions, increase with decreasing crystal radius of the anions and cations, since the leveling effect of the solvation sheaths is absent and ion jumps are easier when the radius is small. In melts constituting mixtures of two salts, positive or negative deviations from additivity are often observed for the values of conductivity (and also for many other properties). These deviations arise for two reasons a change in hole size and the formation of new types of mixed ionic aggregates. 

Finally, no dependence on the temperature, DNA concentration and salt concentration was observed for a temperature jump study using ct-DNA that was not sonicated.27 Based on these results the authors concluded that only large-scale dynamics of the DNA were responsible for the binding kinetics of 1 to DNA, and they suggested that studies with short length DNA may not be relevant for in vivo situations. 

The lowered temperature approach has been linked to flow, temperature jump, photolysis, and nmr methods. Cryoenzymology allows the characterization of enzyme intermediates which have life-times of only milliseconds at normal temperatures, but are stable for hours at low temperatures. Mixed aqueous/organic solvents or even concentrated salt solutions are employed and must always be tested for any adverse effects on the catalytic or structural properties of the enzyme. 

In the polyelectrolyte regime, due to the presence of low-molecular salt, the osmotic pressure of ions becomes less pronounced because the concentration of salt within the network turns out to be less than the concentration of salt in the outer solution n [27]. As the concentration ns grows, the amplitude of the jump of the dependence a(x) decreases and the jump shifts to the region of better solvents (Fig. 2, curve 2). At some critical value of n, the jump on the curve a(x) disappears, i.e. collapse of the network becomes smooth (Fig. 2, curve 3). Under the subsequent increase of n, the curve a(x) becomes closer and closer to the swelling curve of corresponding neutral network (Fig. 2, curves 4). 

At present, we believe that the jump transitions observed in many of the gels studied here represent first order phase transitions. If this is the case, then the gels studied here are among the first found so far in which a first order phase transition occurs near room temperature in pure aqueous solvent with substantial added salt. Early studies by Tanaka s group with poly(acrylamide) based gels required that hydrophobic solvents such as acetone be added for a discontinuous phase transition to be observed near room temperature [6-10]. The more recently studied gels based on poly(n-isopropylacrylamide) [11, 12] and other lower critical solution temperature polymers show discrete phase transitions in water with no salt [11], but the swelling transitions become continuous when moderate amounts of salt are added [12],... 

Figure 7-25 (A) Structural changes occurring upon oxygenation of hemoglobin. After Dickerson144 and Perutz.143 (B) "Rotation at the contact ot, p2 causes a jump in the dovetailing of the CD region of a relative to the FG region of 3 and a switch of hydrogen bonds as shown".143 (C) Some details of the salt bridges.

Enzymatic enantioselectivity in organic solvents can be markedly enhanced by temporarily enlarging the substrate via salt formation (Ke, 1999). In addition to its size, the stereochemistry of the counterion can greatly affect the enantioselectivity enhancement (Shin, 2000). In the Pseudomonas cepacia lipase-catalyzed propanolysis of phenylalanine methyl ester (Phe-OMe) in anhydrous acetonitrile, the E value of 5.8 doubled when the Phe-OMe/(S)-mandelate salt was used as a substrate instead of the free ester, and rose sevenfold with (K)-maridelic acid as a Briansted-Lewis acid. Similar effects were observed with other bulky, but not with petite, counterions. The greatest enhancement was afforded by 10-camphorsulfonic acid the E value increased to 18 2 for a salt with its K-enanliomer and jumped to 53 4 for the S. These effects, also observed in other solvents, were explained by means of structure-based molecular modeling of the lipase-bound transition states of the substrate enantiomers and their diastereomeric salts. 

We now consider a capsule which consists of liquid surrounded by a closed semi-permeable membrane (figure 2) details are provided in [3,4], Water and salt can pass through the membrane from side 1 (inside the capsule) to side 2 (outside), and vice versa, but large polymer molecules cannot. Trapped inside the capsule are n p polyelectrolyte molecules of valence zp and partial molar volume Tip. The resulting Donnan equilibrium is reviewed in [5, 6], Inside the capsule, electroneutrality requires zpn p + z+n + + Z-ri - = 0. We now assume the salt to be monovalent. At equilibrium there is a jump in electrical potential across the membrane inside the capsule x +xi- r X2+X2- x2 with x = (Q =F zpX p) where... 

Time-resolved spectroscopy is performed using a pump-probe method in which a short-pulsed laser is used to initiate a T-jump and a mid-IR probe laser is used to monitor the transient IR absorbance in the sample. A schematic of the entire instrument is shown in Fig. 17.4. For clarity, only key components are shown. In the description that follows, only those components will be described. A continuous-wave (CW) lead-salt (PbSe) diode laser (output power <1 mW) tuned to a specific vibrational mode of the RNA molecule probes the transient absorbance of the sample. The linewidth of the probe laser is quite narrow (<0.5 cm-1) and sets the spectral resolution of the time-resolved experiments. The divergent output of the diode laser is collected and collimated by a gold coated off-axis... 

The formation of the alkaline earth cyanide is the major pathway in the reaction M + BrCN. The other channel (giving MBr + CN) is observed for the reactions of Ba and Sr. The ratio of the cross section is o(BaCN)/ a(BaBr) 25-100 and a(SrCN)/o(SrBr) 250-1000 [363]. It was not possible to measure internal state distribution for the alkaline earth salts, but for the CN product of the minor channel, the vibrational distribution was found to be N(d = 1)/N(p = 0) <. 0.2 and Txot = 1250K for Ba + BrCN and TIot = 750 K for Sr + BrCN. The reaction dynamics appear to be consistent with an electron jump mechanism which would favour the breakup of the M+(BrCN) ion pair to give MCN + Br. 

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