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Woodward–Hoffmann Selection Rules and Microscopic Reversibility01:34

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Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...
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Heterogeneous Catalysis

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Heterogeneous catalysis involves a catalyst in a different phase from the reactants. It is a process where the catalyst and the reactants are in distinct phases, typically solid and gas or liquid.Most heterogeneous catalysts are metals, metal oxides, or acids. The list includes transition metals like iron (Fe), cobalt (Co), nickel (Ni), palladium (Pd), platinum (Pt), chromium (Cr), manganese (Mn), tungsten (W), silver (Ag), and copper (Cu). These metals possess partially vacant d orbitals that...
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Pericyclic reactions are organic reactions that occur via a concerted mechanism without generating any intermediates. The reactions proceed through the movement of electrons in a closed loop to form a cyclic transition state, where rearrangement of the σ and π bonds yields specific products.
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Thermal and Photochemical Electrocyclic Reactions: Overview01:26

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Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
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Thermal Electrocyclic Reactions: Stereochemistry01:17

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The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
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[3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement01:21

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The Cope rearrangement is classified as a [3,3] sigmatropic shift in 1,5-dienes, leading to a more stable, isomeric 1,5-diene. The reaction involves a concerted movement of six electrons, four from two π bonds and two from a σ bond, via an energetically favorable chair-like transition state.
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Squalene-hopene cyclases-evolution, dynamics and catalytic scope.

Per-Olof Syrén1, Sabrina Henche2, Anja Eichler2

  • 1School of Chemical Science and Engineering, Division of Applied Physical Chemistry, KTH Royal Institute of Technology, 100 44 Stockholm, Sweden; Science for Life Laboratory, KTH Royal Institute of Technology, School of Biotechnology, Division of Proteomics and Nanobiotechnology, 171 21 Stockholm, Sweden.

Current Opinion in Structural Biology
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Squalene-hopene cyclases (SHCs) utilize a catalytic aspartic acid for protonation-driven reactions. Understanding enzyme dynamics and active site water location enables biocatalytic production of novel compounds.

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Area of Science:

  • Biocatalysis and Enzyme Engineering
  • Organic Chemistry
  • Molecular Biology

Background:

  • Squalene-hopene cyclases (SHCs) are ancient biocatalysts involved in terpene biosynthesis.
  • These enzymes feature a catalytic aspartic acid residue crucial for protonating isoprenoid substrates.
  • SHCs exhibit a flexible active site capable of catalyzing non-natural reactions.

Purpose of the Study:

  • To review recent mechanistic findings on solvent dynamics in SHC catalysis.
  • To explore the potential of SHCs for biocatalytic synthesis of novel chemical compounds.
  • To understand how active site water influences reaction termination.

Main Methods:

  • Review of existing literature on SHC mechanistic studies.
  • Analysis of the role of solvent dynamics and active site water.
  • Site-directed mutagenesis to reshape the enzyme's active site and explore novel functions.

Main Results:

  • The catalytic strength of the aspartic acid residue enables a wide range of protonation-driven reactions.
  • The spatial arrangement of water molecules in the active site dictates reaction termination pathways (deprotonation or hydration).
  • Enzyme active site engineering via mutagenesis can generate novel catalytic activities.

Conclusions:

  • A deeper understanding of triterpene cyclase dynamics is key to harnessing their catalytic potential.
  • Combining mechanistic insights with chemical expertise can lead to the biocatalytic production of tailored molecules.
  • This approach expands the chemical diversity beyond naturally occurring compounds.