Question

#1)

a) The pK₁, pK₂, and pK_R for the amino acid histidine are 1.8, 9.3, and 6.0, respectively. At pH 4.0 would be charged predominantly as follows:

A	a-carboxylate 0, a-amino +1, imidazole +1, net charge +2
B	a-carboxylate +1, a-amino +1, imidazole −1, net charge +1
C	a-carboxylate −1, a-amino +1, imidazole 0, net charge 0
D	a-carboxylate +1, a-amino 0, imidazole −1, net charge 0
E	a-carboxylate −1, a-amino +1, imidazole +1, net charge +1

b) The pK₁, pK₂, and pK_R for the amino acid arginine are 1.8, 9.0, and 12.5, respectively. At pH 7.0 arginine would be charged predominantly as follows:

A	a-carboxylate +1, a-amino 0, guanidino -1, net charge 0
B	a-carboxylate 0, a-amino 0, guanidino +1, net charge +1
C	a-carboxylate −1, a-amino +1, guanidino +1, net charge +1
D	a-carboxylate −1, a-amino 0, guanidino +1, net charge 0
E	a-carboxylate +1, a-amino −1, guanidino −1, net charge −1

c) The pK₁, pK₂, and pK_R for the amino acid aspartic acid are 2.0, 9.9, and 3.9, respectively. At pH 7.0, aspartic acid would be charged predominantly as follows:

A	a-carboxylate +1, a-amino +1, b-carboxylate +1, net charge +3
B	a-carboxylate 0, a-amino +1, b-carboxylate 0, net charge +1
C	a-carboxylate 0, a-amino −1, b-carboxylate 0, net charge −1
D	a-carboxylate +1, a-amino −1, b-carboxylate +1, net charge +1
E	a-carboxylate −1, a-amino +1, b-carboxylate −1, net charge −1

Answer 1

1. a) Listidine ÞH=4 Neutral at pH= 6 = 20 Ans! . carboxylate (-1), amino (+1) imidazole (+1) net change (+1) Arginine NHO Como NH3 pH = 7 Neutral at pH = 10.8 Ans: o cantorylate (I), amino (+1), guanidino (+1) net charge (+1) Aspartic acid I cool pH = 7 COD СОН Neutral at pH=3 Ans: a cantorglate (-1), amino (+1), b-Catboxylate (-), net charge (-1)