TBR CBT 1 #111 and #144

[Jepstein30]

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#111: Will try to post a picture later tonight but..

I don't get why the methyl group on the right (C-1 of galactose) would be hydrolyzed.. if it's hydrolyzed, why wouldn't all the other methyl groups also be hydrolyzed? There's not much difference between any of them..

I thought A would be the answer there.


#144:

Question asks which media would be best to grow infected bacteria on to identify viral DNA. Each media have the same compounds, just differences in the labeling.

Media I has radioactively labeled Mg, Media II has radioactive P and media III has radioactive N and P.

I wanted to pick 2 and 3 but that was not an answer choice. I decided to go with 3 but now that I think about it, is it the fact that there would be two radioactive signals that makes Media II the only correct (and best) answer? I have limited work with radioactive isotopes, but that makes sense to me now.

Thanks

 

[AFLATPEG]

#111: Will try to post a picture later tonight but..

I don't get why the methyl group on the right (C-1 of galactose) would be hydrolyzed.. if it's hydrolyzed, why wouldn't all the other methyl groups also be hydrolyzed? There's not much difference between any of them..

I thought A would be the answer there.

because the C-1 of galactose is an ether (after the methylation). the anomeric carbon (C-1) is attached to the O, which is attached to the CH3 group. thus, R-O-R' an ether. Ethers can undergo hydrolysis to form an alcohol.

just like when you dehydrate an alcohol, you GET an ether.

R-O-R' + H2O --> 2 HO-R

that's why you see a by-product of the CH3-OH in the figure.

 

[AFLATPEG]

#111: Will try to post a picture later tonight but..

#144:

Question asks which media would be best to grow infected bacteria on to identify viral DNA. Each media have the same compounds, just differences in the labeling.

Media I has radioactively labeled Mg, Media II has radioactive P and media III has radioactive N and P.

I wanted to pick 2 and 3 but that was not an answer choice. I decided to go with 3 but now that I think about it, is it the fact that there would be two radioactive signals that makes Media II the only correct (and best) answer? I have limited work with radioactive isotopes, but that makes sense to me now.

this question isn't testing your knowledge on nuclear chemistry or radioactive isotopes, fyi.


-one hint in the question is "...lactose intolerant bacterium.." so you know it can't live in a medium WITH lactose as its energy source because it can't break it down. thus, medium III is out.
-medium I has labeled Sulfur-35 and medium II has labeled Phosph.-32. you know that Sulfur is usually in proteins (cysteine and methionine) and you know that Phosph. are in DNA. so, the question is asking HOW can we identify the progeny phage DNA? if you wanna look for DNA (which has Phosphorous), then the medium with the P-32 would be helpful. thus, medium II is the answer.

If you chose medium I with the labeled S-35, you wouldn't know if the DNA was taken up or not because looking for Sulfur isn't really proving the existence of DNA.


idk, i hope this helped? this one was harder for me to explain than the other one. haha.

 

[Jepstein30]

this question isn't testing your knowledge on nuclear chemistry or radioactive isotopes, fyi.


-one hint in the question is "...lactose intolerant bacterium.." so you know it can't live in a medium WITH lactose as its energy source because it can't break it down. thus, medium III is out.
-medium I has labeled Sulfur-35 and medium II has labeled Phosph.-32. you know that Sulfur is usually in proteins (cysteine and methionine) and you know that Phosph. are in DNA. so, the question is asking HOW can we identify the progeny phage DNA? if you wanna look for DNA (which has Phosphorous), then the medium with the P-32 would be helpful. thus, medium II is the answer.

If you chose medium I with the labeled S-35, you wouldn't know if the DNA was taken up or not because looking for Sulfur isn't really proving the existence of DNA.


idk, i hope this helped? this one was harder for me to explain than the other one. haha.

Oh.. did Medium III have lactose? Probably skipped right over that.. dang.

Aren't they all ethers though? They are all Backbone-O-Methyl.. I don't get why one -OMethyl would react but the others wouldn't.

 

[AFLATPEG]

Oh.. did Medium III have lactose? Probably skipped right over that.. dang.

