Molecular Biochemistry: Electron Transfer Reactions
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Questions and Answers

What does ΔGº represent in the context of Marcus Theory?

  • The standard free energy of activation
  • The Gibbs energy of the transition state
  • The change in Gibbs energy of the reaction (correct)
  • The difference in energy between reactants and products

    • In Marcus Theory, what condition is indicated by -ΔGº > λ?

  • The reaction is very favorable with significant reorganization energy (correct)
  • The reaction has a high activation energy
  • The transition state is more stable than the reactants
  • The reaction rate increases due to low activation energy

    • What happens to the reaction rate as ΔGº increases beyond λ?

  • The reaction rate remains constant
  • The reaction rate decreases (correct)
  • The reaction rate increases linearly
  • The reaction rate becomes unpredictable

    • What does a situation with -ΔGº = λ imply about the reorganization energy?

    <p>Reorganization energy is equal to the driving force of the reaction</p> Signup and view all the answers

    Which statement is true regarding reactions with no reorganization energy in Marcus Theory?

    <p>The reactions have minimal transition state stability</p> Signup and view all the answers

    What does the symbol λ represent in the equation λ = h / mv?

    <p>The wavelength of the wave/particle</p> Signup and view all the answers

    Which variable in the formula λ = h / mv is affected by the uncertainty principle?

    <p>v (velocity of the particle)</p> Signup and view all the answers

    In quantum mechanics, what does the amplitude of oscillation indicate?

    <p>The probability of finding a particle in a certain state</p> Signup and view all the answers

    Which of the following is NOT a factor in determining the wavelength λ?

    <p>The atomic number of the element</p> Signup and view all the answers

    How is the relationship between energy and distance characterized in the context of the graph shown?

    <p>Exponential decay of energy with distance</p> Signup and view all the answers

    Which model of kinetics is specifically used for reactions involving the making and breaking of bonds?

    <p>Transition State Theory (TST)</p> Signup and view all the answers

    Which model is associated with electron transfer processes?

    <p>Marcus model</p> Signup and view all the answers

    What is the primary focus of the more advanced models of kinetics mentioned?

    <p>Thermodynamics and binding structure</p> Signup and view all the answers

    Which of the following is NOT mentioned as a model of kinetics?

    <p>Folk Theory</p> Signup and view all the answers

    The Marcus like model is primarily used to study which of the following?

    <p>Tunneling of protons</p> Signup and view all the answers

    What is the primary characteristic of electron transfer reactions?

    <p>They are the simplest class of reactions.</p> Signup and view all the answers

    Which species is NOT typically involved in electron transfer reactions?

    <p>Catalysts</p> Signup and view all the answers

    In the equation D + A → D+ + A−, what does D represent?

    <p>An electron donor</p> Signup and view all the answers

    What does the notation D+ signify in the context of electron transfer reactions?

    <p>A positively charged ion of D</p> Signup and view all the answers

    Why might some reactions proceed quickly through an enzyme?

    <p>They lower the activation energy.</p> Signup and view all the answers

    What type of energy change is indicated by ΔG0 in a reaction?

    <p>Change in free energy</p> Signup and view all the answers

    Which outcome is NOT commonly associated with electron transfer reactions?

    <p>Increase in bond energy</p> Signup and view all the answers

    Which statement is true regarding reactions passing through transition states?

    <p>They represent the highest energy point in a reaction.</p> Signup and view all the answers

    What does the rate enhancement formula relate to?

    <p>Differences in transmission coefficients and Gibbs free energy changes</p> Signup and view all the answers

    Which of the following is a typical enhancement range for enzyme-catalyzed reactions?

    <p>1012 - 1015</p> Signup and view all the answers

    How do catalysts affect the transition state of a reaction?

    <p>They lower the energy of the transition state</p> Signup and view all the answers

    What does ΔΔ‡G represent in enzyme kinetics?

    <p>Difference in activation energy between non-catalyzed and catalyzed reactions</p> Signup and view all the answers

    What is the equation for the kinetic rate constant (ket) according to Marcus theory?

    <p>$ket = \frac{2\pi | V(r) |^2 e^{-\frac{(\Delta G + \lambda)_o}{4\lambda k_B T}}}{4\pi\lambda k_B T}$</p> Signup and view all the answers

    Which of the following is true regarding the Gibbs free energy of activation?

