Amino Acid Titration

September 22, 2009 at 7:01 am #11848

Participant

Hello.

I´m having problems understanding some of the key concepts of amino acid titration – I hop you can help me.

As I understand it, the amino acid is at it´s fully protonated form when the solution is at it´s lowest pH value. How can this be, when the definition of a low pH value is a high concentration of H+? Why isn´t the amino acid fully deprotonated instead? And when the amino acid is at it´s highest pH value – why is it fully deprotonated instead of fully protonated? Now that the amino acid is fully deprotonated, the solution must be full of H+-ions, and therefore the pH should be very low.

And one more thing: The amino acids that are negatively or positively charged at neutral pH – what happens to them during titration?

Hope you can help me. Thank you.

September 22, 2009 at 7:14 am #92938

JackBean

Participant

That’s all about equilibria.
Lets have some simple system with simple acid

HA <-> H+ + A-

if you add protons to the mixture, what will happen? You will have lot of protons, so the equilibrium will move to the left. And that is, why is amino acid protonated at low pH. The acidifying protons do not come from the amino acid (otherwise it would be at approx. neutral pH).
The same applies at high pH, just reversed.

About the charged AAs, they behave in accordance to they pK. If you have pH under it, the group will be protonated, if will be higher, the group will be deprotonated. For the same reasons as above 😉

September 22, 2009 at 6:03 pm #92955

micron66

Participant

Thank you. I got it.

April 24, 2010 at 8:09 am #99270

elyateem

Participant

hi
please…could anyone help me???
i have a problem in calculating the net charge for the naa,aaa,baa
i dont know where we put +.5,-.5,+1or -1
plzzzzz help me

April 24, 2010 at 8:40 am #99272

JackBean

Participant

what is naa,aaa,baa? 🙄

where does the stupid practice, not to write 0, comes from? 🙄

April 24, 2010 at 8:49 am #99273

elyateem

Participant

naa=neutral amino acid
aaa=acidic amino acid
baa=basic amino acid
……its not stupid

April 24, 2010 at 8:58 am #99276

JackBean

Participant

it is pretty stupid to write +.5, that’s nothing

just look, what charged groups it have at the particular pH

April 24, 2010 at 9:21 am #99280

elyateem

Participant

iam not talking of ma own pocket
its written like this in the book
if you dont know wt iam talkin about so dont answer without say thats this is stupid question

April 24, 2010 at 9:48 am #99281

JackBean

Participant

I’m saying, it’s stupid to write like +.5, not that the question is stupid (not that it wouldn’t be…)

I gave you the answer…

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