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A tripeptide, Gly-Cys-Pro or GCP, was titrated with a solution of sodium hydroxide ($NaOH$), and a graph was made of pH vs. mL $NaOH$ added. There are regions that are relatively flat (plateaus) as well as regions that are steep. The $pK_a$ values for these amino acids are shown in the table.

Which of the following contains the correct functional group that exists as a mixture on the second plateau of this graph?

Amino Acid $pK_{a1}$ $pK_{a2}$ $pK_{aR}$
Cysteine 1.7 10.8 8.3
Glycine 2.3 9.6 --
Proline 2.1 10.6 --

$\ $

Note: $pK_{a1}$ is for the carboxylic acid group, $pK_{a2}$ is for the amino group and $pK_{aR}$ is for the R group.

A

The amino group of glycine.

B

The carboxylic acid group of cysteine.

C

The amino group of cysteine.

D

The thiol group of cysteine.

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