Substrates, terminology

Imagine that this reaction is catalyzed by substance E. In general the substrate concentration will greatly exceed that of the catalyst. If E is an enzyme, for example, concentrations such as [S]o = 10-3 M and [E]o = 10-9 M are not atypical. The development will continue to be expressed in the terminology used by enzymologists even though the material presented applies to catalytic reactions in general. 

The terminology of L-B films originates from the names of two scientists who invented the technique of film preparation, which transfers the monolayer or multilayers from the water-air interface onto a solid substrate. The key of the L-B technique is to use the amphiphih molecule insoluble in water, with one end hydrophilic and the other hydrophobic. When a drop of a dilute solution containing the amphiphilic molecules is spread on the water-air interface, the hydrophilic end of the amphiphile is preferentially immersed in the water and the hydrophobic end remains in the air. After the evaporation of solvent, the solution leaves a monolayer of amphiphilic molecules in the form of two-dimensional gas due to relatively large spacing between the molecules (see Fig. 15 (a)). At this stage, a barrier moves and compresses the molecules on the water-air interface, and as a result the intermolecular distance decreases and the surface pressure increases. As the compression from the barrier proceeds, two successive phase transitions of the monolayer can be observed. First a transition from the gas" to the liquid state. 

Consider reorientations of a diatomic surface group BC (see Fig. A2.1) connected to the substrate thermostat. By a reorientation is meant a transition of the atom C from one to another well of the azimuthal potential U(qi) (see Fig. 4.4)). The terminology used implies a classical (or at least quasi-classical) description of azimuthal motion allowing the localization of the atom C in a certain well. A classical particle, with the energy lower than the reorientation barrier Awhich does not interact with the thermostat cannot leave the potential well where it was located initially. The only pathway to reorientations is provided by energy fluctuations of a particle which arise from its contact with the thermostat. Let us estimate the average frequency of reorientations in the framework of this classical approach. 

As we saw earlier in this chapter, substrates are the molecules which undergo chemical change as a result of enzyme activity. Many enzymes will only operate when in the presence of essential co-factors or coenzymes. The term coenzyme is not entirely appropriate as it implies that, like enzymes themselves, these compounds do not undergo chemical change. This is not true and more accurate terminology would be co-substrate. Coenzymes are always much smaller than the enzymes with which they operate and are not heat sensitive as are the proteins. 

The manifold intermediates in homogeneous transition-metal catalysis are certainly metal complexes and therefore show a behaviour like ordinary coordination compounds associations of phosphorus donors open up multifarious additional controls. Both, substrates and P ligands are Lewis bases that we have to consider and that compete at the coordination centers of the metal, leading to competitive, non-competitive or uncompetitive activation or inhibition processes in analogy to the terminology of enzyme chemistry... 

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... 

The idea of complementarity in enzyme - substrate interactions was introduced by E. Fischer with his famous lock and key analogy.7 In modem terminology this would represent enzyme-substrate complementarity. The currently favored concept of enzyme-transition state complementarity was introduced by Haldane1 and elaborated by L. Pauling.2... 

For a single substrate-single product reaction with one path (assume path A is insignificant), the pH dependence of ka/Ka reflects the ionization of groups on the free enzyme (Ka, Kb) and free substrate and is not affected by any of the intermediates. The pH dependencies of ka and Kb separately are more complicated and, as well as Kb and Kb, involve KJ and Kb which depend on the number of intermediates and steady state rate constants. Using the terminology of del Rosario and Hammes (499). 

This section presents a summary of the currently preferred conditions for performing Ti-catalyzed asymmetric epoxidations and is derived primarily from the detailed account of Gao et al. [4]. We wish to draw the reader s attention to several aspects of the terminology used here and throughout this chapter. The terms Ti-tartrate complex and Ti-tartrate catalyst are used interchangeably. The term stoichiometric reaction refers to the use of the Ti-tartrate complex in a stoichiometric ratio (100 mol %) relative to the substrate (allylic alcohol). The term catalytic reaction (or quantity) refers to the use of the Ti-tartrate complex in a catalytic ratio (usually 5-10 mol %) relative to the substrate,... 

Diabetes mellitus is a disease caused by insufficient insulin secretion or a decrease in the peripheral effects of insulin. This disease is characterized by a primary defect in the metabolism of carbohydrates and other energy substrates. These metabolic defects can lead to serious acute and chronic pathologic changes. The term diabetes mellitus differentiates this disease from an unrelated disorder known as diabetes insipidus. Diabetes insipidus is caused by a lack of antidiuretic hormone (ADH) production or insensitivity to ADH. Consequently, the full terminology of diabetes mellitus should be used when referring to the insulin-related disease. Most clinicians, however, refer to diabetes mellitus as simply diabetes. ... 

Experimental data from nucleophilic substitution reactions on substrates that have optical activity (the ability to rotate plane-polarized light) shows that two general mechanisms exist for these types of reactions. The first type is called an S 2 mechanism. This mechanism follows second-order kinetics (the reaction rate depends on the concentrations of two reactants), and its intermediate contains both the substrate and the nucleophile and is therefore bimolecular. The terminology S 2 stands for substitution nucleophilic bimolecular. ... 

Enzymology has its own terminology. Reactants and reagents are generally referred to as substrates. Some enzymes only function when bound to additional, non-protein species. These helpers are called cofactors. The combination enzyme + cofactor is called a holoenzyme, while an enzyme that is missing its cofactor is called an apoenzyme. Cofactors can be either inorganic (e.g., metal ions) or organic. If... 

The only terminology specific to the DIDB pertains to drugs/compounds that appear as object or precipitant depending on their role in specific DI. Object refers to a compound that acts as the modified agent (i.e., substrate), and precipitant refers to a compound that acts as the causative agent. A precipitant can be an inhibitor, inducer, or activator, but may not have any effect. 

Thus, in lithium chloride/HMPA the process responsible for the cathodic limit of the SSE is reduction of the HMPA molecule to give the HMPA radical anion = esolv in the normally used terminology. We then encounter exactly the same mechanistic problem with respect to the electroactive species as earlier for anodic processes is the process a direct reduction of the substrate (76) or an indirect one (77) mediated by the solvent (denoted S) radical anion Dealing with the semantic problem does not change the nature of the problem,... 

Substrate configuration — Terminology used in - photovoltaic devices to designate the device architecture in which the light enters the device through the top face of the cell, i.e., oppositely to the substrate. 

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