    <p>It is taken from reactants, not from the enzyme-substrate complex</p> Signup and view all the answers

    What does the term $| V(r) |$ represent in the kinetic rate constant equation?

    <p>The potential energy surface</p> Signup and view all the answers

    What does kcat in the rate enhancement equation represent?

    <p>The turnover number of the enzyme</p> Signup and view all the answers

    What do the symbols $\lambda$ and $k_B$ represent in the Marcus theory equation?

    <p>$\lambda$ is the reorganization energy; $k_B$ is the Boltzmann constant</p> Signup and view all the answers

    What is the significance of the term $e^{-\frac{(\Delta G + \lambda)_o}{4\lambda k_B T}}$ in the equation?

    <p>It describes the temperature dependence of the reaction rate</p> Signup and view all the answers

    What is denoted by ES‡ in enzyme kinetics?

    <p>Energy of the transition state complex</p> Signup and view all the answers

    In the context of the Marcus theory, what does the term $(\Delta G + \lambda)_o$ imply?

    <p>The total energy cost including reorganization and free energy change</p> Signup and view all the answers

    What is the purpose of the transmission coefficient in enzymatic reactions?

    <p>To quantify the probability of successful transitions</p> Signup and view all the answers

    Which term refers to the loss of energy during a reaction due to certain conditions?

    <p>Delta G</p> Signup and view all the answers

    What does the term knon-cat refer to in enzymatic kinetics?

    <p>Rate of non-catalyzed reactions</p> Signup and view all the answers

    Study Notes

    Molecular Biochemistry - Electron Transfer Reactions, Quantum Tunnelling

    • Electron transfer reactions don't involve bond breaking/forming
    • Electron transfer reactions are the simplest class of electron exchange reactions
    • Rate enhancement is a result of differences in the transmission coefficient and the difference between the transition state and reactant Gibbs free energy
    • Enhancements of 1012 - 1015 are typical for enzyme-catalysed reactions
    • Catalysts lower the energy of the transition state
    • Reactions proceed through the same transition state
    • The Gibbs free activation energy is taken from the reactants, not the enzyme-substrate complex
    • Some reactions are too fast to pass through the catalysed transition state (why?)

    Transition States

    • Catalysts function by lowering the transition state energy
    • Reactions proceed through the same transition state
    • The Gibbs free energy of activation is taken directly from the reactants.

    Marcus Theory

    • A new kinetic rate theory

    • Introduces the reorganization energy (λ)

    • Electron transfers are ultrafast (~10−15 seconds)

    • The environment is poorly minimized after electron transfer

    • ∆G⁰ is the difference in Gibbs energy of reactions

    • The rate of electron transfer (ket) is dependent on the electronic coupling element at the closest separation (|V(r)|), Plancks constant (h), Boltzmann's constant (kB), temperature (T) and Reorganisation Energy (λ)

    Quantum Tunnelling

    • Going through a barrier is a quantum mechanical phenomenon (tunnelling)
    • There is a probability that a particle exists in the product potential well even when it cannot classically overcome the barrier
    • Tunnelling is a distance-dependent effect
    • Quantum tunnelling is observed in: Electrons, Isotopes of protons, hydrogen and hydrides
    • There is a vast amount of literature on electron and hydrogen tunnelling in enzyme-catalysed reactions

    Anomalies in Kinetic Observations

    • Kinetic isotope effects are sometimes larger than expected, possibly from zero-point energy differences in isotopes
    • Some reactions exhibit Arrhenius prefactors that are smallest for protonated reactants and largest for tritiated reactants
    • Some reactions proceed despite unfavourable temperatures (why?)

    Wave Particle Duality

    • Electrons can exhibit wave or particle behaviour (Duality)
    • Wavelength (λ) is inversely proportional to momentum (mv) where m = mass and v = velocity

    Tunnelling in Enzymes

    • There is a great deal of literature on electron and hydrogen tunnelling in enzyme-catalysed reactions.

    Models of Kinetics

    • The models have moved from Arrhenius to more advanced models like Transition State Theory (TST) for bond-forming/breaking reactions, Marcus for electron transfer reactions and Marcus-like for proton tunneling
    • Examined thermodynamics and binding structure.

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    Description

    This quiz covers key concepts in molecular biochemistry, focusing on electron transfer reactions and transition states. It delves into the mechanisms of catalysis, the role of Gibbs free energy, and the nature of enzyme-catalyzed reactions. Test your knowledge on these essential biochemical processes.

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