Amino Acid Titration

[micron66]

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.

[JackBean]

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

[micron66]

Thank you. I got it.