Does anyone know anything about Amino acids?

[speg]

Basically, a little quiz online (worth no marks) just to make sure we understand concepts. Gives us a molecule and asks at which pH values (integer) it would exist at.

So they give glycine with the H on the carboxyl group already lost. According to
the notes, this happens at pH 2.3

So we should be able to find this molecule at pH 3 - so far so good.

The notes also says that amino group of glycine will disassociate at pH 9.6

So this molecule should exist from pH 3 - 9

However the correct answer given, is 3,7,8.

I do not understand why it would not exist at pH 4,5,6, or 9.

/exhausted

 

[Babbles]

I Googled up the pka numbers for glycine, and it jives with 2.34 and 9.60. I too do not understand why one couldn't say it exists at all pHs between ~3 and ~9.

Unless the prof is looking for an answer that would be essentially pH neutral, i.e. 7. Were the answers multiple choice?

 

[speg]

Indicate your answers by choosing number(s) from the below list. 1 2 3 7 8 10 12 13 For example, if the pH values for the amino acid are 10 and 12, write 10,12 in the blank with no space between a number and the comma

Correct Answer: 3,7,8

 

[speg]

OMG. I am an idiot. There is no 4,5,6,or 9 on that list :|

Are you freaking kidding me?!!?! Why couldn't he have just told me that 9 emails ago?!

ARGH!!!!!!!

Ok, life can now move on. 🙂

 

[Crusty]

Originally posted by: speg
OMG. I am an idiot. There is no 4,5,6,or 9 on that list :|

Are you freaking kidding me?!!?! Why couldn't he have just told me that 9 emails ago?!

ARGH!!!!!!!

Ok, life can now move on. 🙂

QFT

 

[MrPickins]

Originally posted by: speg
OMG. I am an idiot. There is no 4,5,6,or 9 on that list :|

Are you freaking kidding me?!!?! Why couldn't he have just told me that 9 emails ago?!

ARGH!!!!!!!

Ok, life can now move on. 🙂

:laugh:

The devil is in the details...

 

[uclaLabrat]

Different amino acids have different protons that are acidic in different ranges. Around pH 2-4, carboxylic acids deprotonate. Around 9-11, the ammonium protons come off, as well as the thiol proton on cysteine. Hydroxyl protons come off around pH 14.

 

[Babbles]

Glad you discovered the problem!

Originally posted by: uclaLabrat
Different amino acids have different protons that are acidic in different ranges. Around pH 2-4, carboxylic acids deprotonate. Around 9-11, the ammonium protons come off, as well as the thiol proton on cysteine. Hydroxyl protons come off around pH 14.

Holy cow, there was only around six posts in this entire thread; couldn't be bothered to read it?

Those posting here know what a pka value is - basically what you tried to describe without using the term. Anyhow, the OP's issue was not the lack of understanding the dissociation of a proton but rather why his answers didn't match up with the profs.

 

[Rockinacoustic]

I encourage you to have a strong understanding of where each amino acid (de)protonates at each ionizable group. It will come in handy when you start determining the isoelectric point (pI) values of peptide chains.

 

[GasX]

PEBKAC

 

[uclaLabrat]

Originally posted by: Babbles
Glad you discovered the problem!

Originally posted by: uclaLabrat
Different amino acids have different protons that are acidic in different ranges. Around pH 2-4, carboxylic acids deprotonate. Around 9-11, the ammonium protons come off, as well as the thiol proton on cysteine. Hydroxyl protons come off around pH 14.

Holy cow, there was only around six posts in this entire thread; couldn't be bothered to read it?

Those posting here know what a pka value is - basically what you tried to describe without using the term. Anyhow, the OP's issue was not the lack of understanding the dissociation of a proton but rather why his answers didn't match up with the profs.

Congratulations. Now he knows what pKa's the different protons have, and thus doesn't need to look them up. If he understood that, he would understand that it's just a poorly worded question. I was assuming if he realized that, he wouldn't be making a thread about it. Thanks for playing.

 

[PlasmaBomb]

Glad you sorted the problem and gave us a laugh 🙂