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Related Concept Videos

Molecules with Multiple Chiral Centers02:25

Molecules with Multiple Chiral Centers

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Molecules that possess multiple chiral centers can afford a large number of stereoisomers. For instance, while some molecules like 2-butanol have one chiral center, defined as a tetrahedral carbon atom with four different substituents attached, several molecules like butane-2,3-diol have multiple chiral centers. A simple formula to predict the number of stereoisomers possible for a molecule with n chiral centers is 2n. However, there can be a lower number where some of the stereoisomers are...
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Chirality at Nitrogen, Phosphorus, and Sulfur02:30

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Chirality is most prevalent in carbon-based tetrahedral compounds, but this important facet of molecular symmetry extends to sp3-hybridized nitrogen, phosphorus and sulfur centers, including trivalent molecules with lone pairs. Here, the lone pair behaves as a functional group in addition to the other three substituents to form an analogous tetrahedral center that can be chiral.
A consequence of chirality is the need for enantiomeric resolution. While this is theoretically possible for all...
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Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)

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Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
The central atom need not be NMR-active because its electrons are affected by the electron polarization of the spin-active atoms. However, spin information is transmitted less effectively than in one-bond coupling, and 2J values are usually weaker than 1J values. The energy of...
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¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

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The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene...
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Hybridization of Atomic Orbitals I03:24

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The mathematical expression known as the wave function, ψ, contains information about each orbital and the wavelike properties of electrons in an isolated atom. When atoms are bound together in a molecule, the wave functions combine to produce new mathematical descriptions that have different shapes. This process of combining the wave functions for atomic orbitals is called hybridization and is mathematically accomplished by the linear combination of atomic orbitals. The new orbitals that...
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Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

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Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
The extent of coupling depends on the C‑C bond length, the two H‑C‑C angles, any electron-withdrawing substituents, and the dihedral angle between the involved orbitals. The...
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Updated: Dec 10, 2025

Electron Spin Resonance Micro-imaging of Live Species for Oxygen Mapping
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The Siroheme-[4Fe-4S] Coupled Center.

Isabel Askenasy, M Elizabeth Stroupe

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    |August 28, 2020
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    Summary
    This summary is machine-generated.

    Sulfite reductase enzymes, crucial for microbial survival, utilize a unique siroheme cofactor to convert sulfite to sulfide. Their ancient homology and catalytic mechanisms are key to understanding sulfur metabolism in diverse ecosystems.

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    Area of Science:

    • Biochemistry
    • Microbiology
    • Enzymology

    Background:

    • Sulfur is essential for life, existing in various oxidation states, with reduced forms prevalent in biomolecules.
    • Sulfur reduction occurs via dissimilation and assimilation pathways, both relying on sulfite reductase.
    • Sulfite reductase enzymes are iron metalloenzymes featuring a unique siroheme cofactor coupled to a [4Fe-4S] cluster.

    Purpose of the Study:

    • To explore the role of sulfite reductase in microbial survival across ecosystems.
    • To elucidate the atomic-resolution structures of sulfite reductases and their ancient homology.
    • To detail the catalytic mechanism of the siroheme-[4Fe-4S] cluster in sulfite reduction to sulfide.
    • To investigate siroheme synthesis pathways in various microorganisms.

    Main Methods:

    • Structural analysis of dissimilatory and assimilatory sulfite reductases.
    • Biochemical characterization of the siroheme-[4Fe-4S] active site.
    • Comparative genomics and phylogenetic analysis of sulfite reductase enzymes.
    • Enzymatic assays to study sulfite reduction mechanism.

    Main Results:

    • Sulfite reductase enzymes exhibit ancient homology across diverse microbial groups.
    • The siroheme-[4Fe-4S] cluster catalyzes the six-electron reduction of sulfite to sulfide via a push-pull mechanism.
    • Siroheme is synthesized from uroporphyrinogen III through a pathway involving enzymes homologous to cobalamin synthesis.
    • Environmental microbes utilize sulfite reductase for survival in varied ecosystems.

    Conclusions:

    • Sulfite reductase is a vital enzyme for sulfur metabolism in microorganisms.
    • Understanding sulfite reductase structure and function provides insights into microbial adaptation and evolution.
    • Further research is needed on the assembly of siroheme-[4Fe-4S] clusters into functional enzymes.