activity

Examples of activity in the following topics:

• Enzyme Catalysis

  • Enzymes are proteins that accelerate biochemical transformations by lowering the activation energy of reactions.
  • Enzymes are proteins that are able to lower the activation energy for various biochemical reactions.
  • At the active site, the substrate(s) can form an activated complex at lower energy.
  • This change stabilizes the transition state complex, and thus lowers the activation energy.
  • An enzyme catalyzes a biochemical reaction by binding a substrate at the active site.

• Transition State Theory

  • The species that is formed during the transition state is known as the activated complex.
  • If the rate constant for a reaction is known, TST can be used successfully to calculate the standard enthalpy of activation, the standard entropy of activation, and the standard Gibbs energy of activation.
  • TST is also referred to as "activated-complex theory," "absolute-rate theory," and "theory of absolute reaction rates."
  • The species that forms during the transition state is a higher-energy species known as the activated complex.
  • Once the activated complex is formed, it can then continue its transformation into products, or it can revert back to reactants.

• Rate of Radioactive Decay

  • Total activity (A) is number of decays per unit time of a radioactive sample.
  • Specific activity (SA) number of decays per unit time per amount of substance of the sample at time set to zero (t = 0).
  • The SI unit of radioactive activity is the becquerel (Bq), in honor of the scientist Henri Becquerel.
  • Since sensible sizes of radioactive material contain many atoms, a Bq is a tiny measure of activity; amounts giving activities on the order of GBq (gigabecquerel, 1 x 109 decays per second) or TBq (terabecquerel, 1 x 1012 decays per second) are commonly used.
  • Low activities are also measured in disintegrations per minute (dpm).

• 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.

• Equilibrium Constant Expression

  • where [X] is the activity of X.
  • The activity of X is equal to the concentration of X if it is a gas or liquid.
  • The activity is equal to 1 if its a pure liquid or solid.
  • Their activity is 1, so they do not need to be written in the equilibrium constant.
  • Since activity is a dimensionless quantity, the equilibrium constant, Keq, is also a dimensionless quantity.

• Compounds Having Two or More Chiral Centers

  • The Chinese shrub Ma Huang (Ephedra vulgaris) contains two physiologically active compounds ephedrine and pseudoephedrine.
  • Both compounds are stereoisomers of 2-methylamino-1-phenyl-1-propanol, and both are optically active, one being levorotatory and the other dextrorotatory.
  • Since these two compounds are optically active, each must have an enantiomer.

• Reactions of Substituent Groups

  • The benzylic hydrogens of alkyl substituents on a benzene ring are activated toward free radical attack, as noted earlier.
  • The strongest activating and ortho/para-directing substituents are the amino (-NH2) and hydroxyl (-OH) groups.
  • By acetylating the heteroatom substituent on phenol and aniline, its activating influence can be substantially attenuated.
  • Although the activating influence of the amino group has been reduced by this procedure, the acetyl derivative remains an ortho/para-directing and activating substituent.
  • However, the overall influence of the modified substituent is still activating and ortho/para-directing.

• Reactions of Alkylidene Complexes

  • The next two equations (# 7 & 8) demonstrate the dienophilic activation provided by a Fischer carbene.
  • In most cases the carbene is a stronger activating group than the corresponding ester.

• Reactions of Aryl Diazonium Salts

  • The resulting amine substituent strongly activates an aromatic ring and directs electrophilic substitution to ortho & para locations.
  • The most important application of diazo coupling reactions is electrophilic aromatic substitution of activated benzene derivatives by diazonium electrophiles.
  • (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.

• Predicting if a Metal Will Dissolve in Acid

  • Some metals can be considered to be more "active" than others, in the sense that a more active metal can replace a less active one from a solution of its salt.
  • Here, zinc is more active than copper because it can replace copper in solution.
  • This sequence is known as the electromotive, or activity, series of the metals.
  • The activity series has long been used to predict the direction of oxidation-reduction reactions.
  • Zinc is near the top of the activity series, meaning that this metal has a strong tendency to lose electrons.