Partitioning factor

For these sequences the value of Gj, is less than a certain small value g. For such sequences the folding occurs directly from the ensemble of unfolded states to the NBA. The free energy surface is dominated by the NBA (or a funnel) and the volume associated with NBA is very large. The partition factor <6 is near unify so that these sequences reach the native state by two-state kinetics. The amplitudes in (C2.5.7) are nearly zero. There are no intennediates in the pathways from the denatured state to the native state. Fast folders reach the native state by a nucleation-collapse mechanism which means that once a certain number of contacts (folding nuclei) are fonned then the native state is reached very rapidly [25, 26]. The time scale for reaching the native state for fast folders (which are nonnally associated with those sequences for which topological fmstration is minimal) is found to be... 

We begin our analysis by comparing the surface fluxes. According to the indicated partitioning factors, 74% of the 11 Mg DMS-S/m /h emitted from the ocean surface should be returned as nss-SO in rain. This leads to a predicted wet deposition flux of nss-SO of 8.1 Mg S/(m /h), which is 37% lower than the measured flux of 13 Mg S/(m /h). Since the estimated accuracy of the DMS emission flux is 50% (Andreae, 1986), this is about as good agreement as can be expected. It indicates that our "closed system" assumption is at least a reasonable first approximation. (A more sophisticated treatment would consider sulfur oxida-... 

FIGURE 14.5 Comparison of partition factors for a PAH (biphenyl) and polypeptide (lysozyme) versus percent acetonitrile concentration. 

It is a great deal of work to actually determine a true equilibrium constant and most chemical separation methods speak in terms of values which are proportional to the actual equilibrium constant. At constant flow, the time that a given type of molecule is retained is related to the time for the void volume to pass after the sample is placed in a column or on a plate with the addition of the time for the net retention volume. If the flow remains constant, the temperature of the separation remains constant and no stationary phase is gained or lost, one can attempt qualitative identification using retention times. It is more reasonable to calculate the ratio of net retention volume to the void volume and call the result partition factor or capacity factor, k. ... 

Using the various simplifications above, we have arrived at a model for reaction 11.9 in which only one step, the chemical conversion occurring at the active site of the enzyme characterized by the rate constant k3, exhibits the kinetic isotope effect Hk3. From Equations 11.29 and 11.30, however, it is apparent that the observed isotope effects, HV and H(V/K), are not directly equal to this kinetic isotope effect, Hk3, which is called the intrinsic kinetic isotope effect. The complexity of the reaction may cause part or all of Hk3 to be masked by an amount depending on the ratios k3/ks and k3/k2. The first ratio, k3/k3, compares the intrinsic rate to the rate of product dissociation, and is called the ratio of catalysis, r(=k3/ks). The second, k3/k2, compares the intrinsic rate to the rate of the substrate dissociation and is called forward commitment to catalysis, Cf(=k3/k2), or in short, commitment. The term partitioning factor is sometimes used in the literature for this ratio of rate constants. 

If the overall reaction rate is controlled by step three (k3) (i.e. if that is the rate limiting step), then the observed isotope effect is close to the intrinsic value. On the other hand, if the rate of chemical conversion (step three) is about the same or faster than processes described by ks and k2, partitioning factors will be large, and the observed isotope effects will be smaller or much smaller than the intrinsic isotope effect. The usual goal of isotope studies on enzymatic reactions is to unravel the kinetic scheme and deduce the intrinsic kinetic isotope effect in order to elucidate the nature of the transition state corresponding to the chemical conversion at the active site of an enzyme. Methods of achieving this goal will be discussed later in this chapter. 

In the study of isotope effects, the isotope partitioning factor at a particular site is independent of the isotopic composition at any other site. Thus, an isotope effect for a diisotopically substituted species should be the product of the kinetic isotope effects observed for each of the monoisotopically substituted compounds. 

Values a and b for the fission product isotopes and the partition factors ai and a2 are listed in Table V au for a given isotope, is the fraction which was retained by the local fallout glass particles, and < > is the fraction released to the cloud. Thus, from Table V, i137 is 0.153 which indicates that 15.3% of the 137Cs is retained by the local glass particles. It is interesting to note that the independent yield of cesium in the 137 mass chain is approximately 17%—the balance of the chain is formed as tellurium, iodine, and xenon. 

The equilibrium partition factor is defined as the ratio of the concentration of species A inside and outside the pores. This concept was first introduced by Ferry in terms of a geometric exclusion effect [25]. Since the center of mass of the molecule, assumed to be a hard sphere, cannot be closer to the pore wall than the distance of the molecular radius, Ferry obtained... 

The enrichment or partition factor f is is the relation between the concentrations of an analyte in two phases in our case between the membrane polymer on the one hand and the solvent, gas or aqueous solution, on the other ... 

FIGURE 6.3 (a) Heat partition factors for lubricated sliding contact of polyurethane against copper with a thin CU2O layer, (b) Extracted... 

The small isotope effects found most likely arise from the reversibility of step 1 by a partitioning ejfect The rate at which ArHY+ reverts to ArH should be essentially the same as that at which ArDY" " (or ArTY" ") reverts to ArD (or ArT), since the Ar—H bond is not cleaving. However, ArHY" " should go to ArY faster than either ArDY" " or ArTY", since the Ar—H bond is broken in this step. If k2 3> k-i, this does not matter since a large majority of the intermediates go to product, the rate is determined only by the slow step (ki[ArH][Y ]) and no isotope effect is predicted. However, if kistarting materials is important. If 2 for ArDY" " (or ArTY" ") is < k2 for ArHY+, but k i is the same, then a larger proportion of ArDY reverts to starting compounds. That is, 2/ -1 (the partition factor) for ArDY" " is less than that for ArHY". Consequently, the reaction is slower for ArD than for ArH and an isotope effect is observed. 

One circumstance that could affect the 2/ -1 ratio is steric hindrance. Thus, diazonium coupling of 6 gave no isotope effect, while coupling of 8 gave a kn/kj) ratio of 6.55. For steric reasons, it is much more difficult for 9 to lose a proton (it is harder for a base to approach) than it is for 7, so 2 is greater for the latter. Since no base is necessary to remove ArN2", k- does not depend on steric factors and is about the same for each. Thus the partition factor 2/ -1... 

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