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

Question

Mapub All amino acids have two ionizable functional groups: an a-amino group (average pKe of 9.4) and an acid group (average pK of 2.2). Cysteine has an ionizable side chain (R group) with a a-carboxylic pK, of -8.3.One of the possible ionization states of cysteine is shown below CH2 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 1.5 Scroll down for more of this question. Incorrect. Please select one pH value above and two pH values below 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. 4.8 ] 8.3 9.8 12.3 apma~ s R Give Up & View Solution # Try Again O Next Exit

Explanation / Answer

pK is the pH at which a functional group gets deprotonated.
A group is protonated at a pH lower than its pK.

For cysteine,
pK of amino group = 9.4 (So, it remains protonated at pH < 9.4 and gets deprotonated at pH > 9.4)
pK of carboxyl group = 2.2 (So, it remains protonated at pH < 2.2 and gets deprotonated at pH > 2.2)
pK of R-group = 8.3 (So, it remains protonated at pH < 8.3 and gets deprotonated at pH > 8.3 )

The given structure contains,
Protonated amino group. So, pH must be less than 9.4
Protonated carboxyl group. So, pH must be less than 2.2
Protonated R-group. So, pH must be less than 8.8

So, the pH must be less than 2.2 for all the groups to be in protonated form.
So, pH can 1.5

pK of R-group = 8.3
R-group remains protonataed at pH < 8.3 and gets deprotonated at pH > 8.3 )
So, it remains in charged form (deprotonated form: SH ---> S-) at pH = 9.8 and pH = 12.3

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