Positively acting substance

Each gene would need to have common regulatory elements, promoters, operators, or both, associated with it. In this way, a single control factor, such as a repressor or positively acting substance, could influence all genes simultaneously. 

This example shows the importance of so-called activators, or promoters, for increasing the activity and selectivity of contact mass. These additives can sharply activate the reaction and shift it in a certain direction. Various substances have different effects on the activity of contact mass. For example, antimony has a positive effect on the direct synthesis of organochlorosilanes and increases the total yield of methylchlorosilanes, whereas lead and bismuth reduce the formation of these substances. How-ever, the positive effect of a promoter manifests itself only in a certain concentration, exceeding which transforms a positively acting additive into poison or an inhibitor of the reaction. For example, in a 0.002—0.005% concentration antimony is a promoter of the direct synthesis of methylchlorosilanes on the other hand, in a concentration higher than 0.005% it becomes poison. 

When the hydrogen is in one position, the substance will act as an hydroxy-compound with hydrogen in the other position, as a ketone. Substances possessing this double function are called tautomeric. 

In some cases, the catalyst is a solid substance on whose surface a reactant molecule can be held (adsorbed) in a position favorable for reaction until a molecule of another reactant reaches the same point on the solid. Metals such as iron, nickel, platinum and palladium seem to act in this way in reactions involving gases. There is evidence that in some cases of surface adsorption, bonds of reactant particles are weakened or actually broken, thus aiding reaction with another reactant particle. 

The cluster condensation can be carried on the chains of octahedra sharing edges can be joined to double-strands and finally to layers of octahedra (Fig. 13.18). Every layer consists of metal atoms in two planes arranged in the same way as two adjacent layers of atoms in a closest-packing of spheres. This is simply a section from a metal structure. The X atoms occupy positions between the metal layers and act as insulating layers. Substances like ZrCl that have this structure have metallic properties in two dimensions. 

Monsanto s experience illustrates both the positive and negative impacts on one chemical company under TSCA sections 5 and 8. For a broader perspective, refer to the Chemical Manufacturers Association publication "The First Four Years of the Toxic Substance Control Act." - The authors views are also presented relative to EPA s management of information under section 10. 

Aqueous ammonia is a weak base, so relatively few hydroxide ions form. The position of equilibrium lies to the left. In the forward reaction, the water molecule gives up a proton and acts as an acid. A substance that can act as a proton donor (an acid) in one reaction and a proton acceptor (a base) in another reaction is said to be amphoteric. (Water acts as an acid in tbe presence of a stronger base, and as a base in tbe presence of a stronger acid. 

GHB has met a somewhat different fate in the United States, however. In 1990, the FDA banned the sale of the drug in the United States because of its concerns over possible risks to human health. In view of the generally positive research on the drug s use, the sci-entihc basis of the FDA s decision was not entirely clear at the time to some observers. In any case, the drug s legal status was clarified in 2000 when the DEA classihed it as a Schedule I drug under the Controlled Substances Act. 

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