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Question 18.2: Drawing Zwitterion Forms Look up the zwitterionic structure ......

Drawing Zwitterion Forms
Look up the zwitterionic 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+H^+. At high pH, which is basic, acidic groups may lose H+H^+. In the zwitterion form of an amino acid, the COO−COO^- group is basic and the —NH3+NH_3^+ is acidic.

TABLE 18.3 The 20 𝛂-amino acids found in proteins, with their abbreviations and isoelectric points. The structures are written here in their fully ionized forms. These ions and the isoelectric points given in parentheses are explained in Section 18.4.

Polar , Neutral Side Chains

  H    OH3N+—C∣—C∣∣—O−C∣H3\begin{matrix}   H       O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\ \overset{\mid}{C}H_3\end{matrix}           H    OH3N+—C∣—C∣∣—O−H∣     \begin{matrix}       H       O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\ \overset{\mid}{H}          \end{matrix}             H    OH3N+—C∣—C∣∣—O−               C∣HCH2CH3C∣H3\begin{matrix}       H       O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\                             \overset{\mid}{C}HCH_2 CH_3\\ \overset{\mid}{C}H_3\end{matrix}          H    OH3N+—C∣—C∣∣—O−                C∣H2CHCH3                  C∣H3\begin{matrix}     H       O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\                                \overset{\mid}{C}H_2CHCH_3\\                                    \overset{\mid}{C}H_3\end{matrix}

Alanine, Ala (6.0)                 Glycine, Gly (6.0)                       Isoleucine, Ile (6.0)             Leucine, Leu (6.0)

  H    OH3N+—C∣—C∣∣—O−                 C∣H2CH2SCH3\begin{matrix}    H       O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\                                  \overset{\mid}{C}H_2CH_2SCH_3\\ \\ \\ \end{matrix}        H    OH3N+—C∣—C∣∣—O−      C∣HCH3         C∣H3        \begin{matrix}    H       O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\            \overset{\mid}{C}HCH_3\\                  \overset{\mid}{C}H_3               \\ \\ \\ \\ \\ \end{matrix}

Methionine, Met (5.7)   Phenylalanine, Phe (5.5)   Proline, Pro (6.3)   Tryptophan, Trp (5.9)   Valine, Val (6.0)
Polar , Neutral Side Chains

   H    OH3N+—C∣—C∣∣—O−                  C∣H2C—NH2          O∣∣\begin{matrix}      H       O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\                                   \overset{\mid}{C}H_2C — NH_2\\                   \overset{\mid\mid}{O}\end{matrix}        H    OH3N+—C∣—C∣∣—O−C∣H2    S∣H    \begin{matrix}       H       O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\ \overset{\mid}{C}H_2\\        \overset{\mid}{S}H       \end{matrix}        H    OH3N+—C∣—C∣∣—O−                     C∣H2CH2C—NH2                     O∣∣\begin{matrix}       H       O\\ H_3\overset{+}{N} —\overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\                                          \overset{\mid}{C}H_2CH_2 C — NH_2\\                                          \overset{\mid\mid}{O}\end{matrix}        H    OH3N+—C∣—C∣∣—O−C∣H2O∣H\begin{matrix}      H       O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\ \overset{\mid}{C}H_2\\ \overset{\mid}{O}H\end{matrix}

Asparagine, Asn (5.4)              Cysteine, Cys (5.0)             Glutamine, Gln (5.7)             Serine, Ser (5.7)

   H    OH3N+—C∣—C∣∣—O−         C∣HCH3 O∣H \begin{matrix}     H       O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\                 \overset{\mid}{C}HCH_3\\  \overset{\mid}{O}H\\  \\ \\ \\ \\ \end{matrix}   

Threonine, Thr (5.6)             Tyrosine, Tyr (5.7)
Acidic Side Chains Basic Side Chains

   H    OH3N+—C∣—C∣∣—O−             C∣H2C—O−           O∣∣\begin{matrix}      H       O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\                          \overset{\mid}{C}H_2C — O^-\\                      \overset{\mid\mid}{O}\end{matrix}

Aspartic acid, Asp (3.0)

(Aspartate)

   H    OH3N+—C∣—C∣∣—O−                    C∣H2CH2C—O−                       O∣∣\begin{matrix}      H       O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\                                        \overset{\mid}{C}H_2CH_2C — O^-\\                                             \overset{\mid\mid}{O}\end{matrix}

Glutamic acid, Glu (3.2)
(Glutamate)

H    O                H3N+—C∣—C∣∣—O−           NH2+                  C∣H2CH2CH2NHC∣∣NH2 \begin{matrix}H       O                                \\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-                     NH_2^+\\                                    \overset{\mid}{C}H_2CH_2CH_2NH\overset{\mid\mid}{C}NH_2\\ \\ \\ \\  \end{matrix}

Arginine, Arg (10.8)                                    Histidine, His (7.6)

   H    OH3N+—C∣—C∣∣—O−                                  C∣H2CH2CH2CH2N+H3\begin{matrix}      H       O\\ H_3\overset{+}{N} — \overset{\mid }{C} — \overset{\mid\mid }{C} — O^-\\                                                                    \overset{\mid}{C}H_2CH_2CH_2CH_2\overset{+}{N}H_3\end{matrix}

 

Lysine, Lys (9.7)
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Valine has an alkyl-group side chain that is unaffected by pH. At low pH, valine adds a hydrogen ion to its carboxyl group to give the structure on the left. At high pH, valine loses a hydrogen ion from its acidic —NH3+NH_3^+ group to give the structure on the right.

            O∣∣H3N+—C∣H—C—OH     C∣HCH3  CH3    Low pH                     O∣∣H2N—C∣H—C—O−     C∣HCH3  CH3    High pH\begin{matrix}                        \underset{\mid\mid}{O}\\ H_3\overset{+}{N}—\underset{\mid}{C}H—C—OH\\         \underset{\mid}{C}HCH_3\\   CH_3       \\ Low  pH\end{matrix}               \begin{matrix}                         \underset{\mid\mid}{O}\\ H_2N—\underset{\mid}{C}H—C—O^-\\         \underset{\mid}{C}HCH_3\\   CH_3        \\ High  pH\end{matrix}

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