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Q80

Q80

Postby SeoM » Tue Apr 07, 2020 11:21 pm

In the exam Tyrosine's NH3 group was to the left of the COO group, which was left of the H, which was left of the side chain. This lead me to believe that the substituents were ranked in order going to the right (clockwise). To my understanding this would give a R-Tyrosine.

The Glutamine NH3 group was to the right of the COO group, which lead me to believe the substituents were ranked in order going to the left (anticlockwise). This would give a S-Glutamine.

However, the answer was that it was S-Tyrosine and R-Glutamine. I do not understand why and am hoping that someone could shed light on this please?
SeoM
 
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Re: Q80

Postby goldstanda3269 » Wed Apr 08, 2020 5:02 pm

SeoM wrote:In the exam Tyrosine's NH3 group was to the left of the COO group, which was left of the H, which was left of the side chain. This lead me to believe that the substituents were ranked in order going to the right (clockwise). To my understanding this would give a R-Tyrosine.

The Glutamine NH3 group was to the right of the COO group, which lead me to believe the substituents were ranked in order going to the left (anticlockwise). This would give a S-Glutamine.

However, the answer was that it was S-Tyrosine and R-Glutamine. I do not understand why and am hoping that someone could shed light on this please?


A couple of helpful references:
1) In the videos section of www.gamsat-prep.com, the Org video...
New: Stereochemistry: R/S Configuration
2) ACER's Blue Booklet (GAMSAT Sample Questions), Section 3, Unit 14 (40-41)
3) ACER's Green Booklet (GAMSAT Practice Test 1), Section 3, Unit 27 (83-86)

Here is the good news: You assigned priority properly and determined R and S properly!

Unfortunately, you were not finished!

The structures were presented as Fischer projections which ACER sometimes uses as in the Blue and Green Booklet references above. This means that any atoms bonded horizontally are projecting towards you (see Book 3, ORG 2.3.1/2/3). Both Figure 1 and 2 have H's (lowest priority) facing towards you and so any priority assignment in that position is exactly the opposite.
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