Aren't they all ethers though? They are all Backbone-O-Methyl.. I don't get why one -OMethyl would react but the others wouldn't.

oh yes they are! i'm sorry!

I believe it is because the C-1 of that galactose has a free-end acetal group that is reactive!

 

[AFLATPEG]

Oh.. did Medium III have lactose? Probably skipped right over that.. dang.

Aren't they all ethers though? They are all Backbone-O-Methyl.. I don't get why one -OMethyl would react but the others wouldn't.

ok i google'd the **** out of this! haha

"The first step (methylation) replaces the OH groups off of carbon 2, 3, 4, and 6 of the D-mannose with OCH3. It does the same with the carbons 1, 2, 3, and 6 of the D-galactose. Carbon 1 of the mannose and carbon 4 of the galactose have the glycosidic bond, and no OH, so they are unaffected by the methylation. Then the hydrolysis breaks the bond. It also converts the methoxy group on carbon 1 of the galactose to an OH in the same manner as it breaks the bond between the two monosaccharides. If you think of that carbon 1 of galactose as being bonded to -OR (R = CH3), which is analogous to carbon 1 of mannose being bonded to -OR (R = galactose), then you can see that the same enzyme or catalyst that catalyzed the hydrolysis of the glycosidic bond must do the same to the bond between carbon 1 and the methoxy group. The reason that these that the rest of the OCH3s are unaffected is, I think, because those carbons are not bonded to an oxygen."

 

[Jepstein30]

ok i google'd the **** out of this! haha

"The first step (methylation) replaces the OH groups off of carbon 2, 3, 4, and 6 of the D-mannose with OCH3. It does the same with the carbons 1, 2, 3, and 6 of the D-galactose. Carbon 1 of the mannose and carbon 4 of the galactose have the glycosidic bond, and no OH, so they are unaffected by the methylation. Then the hydrolysis breaks the bond. It also converts the methoxy group on carbon 1 of the galactose to an OH in the same manner as it breaks the bond between the two monosaccharides. If you think of that carbon 1 of galactose as being bonded to -OR (R = CH3), which is analogous to carbon 1 of mannose being bonded to -OR (R = galactose), then you can see that the same enzyme or catalyst that catalyzed the hydrolysis of the glycosidic bond must do the same to the bond between carbon 1 and the methoxy group. The reason that these that the rest of the OCH3s are unaffected is, I think, because those carbons are not bonded to an oxygen."

thanks dude!

weird question honestly.. i mean, I know the hydroxyl group at C-1 is most reactive.. but pretty hard to anticipate whether it would react or not without being given any passage information.

oh well!

 

[AFLATPEG]

thanks dude!

weird question honestly.. i mean, I know the hydroxyl group at C-1 is most reactive.. but pretty hard to anticipate whether it would react or not without being given any passage information.

oh well!

haha no problem! i know, it explains it but doesn't explain WHY. but now we will def. remember this if it ever comes up elsewhere! 🙂

 

[inasensegone]

thanks dude!

weird question honestly.. i mean, I know the hydroxyl group at C-1 is most reactive.. but pretty hard to anticipate whether it would react or not without being given any passage information.

oh well!

Since I'm not sure that you got your answer in this thread, I figured I would let you know that I answered it in the other thread asking about it.

 

[Jepstein30]

Since I'm not sure that you got your answer in this thread, I figured I would let you know that I answered it in the other thread asking about it.

Thanks 😉

a year late but never hurts!

 

[inasensegone]

Thanks 😉

a year late but never hurts!

Haha.... true... I wasn't sure what your status was since I've seen you posting in the MCAT forums. Either way, at least if someone googles your thread, they can be directed to an explanation.

Have you already been accepted to a med school yet? Waiting to hear back?

 

[Jepstein30]

Haha.... true... I wasn't sure what your status was since I've seen you posting in the MCAT forums. Either way, at least if someone googles your thread, they can be directed to an explanation.

Have you already been accepted to a med school yet? Waiting to hear back?

I post to help out, I'm done with taking the MCAT. Rather not share my application details though.. I'm somewhere in the process 😉