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 (D)</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 (A)</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 (B)</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) (D)</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 (B)</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 (C)</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 (A)</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) (B)</p> Signup and view all the answers

Which model is associated with electron transfer processes?

<p>Marcus model (C)</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 (A)</p> Signup and view all the answers

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

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

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

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

What is the primary characteristic of electron transfer reactions?

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

Which species is NOT typically involved in electron transfer reactions?

<p>Catalysts (D)</p> Signup and view all the answers

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

<p>An electron donor (A)</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 (B)</p> Signup and view all the answers

Why might some reactions proceed quickly through an enzyme?

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

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

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

Which outcome is NOT commonly associated with electron transfer reactions?

<p>Increase in bond energy (B)</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. (C)</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 (B)</p> Signup and view all the answers

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

<p>1012 - 1015 (A)</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 (B)</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 (D)</p> Signup and view all the answers

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

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

What does kcat in the rate enhancement equation represent?

<p>The turnover number of the enzyme (C)</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 (A)</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 (D)</p> Signup and view all the answers

What is denoted by ES‡ in enzyme kinetics?

<p>Energy of the transition state complex (D)</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 (A)</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 (A)</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 (A)</p> Signup and view all the answers

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

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

Flashcards

Marcus Theory

A chemical reaction rate theory that describes the rate of electron transfer reactions based on the energy difference between reactants and products.

λ (lambda)

The activation energy for an electron transfer reaction, often referred to as the reorganization energy.

ΔG° (Delta G naught)

The standard Gibbs free energy change associated with an electron transfer reaction.

V(r)

The electronic coupling matrix element, representing the strength of the interaction between the electron donor and acceptor.

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ket

The rate constant for an electron transfer reaction, influenced by the electronic coupling, activation energy, and temperature.

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Electron Transfer Reactions

Reactions that involve the movement of electrons between molecules without breaking or forming chemical bonds. These reactions are important in many biological processes, such as cellular respiration and photosynthesis.

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Electron Exchange Reaction

A type of electron transfer reaction where an electron is transferred from a donor molecule (D) to an acceptor molecule (A).

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Activation Energy (Ea)

The energy difference between the initial state of the reactants and the transition state of the reaction.

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Transition State

The state where bonds are broken and formed in a reaction, representing the highest energy point during the process. It is an unstable intermediate.

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Standard Free Energy Change (ΔG°)

Indicates the change in Gibbs Free Energy (ΔG) at standard conditions (298 K, 1 atm pressure, 1 M concentration). It predicts the spontaneity of a reaction.

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Standard Redox Potential (E°)

A measure of the energy released or absorbed when one mole of electrons is transferred.

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Gibbs Free Energy Equation

A mathematical relationship between the standard free energy change (ΔG°) and the standard redox potential (E°) which helps predict the spontaneity of a reaction. The equation is: ΔG° = -nFE° where n is the number of electrons transferred, F is Faraday's constant (96.49 kJ/mol·V).

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Equilibrium

The state of a system where the Gibbs Free Energy is at its minimum, indicating a spontaneous reaction and the release of energy.

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ΔGº (Standard Gibbs Free Energy Change)

The difference in Gibbs free energy between the reactants and products in an electron transfer reaction.

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ΔG‡ (Gibbs Free Energy of Activation)

The difference in the free energy between the reactants and the transition state of the electron transfer reaction.

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λ (Reorganization Energy)

The minimum amount of energy needed for the reactants to overcome the free energy of activation and reach the transition state.

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ΔGº = λ

When the reorganization energy is equal to the difference in Gibbs free energy between the reactants and products, meaning no additional energy is needed for reaction.

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Rate Enhancement

A measure of how much faster a reaction occurs in the presence of a catalyst compared to the uncatalyzed reaction.

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Activation Energy (Δ‡G)

The energy barrier that must be overcome for a reaction to occur.

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Transition State (ES‡)

A hypothetical state that represents the highest energy point along the reaction pathway. Molecules must reach this state to react.

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Transmission Coefficient (κ)

The factor that accounts for the efficiency of the transition state in converting reactants to products. It's like the probability of actually reaching the top of the mountain pass and successfully crossing.

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Difference in Activation Energy (ΔΔ‡G)

The activation energy difference between the uncatalyzed reaction and the catalyzed reaction. This is the primary factor contributing to rate enhancement by catalysts.

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Activation Energy (Δ‡Gnon-cat)

The energy required for the uncatalyzed reaction to proceed.

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Activation Energy (Δ‡Gcat)

The energy required for the catalyzed reaction to proceed.

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Enzyme-Substrate Complex (ES)

The intermediate formed during the reaction between the enzyme and the substrate, which is involved in reaching the transition state.

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Transition State of the Enzyme-Substrate Complex (ES‡c)

The state that represents the enzyme-substrate complex at the maximum energy point during the reaction. This is the point where the substrate begins to transform into the product.

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Activation Energy (Δ‡Gc) Taken from Reactants

The activation energy of the catalyzed reaction is taken from the reactants, not from the enzyme-substrate complex, meaning the enzyme lowers the activation energy of the reaction without changing the overall energy of the system.

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Reorganization Energy (λ)

The energy needed to rearrange the molecules involved in an electron transfer reaction. It's like the energy needed to build the bridge to cross the energy gap.

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Electronic Coupling Matrix Element (V(r))

The strength of the interaction between the electron donor and acceptor. It's like the bridge's sturdiness.

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Rate Constant (k)

The rate at which the electron transfer happens. It depends on the strength of the interaction, the energy needed for reorganization, and the temperature.

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Wave Function Amplitude and Probability

The probability of finding a particle at a specific location is represented by the amplitude of its wave function. It describes how the wave function oscillates, and the height of those oscillations indicates the likelihood of finding the particle there.

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Quantum Tunneling

An event in quantum mechanics where a particle can pass through a potential barrier even though it doesn't have enough energy to do so classically. This is possible because the wave function of the particle can extend into the barrier region and have a non-zero probability of being found on the other side.

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de Broglie Wavelength Equation

A mathematical equation that describes the relationship between a particle's wavelength, momentum, Planck's constant, and mass. It essentially quantifies the wave-particle duality of matter.

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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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SL22015 Molecular Biochemistry Lecture 6 PDF

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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