Phosphole is the organic compound with the chemical formula C
4H
4PH; it is the phosphorus analog of pyrrole. The term phosphole also refers to substituted derivatives of the parent heterocycle. These compounds are of theoretical interest but also serve as ligands for transition metals and as precursors to more complex organophosphorus compounds.
Triphosphole, C
2H
3P
3, is a heterocycle with 3 phosphorus atoms.
Pentaphosphole, P
5H, is a cyclic compound with 5 phosphorus atoms.
https://en.wikipedia.org/wiki/Phosphole
An oxyanion hole is a pocket in the active site of an enzyme that stabilizes transition statenegative charge on a deprotonated oxygen or alkoxide.[1] The pocket typically consists of backbone amides or positively charged residues. Stabilising the transition state lowers the activation energy necessary for the reaction, and so promotes catalysis.[2] For example, proteases such as chymotrypsin contain an oxyanion hole to stabilise the tetrahedral intermediate anion formed during proteolysis and protects substrate's negatively charged oxygen from water molecules.[3] Additionally, it may allow for insertion or positioning of a substrate, which would suffer from steric hindrance if it could not occupy the hole (such as BPG in hemoglobin). Enzymes that catalyse multi-step reactions can have multiple oxyanion holes that stabilise different transition states in the reaction.[4]
https://en.wikipedia.org/wiki/Oxyanion_hole
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