active site

Examples of active site in the following topics:

• Enzyme Catalysis

  • They do this by binding the reactant(s), known as the substrate(s), to an active site within the enzyme.
  • At the active site, the substrate(s) can form an activated complex at lower energy.
  • Once the reaction completes, the product(s) leaves the active site, so the enzyme is free to catalyze more reactions.
  • This model proposes that the binding of the reactant, or substrate, to the enzyme active site results in a conformational change to the enzyme.
  • An enzyme catalyzes a biochemical reaction by binding a substrate at the active site.

• Biomolecules

  • Coordination complexes are found in many biomolecules, especially as essential ingredients for the active site of enzymes.
  • The transition metals, particularly zinc and iron, are often key components of enzyme active sites.
  • As with all enzymes, the shape of the active site is crucial.
  • The structure of the active site in carbonic anhydrases is well known from a number of crystal structures.
  • Active site of carbonic anhydrase.

• Iron

  • Iron, the active site of many redox enzymes, has many oxidation states, but ferrous (Fe2+) and ferric (Fe3+) are the most common.
  • Iron is also the metal used at the active site of many important redox enzymes dealing with cellular respiration, oxidation, and reduction in plants and animals.

• Characteristics of Specific Substitution Reactions

  • However, the presence of a second strongly-activating substituent group permits acylation; the site of reaction is that favored by both substituents.
  • Friedel-Crafts alkylation, on the other hand, introduces an activating substituent (an alkyl group), so more than one substitution may take place.
  • The fourth example illustrates the poor orientational selectivity often found in alkylation reactions of activated benzene rings.
  • The bulky tert-butyl group ends up attached to the reactive meta-xylene ring at the least hindered site.
  • This may not be the site of initial bonding, since polyalkylbenzenes rearrange under Friedel-Crafts conditions (para-dipropylbenzene rearranges to meta-dipropylbenzene on heating with AlCl3).

• Substitution Reactions of Benzene Derivatives

  • In both cases the charge distribution in the benzene ring is greatest at sites ortho and para to the substituent.
  • Since a mono-substituted benzene ring has two equivalent ortho-sites, two equivalent meta-sites and a unique para-site, three possible constitutional isomers may be formed in such a substitution.
  • Thus, substituents that activate the benzene ring toward electrophilic attack generally direct substitution to the ortho and para locations.
  • Clearly, the alkyl substituents activate the benzene ring in the nitration reaction, and the chlorine and ester substituents deactivate the ring.
  • Consequently, all these substituents direct substitution to ortho and para sites.

• Electrophilic Substitution of Disubstituted Benzene Rings

  • The activation or deactivation of the ring can be predicted more or less by the sum of the individual effects of these substituents.
  • The site at which a new substituent is introduced depends on the orientation of the existing groups and their individual directing effects.
  • Symmetry, as in the first two cases, makes it easy to predict the site at which substitution is likely to occur.
  • Note that if two different sites are favored, substitution will usually occur at the one that is least hindered by ortho groups.
  • The strongly activating hydroxyl (–OH) and amino (–NH2) substituents favor dihalogenation in examples 5 and six.

• Three-Membered Rings

  • Example 2 reflects the SN2 character of nucleophile (chloride anion) attack on the protonated aziridine (the less substituted carbon is the site of addition).
  • The α-lactone intermediate shown in the solvolysis of optically active 2-bromopropanoic acid (example 9) accounts both for the 1st-order kinetics of this reaction and the retention of configuration in the product.
  • The π-electron system of the substituent assists development of positive charge at the adjacent oxirane carbon, directing nucleophilic attack to that site.

• Reactions of Aryl Diazonium Salts

  • The resulting amine substituent strongly activates an aromatic ring and directs electrophilic substitution to ortho & para locations.
  • (i) At acid pH (< 6) an amino group is a stronger activating substituent than a hydroxyl group (i.e. a phenol).
  • At alkaline pH (> 7.5) phenolic functions are stronger activators, due to increased phenoxide base concentration.
  • (ii) Coupling to an activated benzene ring occurs preferentially para to the activating group if that location is free.
  • (iii) Naphthalene normally undergoes electrophilic substitution at an alpha-location more rapidly than at beta-sites; however, ortho-coupling is preferred.

• Reaction Rates and Kinetics

  • The potential energy of a reacting system changes as the reaction progresses.The overall change may be exothermic ( energy is released ) or endothermic ( energy must be added ), and there is usually an activation energy requirement as well.
  • The distribution of electrons at sites of reaction (functional groups) is a particularly important factor.

• Reaction of Amines with Nitrous Acid

  • Depending on ring substitution, 3º-Aryl amines may undergo aromatic ring nitrosation at sites ortho or para to the amine substituent.
  • The nitrosonium cation is not sufficiently electrophilic to react with benzene itself, or even toluene, but highly activated aromatic rings such as amines and phenols are capable of substitution.
  • Once nitrosated, the activating character of the amine nitrogen is greatly diminished; and N-nitrosoaniline derivatives, or indeed any amide derivatives, do not undergo ring nitrosation.