foxstudy
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Why can it also be referred to as the test for peptide bonds?
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BobbJo
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It gives a positive test for proteins but a negative test for amino acids. Hence it tests for the presence of peptide bonds.
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foxstudy
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(Original post by BobbJo)
It gives a positive test for proteins but a negative test for amino acids. Hence it tests for the presence of peptide bonds.
Is it also due to the fact that the copper hydroxide interacts with peptide bonds? Not sure if this is correct though.
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BobbJo
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(Original post by foxstudy)
Is it also due to the fact that the copper hydroxide interacts with peptide bonds? Not sure if this is correct though.
Copper (II) hydroxide is a blue ppt and is seen if you add excess CuSO4 rather than few drops.

The reason for the purple colouration is the formation of a coordination complex between Cu2+ and 4 N atoms of peptide bonds with 2 O of H2O, forming an octahedral complex which absorbs in the 540nm range, hence giving a violet colour. Lone pairs on 4 N atoms are donated to the central Cu2+ ion. It is called a chelate complex. (Chelate because the ligand is bidentate; you need 2 peptide bonds on a single molecule). KOH is essential as it provides an alkaline medium which is necessary for the reaction to occur.
https://www.onlinebiologynotes.com/w...-principle.png
Amino acids and dipeptides do not respond to the test. With dipeptides, the orientation can never be right because there'd be just one electron pair per molecule, thus 4 molecules would be required, and these bulky molecules can't huddle together around the Cu.
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foxstudy
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(Original post by BobbJo)
Copper (II) hydroxide is a blue ppt and is seen if you add excess CuSO4 rather than few drops.

The reason for the purple colouration is the formation of a coordination complex between Cu2+ and 4 N atoms of peptide bonds with 2 O of H2O, forming an octahedral complex which absorbs in the 540nm range, hence giving a violet colour. Lone pairs on 4 N atoms are donated to the central Cu2+ ion. It is called a chelate complex. (Chelate because the ligand is bidentate; you need 2 peptide bonds on a single molecule). KOH is essential as it provides an alkaline medium which is necessary for the reaction to occur.
https://www.onlinebiologynotes.com/w...-principle.png
Amino acids and dipeptides do not respond to the test. With dipeptides, the orientation can never be right because there'd be just one electron pair per molecule, thus 4 molecules would be required, and these bulky molecules can't huddle together around the Cu.
Isn't the peptide bond just a bond not 4 N atoms??
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BobbJo
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(Original post by foxstudy)
Isn't the peptide bond just a bond not 4 N atoms??
peptide bond is CONH

N atom is N atom

N atom of peptide bond is the N atom in CONH

did you look at the picture
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foxstudy
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(Original post by BobbJo)
peptide bond is CONH

N atom is N atom

N atom of peptide bond is the N atom in CONH

did you look at the picture
Ahh yea but if it was a complex between the Cu2+ ion and nitrogen atoms then surely it could detect an amino acid?
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BobbJo
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(Original post by foxstudy)
Ahh yea but if it was a complex between the Cu2+ ion and nitrogen atoms then surely it could detect an amino acid?
No it couldn't. You need at least 2 peptide bonds on one molecule. There is too much steric hindrance for 4 amino acids to be in the right orientation to be donating their lone pair on the N atom to the central Cu2+. You need at least 2 peptide bonds on one molecule.
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