Sticky substrates

ISOTOPE TRAPPING STICKY SUBSTRATES LIGAND EXCLUSION MODEL LIGAND FIELD SPLITTING CRYSTAL FIELD SPLITTING LIGAND-INDUCED CONFORMATIONAL CHANGE... 

BIOCHEMICAL NOMENCLATURE STEROID SULFOTRANSFERASE STICKY SUBSTRATES ISOTOPE TRAPPING STIFFNESS (or Stiffness Instability)... 

ISOTOPE TRAPPING STICKY SUBSTRATES Substrate-induced conformational change, INDUCED FIT MODEL SUBSTRATE INHIBITION ABORTIVE COMPLEX FORMATION LACTATE DEHYDROGENASE LEE-WILSON EQUATION... 

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. 

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... 

Nearest the sap inlet of the back pan it is composed of a protein-rich, sticky substrate that is rich in calcium malate and is probably largely caused by the denaturing of organic substances and microbes. 

Although the pH profiles for nonsticky substrates have simpler shapes and are easier to interpret, there is useful information in the pH profiles for a sticky substrate, and thus one must know what to expect, and how to interpret the results. The pK values may be perturbed in the V/K profile for a sticky substrate, and both the V/K and V profiles may show a hollow, or at least a flatter shape, near the pK. Consider the following mechanism in which protonation of a group with a pK less than 7 prevents catalysis ... 

Particle size may be difficult to characterize exactly, especially if the particles have irregular shapes. Possibly the best way to characterize particles is to determine the characteristic length (the ratio of voliune to siuface area) since this can be used to extrapolate between different shapes. Particle size distributions can be done by passing the substrate through a series of meshes with different aperture sizes, although this can be difficult with moist or sticky substrates, and in any case will work most effectively for roughly cubic or spherical particles. 

If the substrate is sticky (i.e., dissociates more slowly than it reacts to give products), the plQ values will not be seen in the correct position on the profile, but will be displaced outward (i.e., to lower pH when protonation decreases activity, and to higher pH when protonation increases V/K). The amount of displacement will be Iog(i + k /kz), where is the net rate constant for reaction of the collision complex to yield products, and is for dissociation. With a sticky substrate, the displacement can be a pH unit or more, although values of 0.5-1.0 pH unit are more common. 

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). 

Substrates are normally sticky in the reaction direction with the higher maximum velocity, and not always then. In the direction with the lower maximum velocity, substrates cannot normally be sticky because their release rates from the enzyme must exceed the more rapid maximum velocity in the reverse direction. Thus, one needs to use the pH profiles of slow substrates only in the direction with the fastest maximum velocity in the reverse direction, one can use the profiles of the normal substrates for comparison. Further, one should use the V/K profiles for nonsticky substrates to determine the pK a values. Then, one should determine the nature of the catalytic groups by solvent perturbation and temperature variation of the pJta values, and then, by comparison with the V/K profile of the normal sticky substrate determine the stickiness and, from the shape of the profile in the vicinity of the pJCn, how rapidly the protonation state of the group is equilibrated when the substrate is present (Rose et al, 1974 Cleland, 1982). 

The definition is intended to differentiate these adhesives from merely sticky materials like flypaper or materials that may have only substrate specific adhesion. 

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]. 

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... 

If the substrate is also sticky k2 ks), we arrive at the diffusion-controlled parameter ki... 

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]. 

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]. 

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. 

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... 

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