AAMC 9 - Question #146 - Amino Acids & Charge

[Fivo]

Can someone please explain this question and answer to me please? I ran out of time and just put a ranom answer. Upon reading the question afterwards, I kind of get it but not for sure

thanks

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[dsoz]

Look at the bottom unit for each (the end of the R-group). The rest of the molecule is the same, so ignore it for now.

In lysine the end group is NH2. But under acidic pH, it will really be NH3 (+) because the extra protons that are floating around (acid = proton + charge) will hook up with the lone pair on the nitrogen

In aspartic acid it is COOH, and under acidic conditions it will be COOH (no charge) because the proton will probably not dissociate from the acid.

Therefore aspartic acid (neutral) will have a charge one less than lysine (1+)

hth
dsoz

 

[Fivo]

omg wow. Thanks a lot. makes a lot more sense...fml lol.

Cheers

 

[MedPR]

I took AAMC 9 on March 26 and I remember this question. fml.

 

[ilovemedi]

what chapter is this info in for the tbr books? acid/base chapter doesn't really discuss this.. =/'

nvm.. it's just a basic acid/bsae def,,,, acid's donate H... bases accept H.. but why doesn't the COOH disassociate in the acid? is it because it's an acid in an acidic solution?

 

[indianjatt]

what chapter is this info in for the tbr books? acid/base chapter doesn't really discuss this.. =/'

nvm.. it's just a basic acid/bsae def,,,, acid's donate H... bases accept H.. but why doesn't the COOH disassociate in the acid? is it because it's an acid in an acidic solution?

Yeah. Exactly that. We have an acid (carboxylic acid), which is in an even more acidic solution making the COOH a base actually..... which is why it's fully protonated. The acidic medium has a lower pH than the pKa of the COOH, which is why the COOH remains protonated.

 

[BestDoctorEver]

I just took AAMC 9 from TPR class and I did not get that question on it...Are these AAMCs different?