Amino Acids and pH

[bubbles21]

Okay so i was just going through insight 2011, and i stumbled across a few problems.
First off Q4 multichoice. A dipeptide is produced by a reaction between 10.0g each of the amino acids cysteine (M=121.1 g mol^(-1)) and glycine (M=75.1 g mol^(-1)). The mass, in g, of water eliminated in the reaction is:
A .0825
B 1.49 (correct answer according to insight)
C 2.39
D 2.98
Initially i went B because cysteine is the limiting reagent, then mol*18 = 1.49, but then i thought wouldn't the excess glycine just react with itself? Producing 2.39 grams of water.

Second question, i'm tired of typing already.... 
Q12 multi: Which of the following amino acids will have the highest pH when dissolved in solution?
A aspartic acid
B histidine
C lysine
D Methionine
i went with B because it has two Amine groups even if one is tertiary and the other is secondary, and they said C, saying that histidine has no acidic or basic groups and would be neutral.
http://www.elmhurst.edu/~chm/vchembook/561aminostructure.html
and this ancient website agrees 
And lastttt query, in water would asparagine be basic or neutral, insight says basic, that website says neutral. wiki says it is uncharged which means neutral as well.
So in essence, i am thoroughly confuzzled.
Cheers for any help in advance

[Water]

Q4, It asks for the "water" produced by the reaction in the "production" of dipeptide




2) After checking the website, my possible hypothesis is that Lysine will readily accept the proton more than the Histidine  thats indicated.


PS: Though to be honest, within my knowledge of VCE Chemistry, I'd pick Lysine purely because  Histidine N-H, would be non feasible, as it would not gain a proton, whilst NH2 would be more prone to accept protons from the water solution.

[jane1234]

3. Think it's basic because the two NH2 groups will be likely to form NH3+ groups by adding a hydrogen from water, making the solution basic (similar to question 2).

[bubbles21]

Thanks for quick reply!!!
Okay, might have to see what the chem teacher reckons.

With asparagine however, it comes under wiki as polar and uncharged, which could only be true if the NH2 in its side chain didn't accept any protons? Generally i don't trust wiki, but when the other website is saying the same....

[bubbles21]

oh and Water you would be right about Lysine having a greater tendency to accept.

http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/proteins.htm

if you scroll down about 1/3 to 1/2 way down to the pKa values of the amino acids, the pKa of Histidine is lower than lysine

[jane1234]

Ah, my bad.

Asparagine is an amide group, not amine so it probably won't produce a basic solution. Still not 100% sure, but think wiki, google and the other site are right in saying it's neutral.

[RobM8]

Okay so i was just going through insight 2011, and i stumbled across a few problems.
First off Q4 multichoice. A dipeptide is produced by a reaction between 10.0g each of the amino acids cysteine (M=121.1 g mol^(-1)) and glycine (M=75.1 g mol^(-1)). The mass, in g, of water eliminated in the reaction is:
A .0825
B 1.49 (correct answer according to insight)
C 2.39
D 2.98
Initially i went B because cysteine is the limiting reagent, then mol*18 = 1.49, but then i thought wouldn't the excess glycine just react with itself? Producing 2.39 grams of water.

Is it possible for the same amino acid to condensation polymerise with itself?

so you could have for example: cys cys cys cys cys.....

[davidle_10]

Yep for sure.

[acinod]

Ah, my bad.

Asparagine is an amide group, not amine so it probably won't produce a basic solution. Still not 100% sure, but think wiki, google and the other site are right in saying it's neutral.

Insight 2011 said that the dipeptide formed from Asparagine and Alanine is 'Basic because Asparagine contains a basic NH2 functional group.'

[scocliffe09]

Ah, my bad.

Asparagine is an amide group, not amine so it probably won't produce a basic solution. Still not 100% sure, but think wiki, google and the other site are right in saying it's neutral.

Insight 2011 said that the dipeptide formed from Asparagine and Alanine is 'Basic because Asparagine contains a basic NH2 functional group.'

amide groups are incredibly weak bases which are so weak that they're basically (essentially) neutral (lol). But that said, anything which forms a pH of 7.1 I suppose you could technically call basic...

[bubbles21]

If that comes up in the exam, i think we would all have the right to plan a mass mutiny on vcaa.