Oxocarbenium ion - Revision history

Spencer Williams: /* Conformation */

2017-03-24T11:32:25Z

Conformation

2017-03-24T11:32:05Z

Conformation

2017-03-19T07:29:05Z

Conformation

2017-03-19T07:28:47Z

Conformation

2017-03-19T07:27:52Z

2017-03-19T07:27:05Z

2017-03-19T04:38:06Z

2017-03-19T04:36:59Z

2017-03-18T05:27:56Z

2017-03-17T21:42:34Z

Conformation

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 Because the oxocarbenium ion has double-bond character between the anomeric carbon and the ring oxygen, this part of the structure must be planar <cite>Sinnott1990</cite>. Thus for a 6-membered ring, C5, O5, C1 and C2 must lie in a plane. This arrangement can be accommodated in a 6-membered ring in the <sup>4</sup>''H''<sub>3</sub> or <sup>3</sup>''H''<sub>4</sub> half chair conformations; <sup>4</sup>''E'', ''E''<sub>4</sub>, <sup>3</sup>''E'', or ''E''<sub>3</sub> envelope conformations; or <sup>2,5</sup>''B'' or ''B''<sub>2,5</sub> boat conformations.
-[[Image:Oxocarbenium_conformations.png|center|600px]]
+[[Image:Oxocarbenium_conformations.png|center|800px]]
 The complete conformational pseudorotational itinerary for a pyranosyloxocarbenium ion is the same as that for cyclohexene, but with different descriptors:

@@ Line 15: / Line 15: @@
 Because the oxocarbenium ion has double-bond character between the anomeric carbon and the ring oxygen, this part of the structure must be planar <cite>Sinnott1990</cite>. Thus for a 6-membered ring, C5, O5, C1 and C2 must lie in a plane. This arrangement can be accommodated in a 6-membered ring in the <sup>4</sup>''H''<sub>3</sub> or <sup>3</sup>''H''<sub>4</sub> half chair conformations; <sup>4</sup>''E'', ''E''<sub>4</sub>, <sup>3</sup>''E'', or ''E''<sub>3</sub> envelope conformations; or <sup>2,5</sup>''B'' or ''B''<sub>2,5</sub> boat conformations.
-[[Image:Oxocarbenium_conformations.png|center|300px]]
+[[Image:Oxocarbenium_conformations.png|center|600px]]
 The complete conformational pseudorotational itinerary for a pyranosyloxocarbenium ion is the same as that for cyclohexene, but with different descriptors:

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 The complete conformational pseudorotational itinerary for a pyranosyloxocarbenium ion is the same as that for cyclohexene, but with different descriptors:
-... <-> <sup>3</sup>''E'' <-> <sup>3</sup>''H''<sub>4</sub> <-> ''E''<sub>4</sub> <-> <sup>2,5</sup>''B'' <-> ''E''<sub>3</sub> <-> <sup>4</sup>''H''<sub>3</sub> <-> <sup>4</sup>''E'' <-> ''B''<sub>2,5</sub> <-> <sup>3</sup>''E'' <->
+... <-> <sup>3</sup>''E'' <-> <sup>3</sup>''H''<sub>4</sub> <-> ''E''<sub>4</sub> <-> <sup>2,5</sup>''B'' <-> ''E''<sub>3</sub> <-> <sup>4</sup>''H''<sub>3</sub> <-> <sup>4</sup>''E'' <-> ''B''<sub>2,5</sub> <-> <sup>3</sup>''E'' <-> ...
 ==See also==

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 The complete conformational pseudorotational itinerary for a pyranosyloxocarbenium ion is the same as that for cyclohexene, but with different descriptors:
-<-> <sup>3</sup>''E'' <-> <sup>3</sup>''H''<sub>4</sub> <-> ''E''<sub>4 <-> <sup>2,5</sup>''B'' <-> ''E''<sub>3</sub> <-> <sup>4</sup>''H''<sub>3</sub> <-> <sup>4</sup>''E'' <-> ''B''<sub>2,5</sub> <-> <sup>3</sup>''E'' <->
+... <-> <sup>3</sup>''E'' <-> <sup>3</sup>''H''<sub>4</sub> <-> ''E''<sub>4</sub> <-> <sup>2,5</sup>''B'' <-> ''E''<sub>3</sub> <-> <sup>4</sup>''H''<sub>3</sub> <-> <sup>4</sup>''E'' <-> ''B''<sub>2,5</sub> <-> <sup>3</sup>''E'' <->
 ==See also==

