Sliding activity path

Fixed-activity and sliding-activity paths (Sections 14.2-14.3) are analogous to their counterparts in fugacity, except that they apply to aqueous species instead of gases. Fixed-activity paths are useful for simulating, for example, a laboratory experiment controlled by a pH-stat, a device that holds pH constant. Sliding-... 

We set a 0.1 molal NaCl solution and define a sliding activity path in which pH varies from 4 to 12... 

In a final example of the use of a sliding activity path, we calculate a speciation diagram, plotted versus pH, for hexavalent uranium in the presence of dissolved phosphate at 25 °C. We take a 10 mmolal NaCl solution containing 1 mmolal each ofUO +, the basis species for U(VI), and HPO4... 

Fig. 14.12. Speciation diagram at 25 °C for a 1 mmolal solution of hexavalent uranium containing 1 mmolal dissolved phosphate, calculated as a sliding activity path.

FIG. 12.11 Fractions of various components sorbed onto a hydrous ferric oxide surface at 25°C, as functions of pH, for the sliding activity path shown in Fig. 12.10. 

Sliding activity and sliding fugacity paths are similar to fixed activity and fixed fugacity paths, except that the model varies the buffered activity or fugacity over the reaction path rather than holding it constant. Once the equilibrium state of the initial system is known, the model stores the initial activity a° or initial fugacity / / of the buffered species or gas. (The modeler could set this value as a constraint on the initial system, but this is not necessary.)... 

The relative magnitude of these two activation free energies determines the size and shape of the critical nucleus, and hence of the resulting crystal. If sliding diffusion is easy then extended chain crystals may form if it is hard then the thickness will be determined kinetically and will be close to lmin. The work so far has concentrated on obtaining a measure for this nucleus for different input parameters and on plotting the most likely path for its formation. The SI catastrophe does not occur because there is always a barrier against the formation of thick crystals which increases with /. 

For cooperative functions of the polymerase and exonuclease, the 3 terminus of DNA is presumed to shuttle rapidly without dissociation between the two active sites which are 30 A apart. The sliding path involves 4—5 bp of duplex DNA plus four to five bases of the single-stranded frayed end. Photolabeling studies with azido-DNA also suggest that the polymerase active site makes contacts with five to seven bases of duplex DNA (60). When a duplex contains unpaired bases in the primer strand, the 3 primer terminus resides predominantly at the exonuclease site. The RDS for exonuclease activity of Pol Ik is thought to be the transfer of the primer terminus from the polymerase to the exonuclease site. 

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