Question

In: Chemistry

On amino acid titration curves, using pH, pKa, etc, how do we know which hydrogens to...

On amino acid titration curves, using pH, pKa, etc, how do we know which hydrogens to remove at which states? I know it has to do with the acidity, and that at low pH the structure gets protonated and whatnot, but how do I know the order in which to protonate/deprotonate?

Solutions

Expert Solution

amino acids have pKa values, which are mainly:

the pH value at which the specie get protonated

if pKa > pH; then the species will be PROTONATED, that is COO- + H+ = COH and NH2 + H+ = NH3+

if pKa < pH; then the species are NOT protonated, there is not enough H+ in solution to do this, so the sample will doante H+ from its own, COOH = COO- + H+ and NH3+ = NH2 + H+

therefore, knowing the pKa values of the amino acid, we can set the values at which the titration will "shift" and also:

- using buffer equaiton

pH = pKa + log(A-/HA)

relate the protonated vs the deprotoanted state

pH = pKa + log(COO-/COOH)

pH = pKa + log(NH2/NH3+)


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