Kinetic window

Using sub-ambient temperatures for preparing the protein-ligand equilibrium mixtures and for centrifugation of the GPC spin column, the dissociation rate constant decreases and the off-rate diminishes, thereby expanding the kinetic window observable with GPC spin column screening to even weaker binders with Kd values >20 pM. 

There are a number of non-electrochemical techniques that have proven invaluable in combination with electrochemical results in understanding the chemistry and the kinetics. Laser flash photolysis (LFP) is a well-established technique for the study of the transient spectroscopy and kinetics of reactive intermediates. The technique is valuable for the studying of the kinetics of the reactions of radical anions, particularly those that undergo rapid stepwise dissociative processes. The kinetics of fragmentation of radical anions can be determined using this method if (i) the radical anion of interest can be formed in a process initiated by a laser pulse, (ii) it has a characteristic absorption spectrum with a suitable extinction coefficient, and (iii) the rate of decay of the absorption of the radical anion falls within the kinetic window of the LFP technique typically this is in the order of 1 x 10" s to 1 X 10 s . 

The approximate kinetic window of DPSCA at electrodes with a diameter in the 0.5-1 mm range is given by the expression 0.3 s-1 < k < 300 s 1, where k is a first-order or pseudo-first-order rate constant. With the application of microelectrodes, the upper limit can be extended considerably. 

FIGURE 9.6 Photosensitized hydrogen abstraction from triethylamine DH by 9,10-anthra-quinone A (for the formulas, see Chart 9.3). The rate constant of in-cage deprotonation as obtained from the polarity pattern, is shown as a function of the relativity permittivity e of the reaction medium (mixtures of acetonitrile and chloroform). The timescale of the CIDNP effect provides a kinetic window, within which such a quantitative treatment is apphcable. Further explanation, see text. 

Figure 21 CIDNP investigation of the in-cage deprotonation of the triethylamine radical cation by the radical anion of 9,10-anthraquinone (solid line) and 2,7-dinitrofluorenone (broken line). The graph shows the deprotonation rate constant as function of the relative permittivity of the solvent. The shaded areas denote the approximate boundaries of the kinetic window accessible by the CIDNP experiments. Further explanation, see text and Chart 5. Reproduced from Ref. 60 with permission copyright (2003) American Chemical Society.

To keep within the kinetic window of time-resolved CIDNP, the reaction rate is controlled by the alkene concentration. The method was reported to... 

Investigation of chemical kinetics meet two limitations in direct current polarography (DCP) operating with dropping mercury electrodes the first is the narrow kinetic window given by the drop time 0.1 s < < 5 s... 

In polymers, motions are slowed down compared to when they occur in low molar mass liquid crystals. This often brings processes that are too fast in low molar mass liquid crystals into the kinetic window of line shape analysis or even 2D exchange experiments. Thus motional processes that cannot be studied in low molar mass liquid crystals become accessible in polymers [19]. Examples of NMR applications to liquid-crystalline polymers are given in the following section. 

Inverted regions have been observed in very exothermic inter-molecular electron transfers and in other systems where the relative motions of donor and acceptor are somewhat restricted. In such conditions, diffusion is not the rate-determining step and a wider kinetic window is open for the observation of the inverted region. Another example of such systems is the charge recombination of contact radical-ion pairs formed after charge... 

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