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 [[Image:Oxocarbenium.png|center|600px]]
 For [[glycosyltransferases]] the situation is less clear, and there remains the possibility that an oxocarbenium ion may represent a bonafide reaction intermediate. For this to be the case the active site must effectively exclude water and must not contain nucleophilic residues in close proximity to the reactive centre.
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 [[Image:Oxocarbenium_conformations.png|center|300px]]
 ==See also==

← Older revision		Revision as of 07:27, 19 March 2017
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	[[Image:Oxocarbenium.png\|center\|600px]]		[[Image:Oxocarbenium.png\|center\|600px]]

		+	The complete conformational pseudorotational itinerary for a pyranosyloxocarbenium ion is the same as that for cyclohexene, but with different descriptors:
		+
		+	<-> <sup>3</sup>''E'' <-> <sup>3</sup>''H''<sub>4</sub> <-> ''E''<sub>4 <-> <sup>2,5</sup>''B'' <-> ''E''<sub>3</sub> <-> <sup>4</sup>''H''<sub>3</sub> <-> <sup>4</sup>''E'' <-> ''B''<sub>2,5</sub> <-> <sup>3</sup>''E'' <->
	For [[glycosyltransferases]] the situation is less clear, and there remains the possibility that an oxocarbenium ion may represent a bonafide reaction intermediate. For this to be the case the active site must effectively exclude water and must not contain nucleophilic residues in close proximity to the reactive centre.		For [[glycosyltransferases]] the situation is less clear, and there remains the possibility that an oxocarbenium ion may represent a bonafide reaction intermediate. For this to be the case the active site must effectively exclude water and must not contain nucleophilic residues in close proximity to the reactive centre.

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 == Conformation ==
-Because the oxocarbenium ion has double-bond character between the anomeric carbon and the ring oxygen, this part of the structure must be planar <cite>Sinnott1990</cite>. Thus for a 6-membered ring, C5, O5, C1 and C2 must lie in a plane. This arrangement can be accommodated in a 6-membered ring only in the <sup>4</sup>''H''<sub>3</sub> or <sup>3</sup>''H''<sub>4</sub> half chair conformations; <sup>4</sup>''E'', ''E''<sub>4</sub>, <sup>3</sup>''E'', or ''E''<sub>3</sub> envelope conformations; or <sup>2,5</sup>''B'' or ''B''<sub>2,5</sub> boat conformations.
+Because the oxocarbenium ion has double-bond character between the anomeric carbon and the ring oxygen, this part of the structure must be planar <cite>Sinnott1990</cite>. Thus for a 6-membered ring, C5, O5, C1 and C2 must lie in a plane. This arrangement can be accommodated in a 6-membered ring in the <sup>4</sup>''H''<sub>3</sub> or <sup>3</sup>''H''<sub>4</sub> half chair conformations; <sup>4</sup>''E'', ''E''<sub>4</sub>, <sup>3</sup>''E'', or ''E''<sub>3</sub> envelope conformations; or <sup>2,5</sup>''B'' or ''B''<sub>2,5</sub> boat conformations.
 [[Image:Oxocarbenium_conformations.png|center|300px]]

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 ----
-An '''oxocarbenium ion''' is the species that is formed upon formal ‘ionization’ of a glycosidic bond. Also called an oxacarbenium ion, or glycosylium ion, they are a positively charged carbocation that is stabilized by resonance.
+An '''oxocarbenium ion''' is the species that is formed upon formal ‘ionization’ of a glycosidic bond. Also called an oxacarbenium ion, or glycosylium ion, it is a positively charged carbocation that is stabilized by resonance.
 == Lifetime and involvement in enzymatic mechanisms ==
 Lifetimes of oxocarbenium ions in water are estimated at around 10<sup>-12</sup> sec: right at the limit of existence as free species. Further, in the presence of anionic nucleophiles their lifetime was shown to be considerably shorter <cite>Banait1991</cite>. Therefore it seems unlikely that oxocarbenium ion intermediates exist in the active sites of [[glycoside hydrolases]], which contain nucleophilic carboxylate residues. Indeed there is no experimental evidence for their existence. However, kinetic isotope effect studies have shown that [[transition state]]s for glycosyl transfer have considerable oxocarbenium ion character.