Question 18.2: Look up the zwitterion structure of valine in Table 18.3. Dr...
Look up the zwitterion structure of valine in Table 18.3. Draw valine as it would be found (a) at low pH and (b) at high pH.
ANALYSIS At low pH, which is acidic, basic groups may gain H^+ and at high pH, which is basic, acidic groups may lose H^+. In the zwitterion form of an amino acid, the – COO^- group is basic and the – NH_3 ^+ is acidic.
Valine has an alkyl-group side chain that is unaffected by pH. At low pH, which is acidic, valine adds a hydrogen ion to its carboxyl group to give the structure on the left below. At high pH, which is basic, valine loses a hydrogen ion from its acidic – NH_3 ^+ group to give the structure on the right below.
\underset{Low pH}{\begin{array}{r c} O \qquad \enspace \; \\ _+ \qquad \qquad || \qquad \enspace \; \\ H_3N – CH – C – OH \\ | \qquad \qquad \quad \enspace \\ CHCH_3 \qquad \; \\ | \qquad \qquad \quad \enspace \\ CH_3 \qquad \qquad \end{array}} \quad \underset{High pH}{\begin{array}{r c} O \qquad \; \\ || \qquad \enspace \\ H_2N – CH – C – O^- \\ | \qquad \qquad \quad \enspace \\ CHCH_3 \qquad \; \\ | \qquad \qquad \quad \enspace \\ CH_3 \qquad \qquad \end{array}}Related Answered Questions
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