Question 3 The amino acid histidine is very important biologically and can also be used as a buffer
H3N—CH I CH2 1 H C—NN 1\ , CH CH —N, H A
OOH H2N—CH I CH2 z H C—N 11 NCH CH —N .1
OOH H3N—CH I CH2 1 H C—NN II + CH CH—N’
COO-_± I H3N—CH I CH2 1 H C—NN 11 CH CH—N,
co971/ I H2N—CH I CH2 1 H C—NN 11 CH CH—N,
B C D E
a. (4 pts) The structures above show forms of histidine. Using the letters “A, B, C, D, E” to name the forms, fill in the blanks in the acid/base equilibria below.
Form:
pKR 4-o.
pK2 41—-1.
b. (4 pts) The pl of histidine is 7.6. What is the predominant form of histidine at pH 7.6 and what is its net charge? Give your answer as “A, B, C, D, E.”
Form
Net Charge
c. (5 pts) There are two Henderson-Hasselbach equations you cou. win., Lui ifistidine buffer at pH 7.6. Write them both, using the relevant term pKi, pK2, or pKR, and the correct letters (A, B, C, D or E) for the conjugate acid and base forms.
d. (3 pts) Using “D” as the conjugate base species, calculate the ratio of conjt____Ilatel)asep its weak acid at pH 7.6. Show all work
