Question 16.CC.3: Ionized Forms of Amino Acids Consider the amino acid cystein......
Ionized Forms of Amino Acids
Consider the amino acid cysteine.
a. What is the pI of cysteine and what does it mean?
b. At a pH of 2.0, how does the zwitterion of cysteine change?
c. At a pH of 8.0, how does the zwitterion of cysteine change?
a. From Table 16.2, the pI of cysteine is 5.1, which means that at a pH of 5.1, cysteine exists as a zwitterion with an overall charge of zero.
b. Because a pH of 2.0 is more acidic and below the pI of cysteine, the —COO¯ gains H^+ to give —COOH . The remaining — {\mathrm{NH}_{3}}^+ gives cysteine an overall positive charge (1+).
c. Because a pH of 8.0 is more basic and above the pI of cysteine, the —{\mathrm{NH}_{3}}^+ loses H^+ to give — \mathrm{NH}_{2} The remaining —COO¯ gives cysteine an overall negative charge (1-).
| TABLE 16.2 The 20 Amino Acids (Ionized) in Proteins | |||||
| Nonpolar Amino Acids | |||||
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| Polar Amino Acids (Neutral) | |||||
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| Acidic Amino Acids | Basic Amino Acids | ||||
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