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Free radicals

Dimer formation

BINDING MODEL

Gibbs free energy

  Literature shows that coniferyl alcohol (monolignol G) forms resonance structure after creating a free radical, these resonance structures would form dimers (β-5, β-O-4, β-β).
  The reactions starts from the catalytic of the enzyme and the addition of water. We first assumed that our conditions may be different as the literature, so we decided to confirm the feasibility of the reaction through Gibbs free energy calculation.
(Calculation method using Spartan 16)

(1) From monomeric alcohol to carrying free radicals

Figure 1: (Left) Three resonance forms(Wang, Y. et al. 2013), (Right) Reaction diagram (Vanholme, R. et al. 2010)

Figure 2: Monolignol as the free energy benchmark (1 hartree ≈ 2625.5 kJ/mole)

  In Figure 2. , we found that the free energy of this process is positive, which means it's not spontaneous. Thus, we think the way to make the reactions spontaneous is to use enzymes such as laccase and peroxidase.

(2) Free radical state of dimer formation

Figure 3: The green indicator is β-O-4; the blue indicator is β-β; the red indicator is β-5 (Barceló, A. R. et al. 2004)

Figure 4: β-β free energy changes, (a) is the two resonant monolignols, (b) is the two resonant monolignol bonds, (c) is β-β formation (1 hartree ≈ 2625.5 kJ/mole)

Figure 5: β-5 free energy changes, (a) is the two resonant monolignols, (b) is the two resonant monolignol bonds, (c) is β-5 formation (1 hartree ≈ 2625.5 kJ/mole)

Figure 6: β-O-4 free energy change, (a) is two resonance monolignols, (b) is two resonance monolignol bonding, (c) is formed by β-O-4, and water is added to the reaction at 2 to 3 (1 hartree ≈ 2625.5 kJ/mole)

  Through these simulations, it's obvious that these reactions are thermodynamically spontaneous at room temperature with catalysts, and it's also feasible to synthesize these bonds by using our enzymes.

Figure 7: Calculation data-coniferyl alcohol to resonance form

Figure 8: Calculation data-resonance form to dimer

Reference

Barceló, A. R., Ros, L. G., Gabaldón, C., López-Serrano, M., Pomar, F., Carrión, J. S., & Pedreño, M. A. (2004). Basic peroxidases: the gateway for lignin evolution? . Phytochemistry Reviews. (2004), 3(1-2), 61-78

Vanholme, R., Demedts, B., Morreel, K., Ralph, J., & Boerjan, W. (2010). Lignin biosynthesis and structure.(2010) Plant physiology, 153(3), 895-905.

Wang, Y., Chantreau, M., Sibout, R., & Hawkins, S. (2013). Plant cell wall lignification and monolignol metabolism. Frontiers in plant science, 4, 220