Obtain the three pK_a values of the a-amino acid histidine from Table 27.1. Because there are three pK_a values, the dissociation equilibria contain four species.

(a) With the species present at low pH on the left, and the fully dissociated species on the right, draw and label by letter all of the species involved in the acid–base dissociation equilibria of histidine.

(b) Sketch a graph similar to Fig. 27.1 in which the fraction of all of the species are shown as a function of pH. Label each curve with a letter corresponding to one of the forms you drew in part (b). Indicate the pH values at which the fractions of two species are equal.

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TABLE 27.1 Names, Structures, Abbreviations, and Properties of the Twenty Common Naturally Occurring Amino Acids Name and abbreviations R* Amino acids with simple allphatic side chains glycine, Gly, G alanine, Ala, A valine, Val, V leucine, Leu, L isoleucine, lle, I -H -CH3 -CH(CH3)₂ -CH₂CH(CH3)₂ -CH-CH₂CH3 CH3 [S configuration] General structure: Optical rotation of Lenantiomer in H₂O (sign of [a],) E EIE (+) (+) (-) (+) H₂N-CH- pka1 lo -0- 2.34 2.35 2.32 2.36 2.36 PK₂2 9.60 9.69 9.62 9.60 9.68 РКаз Isoelectric point, pl 5.97 6.02 5.97 5.98 6.02