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Bioorganic Chemistry  185
411. The amino acid valine has a pKa = 2.3 for the carboxyl end and a pKa = 9.6 
for the amino end. This means that the pI for valine is 5.95 (the average 
of the two pKa values). Which of the following is true concerning a 1 mM 
valine solution buffered at pH 2.3?
(A) There are equal concentrations of the zwitterion form and the form 
with a +1 net charge.
(B) There are equal concentrations of the zwitterion form and the form 
with a –1 net charge.
(C) Only the fully protonated form is present.
(D) Only the fully deprotonated form is present.
412. The amino acid valine has a pKa = 2.3 for the carboxyl end and a pKa = 9.6 
for the amino end. This means that the pI for valine is 5.95 (the average 
of the two pKa values). Which of the following is true concerning a 1 mM 
valine solution buffered at pH 12?
(A) 
O
CH
CH CH3
CH3
OH3N C
+ –
(B) 
O
CH
CH CH3
CH3
OHH2N C
(C) 
O
CH
CH CH3
CH3
H2N C O
–
(D) 
O
CH
CH CH3
CH3
H3N C OH
+