All amino acids have two ionizable functional groups: an a-amino group (average

Question

All amino acids have two ionizable functional groups: an a-amino group (average pKa of 9.4) and an a-carboxylic acid group (average pKa of 2.2). Cysteine has an ionizable side chain (R group) with a pKa of -8.3. One of the possible ionization states of cysteine is shown below. H3N CH -C OH CH SH At what pH would the above structure be the predominant ionization state? Note: Consider the ionization state of all three of the functional groups. pH Select answer Scroll down for more of this question. The protonated form of the R group of cysteine is shown in the structure above. The ratio of the protonated form to the charged (deprotonated) form depends on the pKa of the R group and the pH of the solution. Select all the pH values at which the charged form of the R group would predominate. 4.3 8.3 9.8 11.8

Explanation / Answer

the NH2 group is protonated, so the pH must be lower than the pKa2

the COOH grop is protonated so pH must be lower pKa1

therefore, any pH < 2.2

For a charged group of R...

SH + H+ --> SH2+

so..

this must be rich in H+ ions, so it must have lower pH than the pKa value

anything lower than 8.3 will do

4.3 is a good guess, 8.3 will be 50%-50% approx

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