Stickiness of substrates

This approach is a very powerful one both for determining kinetic mechanism and for measuring the stickiness of substrates. References (104) and (119) give examples of its use, and Ref. (120) gives a description of some of the experimental problems caused by impurities in labeled or unlabeled substrates, and how to avoid them. 

The stickiness of substrate is determined by the ratio of the net rate constant for reaction of the first collision complex through the first irreversible step, to the rate constant for dissociation of the collision complex. This ratio can be expressed as... 

Similarly, comparing diminished internal to diminished limiting isotope effects provides a measure of the "stickiness" of substrates (Eqs. (17.36) and (17.37)). 

A designation given to a substrate (or ligand) that displays a strong tendency to remain bound to its binding site, and, in the case of enzymes, to undergo enzymatic catalysis with greater ease than to dissociate. Cleland states that the stickiness of a substrate is a measure of the ratio of the net constant for reaction of the first... 

These hydrogenations are rapid and quantitative. Conversely, the gas-solid hydrogenations of alkenes that were doped by platinum metal compounds on their recrystallization maybe incomplete as, for example, the hydrogenation of trans-cinnamic acid or N-vinylisatin [58,61], which should be milled for completion. Furthermore, the spillover technique by mixing powders of substrate and catalyst followed by application of hydrogen and several hours or days rest [69,70] appears inappropriate for quantitative conversions. Milling is, however, not applicable if volatile liquids have to be constantly pumped off [69] or if the products become liquid or sticky [70]. 

The simple treatment given above is based on the assumption that all proton dissociations are rapid compared to /ccat, that enzyme in only one state of protonation binds substrate, and that ES in only one state of protonation yields products. These assumptions are not always valid. It also assumes that both binding and dissociation of substrate are rapid, that is, to use Cleland s terminology the substrate is not "sticky." For a sticky substrate that dissociates more slowly than it reacts to form products (/c3 > /c2 Eq. 9-54), the values of pKlE will be lowered and pK1e of Eq. 9-53 will be raised by log (1 + k3 / /c2).65/66 In addition to the articles by Cleland, other detailed treatments of pH effects have been prepared by Brocklehurst and Dixon69 and Tipton and Dixon.70... 

Pretreated samples in our study were washed with water until neutralized to pH 7.0, but this step required a lot of water, about 300 times the weight of substrate used, because the pretreated sample was sticky and swollen. It was theorized that there was still a small amount of surfactant left in the washed fibers, and that residual surfactant may help enzymatic hydrolysis. Figure 4 shows the effect of residual surfactant after pretreatment on digestibility at 72 h. To see this effect, newspaper was pretreated without surfactant, and then a given amount of surfactant was added to... 

The main use of adhesives in labelling applications is in the form of pressure-sensitives, i.e., sticky labels attached either directly or indirectly (behind a potential barrier layer) to a foodstuff. Pressure-sensitive adhesives are a distinct category of adhesives that in dry form are permanently tacky at room temperature. These adhesives will adhere to a variety of substrates when applied with pressure they do not require activation by water, heat or solvents and they have sufficient cohesive strength to be handled with the fingers or by mechanical means in labelling stations. 

The relative rates of steps in the mechanism, which is quantitative information on which steps are rate limiting at high or low concentrations of substrates. Are substrates sticky (that is, react once they combine faster than they dissociate) ... 

The log(V/K) profile shows the pKs of groups in substrate or enzyme required for binding, as well as ones involved in catalysis (Fig. 5). Thus, if a substrate binds only as a dianion, the V/K profile will decrease a factor of 10 per pH nnits below the pK of the substrate. If a group on the enzyme has a required protonation state for catalysis, this pK will also appear. The pKs seen in V/K profiles may not appear at their true values if the substrate is sticky (that is, reacts to give products as fast or faster than it dissociates). The pK will be displaced ontward on the profile by log (1 + Sr), where Sr is the stickiness ratio (the ratio of the net rate constant to produce the first product and the off-rate constant for the substrate). Comparison of pKs in V/K and pKi profiles allows one to determine stickiness of the substrate. 

As long as EA and EAH can be interconverted directly (i.e., ks and have finite values) and EAH forms, the low-pH asymptote with a slope of 1 intersects the high-pH plateau at lpA(i - log(l + 3//t2)l- Thus, the apparent pK is displaced outward by the stickiness of the substrate. In a mechanism more complicated than Mechanism (76) the displacement is log(l + 8]), where S, is a stickiness ratio that is slightly different from that determined by the isotope partitioning method in Section V1,A above [the stickiness of the first product released also contributes to the ratio see Ref. (i)j, but in a simple mechanism such as Mechanism (76) S, and S, are the same. 

If EAH does not form in Mechanism (76) (in which case the pK is seen in the V/K profile, but not in the V profile), or if EAH is only a dead-end complex and ks and kf, are zero, the V/K profile has the simple shape predicted by Eq. (73), and the pK is not perturbed by the stickiness of the substrate and is seen in its correct position. Thus, the displacement of a pK by the stickiness of a substrate requires that incorrectly protonated E-substrate complexes form and that, without dissociation of substrate, they be interconverted at a reasonable rate to a... 

