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Prep101 Practice Sets: Class 1, Question 4
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Josh_4080
#1 Posted : Monday, June 22, 2020 10:32:58 PM
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I am a little bit confused as to the wording of the question here. I know this question is super simple, but I thought the "addition of an acetyl" group meant it added an acetyl group to the NH3 group, not that it subbed out the NH3 group for the acetyl. How do we know when the question is asking for subbing out and one is adding?

Also it says NH3 has a positive charge, but I thought NH3 was neutral and NH4 has a positive charge?

Thanks!!
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INSTR_Shinthujah_68
#2 Posted : Tuesday, June 23, 2020 6:27:28 PM
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Hi Josh,

The process of acetylation refers to the attachment of an acetyl group to an alpha-amino group as amino groups are highly reactive. The NH3 is not subbed out, this will be the case for all acetylation reactions.

This diagram might help you better understand the reaction: https://media.springerna..._2018_116_Fig1_HTML.png

You need to remember that the amino group (NH3) is attached to the alpha carbon (NH3-C-R) therefore it has 4 groups surrounding the nitrogen so it is positively charged (NH3+). Once we acetylate the amino group, we now have (CH3CO-NH-C-R), therefore we have lost the charge.

I hope this helps :)

As a side note: coming to your answer through the process of elimination would have given you this answer as well, as the others are incorrect. I would highly recommend this approach if you're unsure!

- Shin
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Josh_4080
#3 Posted : Wednesday, June 24, 2020 8:48:19 PM
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Thanks for that photo! That makes much more sense. I am still a little confused as in the answer explanations it says "Acetylation of the N-terminus would involve replacing the NH3+ group at the extreme N-terminus with a CH3 group". So this means replacing the NH3+ group with an CH3CO-NH group?

Thanks!
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INSTR_Katerina_102
#4 Posted : Thursday, June 25, 2020 10:43:59 PM
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Yes, that is correct.
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