966 CHAPTER 23 Amino Acids, Peptides, and Proteinson one atom and a positive charge on a nonadjacent atom. (The name comes from
zwitter, German for “hermaphrodite”or “hybrid.”)
A few amino acids have side chains with ionizable hydrogens (Table 23.2). The
protonated imidazole side chain of histidine, for example, has a of 6.04. Histidine,
therefore, can exist in four different forms, and the form that predominates depends on
the pH of the solution.PROBLEM 4Why are the carboxylic acid groups of the amino acids so much more acidic
than a carboxylic acid such as acetic acidPROBLEM 5 SOLVEDDraw the form in which each of the following amino acids predominantly exists at physio-
logical pH (7.3):a. aspartic acid c. glutamine e. arginine
b. histidine d. lysine f. tyrosineSOLUTION TO 5a Both carboxyl groups are in their basic forms because the pH is
greater than their. The protonated amino group is in its acidic form because the pH
is less than itsPROBLEM 6Draw the form in which glutamic acid predominantly exists in a solution with the follow-
ing pH:a. b. c. d.PROBLEM 7a. Why is the of the glutamic acid side chain greater than the of the aspartic acid
side chain?
b. Why is the of the arginine side chain greater than the of the lysine side chain?23.4 The Isoelectric Point
The isoelectric point(pI) of an amino acid is the pH at which it has no net charge. In
other words, it is the pH at which the amount of positive charge on an amino acid
exactly balances the amount of negative charge:pI (isoelectric point)pH at which there is no net chargepKa pKapKa pKapH= 0 pH= 3 pH= 6 pH= 11+NH
3OO
−OCCH
2 CHCO−pKa.pKa’s(pKa=4.76)?(pKa'2)pKa+NH
3+NH
3+NH
HN NH^3CH 2 CHCOH+ HN NHCH 2 CHCO−+ NNHCH 2 CHCO−NNHCH 2 CHCO−NH 2OOOOpH = 0pH = 4pH = 8pH = 12
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