When the p T in Mechanism (76) is above 7, the interconversion of EA and EAH will involve reaction with hydroxide, rather than with protons, k(, will be unimolecular, and fc5[OH ] will replace k. The p f now is displaced by the stickiness of the substrate only when EA and EAH are rapidly equilibrated, and the degree of displacement decreases as kf, becomes equal to or less than 2 [see Ref. (i) for a fuller discussion]. 

Mechanism (77) applies to mechanisms where pKf is above 7. If it is below 7 and equilibration of EA and EAH involves protons, rather than hydroxide, the stickiness of the substrate will displace the pAT in the V/K profile only if the interconversion of EA and EAH is fast (5). 

With alanine dehydrogenase, (V/ Taianine) was 1.4 at the pH optimum and increased to 2.0 below pH 7, whereas (WATserine) was 2.0 at all pH values (serine is a slow, nonsticky substrate) (Si). The internal commitments appear to be the same for both substrates and are sizeable, since the intrinsic isotope effect is likely to be 5 or 6. In cases like this, a rough estimate of the stickiness of the substrate can be obtained from the following equation ... 

While the displacement of the plC is a sufficient problem, a further difficulty with sticky substrates is the alteration in shape of the pH profile in the vicinity of pJEa. which can occur with certain values of the rate constants. This alteration in shape can take the form of a hoUow, and very infrequently a form of a hump. The hollow in the pH profile results when proton movement into and out of the active site is restricted, so that the state of protonation of the enzyme-substrate complex is not equiUbrated rapidly with respect to the rate of reaction to give products or the rate of substrate dissociation (Section 14.4). 

Thus, the main cause of perturbation of p/Ca values in the V profiles is the fact that the chemical reaction is not rate-limiting. However, when the stickiness of a substrate does not displace the pK a values in V profiles, it can lead to more complex shapes than the simple one corresponding in algebraic form to equation = (V /i )/(i-i-H /i a)i if proton access to the groups involved is restricted in the central productive complex of enzyme wiA substrates. When both the substrate that dissociates most rapidly from the central complex and the proton on the group in question are sticl, the curve will have a hollow in the vicinity of the pK. ... 

The method of isotope trapping allows one to detemrine the stickiness of all substrates but the last to combine in an ordered mechanism and of all substrates in a random mechanism (Rose et al, 1974 Rose 1995). This method was developed initially by Rose for the yeast hexokinase reaction, and it is essentially a single turnover experiment in which one detemtines analytically the proportion of an enzyme-substrate complex that reacts to give products, as opposed to dissociating. 

The tack of the adhesive indicates the stickiness of the hot-melt as it changes from a liquid to a solid state. This property affects the ability of the adhesive to hold the substrates together. 

In some cases, plasticization of a PSA may be detrimental to its performance. A well-known example is the deterioration of the performance of an adhesive applied to plasticized PVC. Migration of the plasticizer from the flexible vinyl into the PSA often softens the adhesive to the point where it fails cohesively from the vinyl, leaving sticky residue behind during removal of the adhesive-coated article from the substrate. One way to address this detrimental effect of plasticizer migration is to formulate an already plasticized PSA, perhaps because a better balance exists between the plasticizer in the PVC substrate and the PSA in contact with it [101]. 

As enzymatic oxidative transformation of the PVA polymer can act as a multiple simultaneous event on the polymer with concurrent chain fission by the appropriate enzymes, the polymer can be broken down into small oligomers that can be channelled into the primary metabolism. This picture is not complete because PVA is usually more or less acetylated. The DH is a pivotal factor in almost every aspect of PVA application. Surprisingly there are very few data dealing with the enzymes involved in the deacetylation of not fuUy hydrolysed PVA polymer. In technical processes, esterase enzymes are widely applied to deal with PVAc structures. A good example is from the pulp and paper industry [85], where PVAc, a component of stickies , is hydrolysed to the less sticky PVA. Esterases from natural sources are known to accept the acetyl residues on the polymer as substrate but little detailed knowledge exists about the identity of acetyl esterases in the PVA degradative environment [86]. 

The limits to the areal density of deposit for filters are set by clogging of the filter that sets in at typically 100 xg/cm2. The limit of areal density for impactors is set by the problem of particle bounce. This is a serious problem for coarse, dry aerosols but less so for fine, wet, secondary aerosols. Nevertheless, sticky substrates are universally used (19), and deposits are generally limited to a few monolayers of particles for a 2.5- xm particle. This limit amounts to no more than 7 xm of deposit, or, for 1.5- xg/m3 aerosols (per stage), about 1000 xg/cm2 of deposit. 

Extracts dried on to a water-soluble base are useful in the preparation of powdered drinks. Soft extracts tend to contain about 70% solid matter and can be mixed into a slun y with the substrate and spray-dried or dried in a vacuum oven. The substrate is usually essential to prevent the dried extract from reabsorbing moisture and turning back into a hard or sticky mass. The dried extracts can be dry-blended with other ingredients in a powdered drink formulation. 

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