Free Version
Moderate

# Amino Acid Titration Curves

BIOCHM-KREEMQ

Consider the titration curve below for the amino acid Histidine (His) titrated using $NaOH$. The fully protonated structure of His, which predominates at pH=0, is shown as an inset in the upper left-hand corner. A chart of amino acid pK values is also provided.

Using your understanding of the acid–base properties of amino acids and the information that can be gleaned from titration curves and known pK values, determine which of the following statements related to the titration of His is BEST supported by the information provided above.

Amino Acid $α-COOH \quad$ $α-NH_{3}^{+} \quad$ R-Group
$pK_{1}$ $pK_{2}$ $pK_{R}$
Gly 2.34 9.60 -
Arg 2.17 9.04 12.48
Lys 2.18 8.95 10.53
Tyr 2.20 9.10 10.10
Cys 1.80 10.80 8.30
His 1.82 9.17 6.00
Glu 2.19 9.67 4.25
Asp 1.88 9.60 3.65
A

The isoelectric point (pI) of His = 7.0.

B

The net charge on His at pH = 3.5 is a negative one (-1).

C

His can provide significant buffering capacity in human cells (pH ~ 7), whereas Asp and Glu cannot.

D

His is present at pH 6.0 in approximately equal amounts of the following 2 structures: