Química Orgânica 2 · Aminoácidos e conectividade dos peptídeos
Glycine and alanine from acid hydrolysis of Gly-Ala
Condições: 6 M HCl, 110 °C
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O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
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Protonate the N-terminal amine
In 6 M HCl the hydronium ion protonates the basic N-terminal amine first; it stays as an ammonium group throughout.
As setas curvas
- The amine lone pair takes a proton from hydronium.
- The O–H pair stays on oxygen as water.
Protonate the amide oxygen
Amides protonate on oxygen, not nitrogen, because the cation is resonance-stabilized by the nitrogen lone pair. Protonation makes the very unreactive amide carbonyl electrophilic.
As setas curvas
- The carbonyl oxygen lone pair takes a proton from hydronium.
- The O–H pair stays on oxygen.
Water adds to the carbonyl carbon
Water attacks the protonated amide carbon and the C=O π pair moves onto the protonated oxygen, giving a tetrahedral intermediate.
As setas curvas
- A water lone pair attacks the carbonyl carbon.
- The C=O π pair moves onto oxygen.
Water removes a proton
A solvent water molecule removes the extra proton from the added oxygen, giving a neutral tetrahedral intermediate.
As setas curvas
- A water lone pair takes the O–H proton.
- The O–H pair stays on oxygen.
Protonate the nitrogen
Now the nitrogen is an amine-like sp3 nitrogen, no longer delocalized, so it is the most basic site; hydronium protonates it to make the alanine nitrogen a neutral leaving group.
As setas curvas
- The nitrogen lone pair takes a proton from hydronium.
- The O–H pair stays on oxygen.
Leaving group
A neutral amine leaves, not an amide anion.
The C–N bond breaks
An OH lone pair re-forms the C=O bond and pushes out the neutral alanine nitrogen, giving the protonated carboxylic acid of glycine.
As setas curvas
- An oxygen lone pair forms the C=O π bond.
- The C–N pair stays on nitrogen as the amine lone pair.
Deprotonate the carboxylic acid
Water removes the proton from the protonated carbonyl, giving glycine's neutral carboxylic acid group.
As setas curvas
- A water lone pair takes the O–H proton.
- The O–H pair stays on the carbonyl oxygen.
Protonate the alanine nitrogen
In 6 M HCl the freed alanine amine is protonated too.
As setas curvas
- The amine lone pair takes a proton from hydronium.
- The O–H pair stays on oxygen.
Glycine and alanine hydrochlorides
Hydrolysis gives glycine and alanine, both as ammonium salts in the hot acid; the alanine stereocentre is untouched. Amino-acid analysis of such a hydrolysate gives a peptide's composition but not its sequence.
O que você vai praticar
Draw the acid-catalysed hydrolysis of the Gly-Ala peptide bond, from N-terminal and carbonyl protonation to loss of alanine.
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