Chapter 21, Problem 25P

For each amino acid: [1] draw the L enantiomer in a Fischer projection; [2] classify the amino acid as neutral, acidic, or basic; [3] give the three-letter symbol; [4] give the one-letter symbol.

1. leucine
2. tryptophan
3. lysine
4. aspartic acid

Interpretation Introduction

(a)

Interpretation:

The L-isomer of leucine in Fisher projection needs to be drawn and should be identified as acidic, basic or neutral. The three letter and one symbol for it needs to be determined.

Concept Introduction:

Like monosaccharides, the prefixes D and L are used to designate the arrangement of groups to the chiral center of amino acid, when drawn with a vertical carbon chain having $-{\text{COO}}^{-}$ group at the top and R group at the bottom.

Amino acids that have the ${\text{-NH}}_{\text{3}}{}^{+}$ group on the left side of Fisher projection is called as L isomer. For example,

Answer to Problem 25P

• L-isomer of leucine is as follows:

• Leucine is neutral.
• Three-letter symbol for leucine is Leu.
• One-letter symbol for leucine is L.

Explanation of Solution

For leucine the R group is ${\text{CH}}_{\text{2}}\text{CH}{\left({\text{CH}}_{\text{3}}\right)}_{\text{2}}$ and in L-isomer the amino group $\left({\text{-NH}}_{\text{3}}{}^{+}\right)$ group present in the left side of Fisher projection. So, L-isomer of leucine is as follows:

The amino acid leucine has one positive charge on amino group $\left({\text{-NH}}_{\text{3}}{}^{+}\right)$ and one negative charge on carboxyl group $\left(-{\text{COO}}^{-}\right)$ , so, the net charge is zero. Hence, the amino acid leucine is neutral.

One letter symbol for leucine is L and three-letter symbol for leucine is Leu.

Interpretation Introduction

(b)

Interpretation:

To draw L-isomer of tryptophan (amino acid) in Fisher projection, to identify it is acidic, basic or neutral and to give three letter and one symbol for it

Concept Introduction : :

Like monosaccharides, the prefixes D and L are used to designate the arrangement of groups to the chiral center of amino acid, when drawn with a vertical carbon chain having $-{\text{COO}}^{-}$ group at the top and R group at the bottom.

Amino acids that have the ${\text{-NH}}_{\text{3}}{}^{+}$ group on the left side of Fisher projection is called as L isomer. For example,

Answer to Problem 25P

• L-isomer of tryptophan is as follows:

• Amino acid tryptophan is neutral.

Explanation of Solution

For tryptophan the R group is as follows:

And in L-isomer the amino group $\left({\text{-NH}}_{\text{3}}{}^{+}\right)$ group present in the left side of Fisher projection. So, L-isomer of tryptophan is as follows:

The amino acid tryptophan has one positive charge on amino group $\left({\text{-NH}}_{\text{3}}{}^{+}\right)$ and one negative charge on carboxyl group $\left(-{\text{COO}}^{-}\right)$ , so, the net charge is zero. Hence, the amino acid tryptophan is neutral.

One letter symbol for tryptophan is W and three-letter symbol for tryptophan is Trp.

Interpretation Introduction

(c)

Interpretation:

The L-isomer of lysine in Fisher projection needs to be drawn, whether it is acidic, basic or neutral needs to be determined and the three-letter and one symbol for it needs to be given.

Concept Introduction:

Like monosaccharides, the prefixes D and L are used to designate the arrangement of groups to the chiral center of amino acid, when drawn with a vertical carbon chain having $-{\text{COO}}^{-}$ group at the top and R group at the bottom.

Amino acids that have the ${\text{-NH}}_{\text{3}}{}^{+}$ group on the left side of Fisher projection is called as L isomer. For example,

Answer to Problem 25P

• L-isomer of lysine is as follows:

• The amino acid lysine is basic in nature.
• The three letter symbol for lysine is Lys.
• The one letter symbol for lysine is K.

Explanation of Solution

For lysine the R group is ${\left({\text{CH}}_2\right)}_4{\text{NH}}_{\text{3}}{}^{+}$ and in L-isomer the amino group $\left({\text{-NH}}_{\text{3}}{}^{+}\right)$ group present in the left side of Fisher projection. So, L-isomer of lysine is as follows:

The amino acid lysine has one positive charge on amino group $\left({\text{-NH}}_{\text{3}}{}^{+}\right)$ and one negative charge on carboxyl group $\left(-{\text{COO}}^{-}\right)$ and one positive charge on ${\left({\text{CH}}_2\right)}_4{\text{NH}}_{\text{3}}{}^{+}$ , so, the net charge is one. Hence, the amino acid lysine is basic in nature.

The three letter symbol for lysine is Lys and one letter symbol for lysine is K.

Interpretation Introduction

(d)

Interpretation:

The L-isomer of aspartic acid in Fisher projection needs to be drawn, whether it is acidic, basic or neutral needs to be determined and the three-letter and one symbol for it needs to be given.

Concept Introduction:

Like monosaccharides, the prefixes D and L are used to designate the arrangement of groups to the chiral center of amino acid, when drawn with a vertical carbon chain having $-{\text{COO}}^{-}$ group at the top and R group at the bottom.

Amino acids that have the ${\text{-NH}}_{\text{3}}{}^{+}$ group on the left side of Fisher projection is called as L isomer. For example,

Answer to Problem 25P

• L-isomer of aspartic acid is as follows:

• The aspartic acid is acidic in nature.
• The three letter symbol for aspartic acid is Asp.
• The one letter symbol for aspartic acid is D.

Explanation of Solution

For aspartic acid the R group is ${\text{CH}}_{\text{2}}{\text{COO}}^{-}$ and in L-isomer the amino group $\left({\text{-NH}}_{\text{3}}{}^{+}\right)$ present in the left side of Fisher projection. So, L-isomer of aspartic acid is as follows:

The aspartic acid has one positive charge on amino group $\left({\text{-NH}}_{\text{3}}{}^{+}\right)$ and one negative charge on carboxyl group $\left(-{\text{COO}}^{-}\right)$ , and one more negative charge on ${\text{CH}}_{\text{2}}{\text{COO}}^{-}$ group so, the net charge is negative. Hence, the aspartic acid is acidic in nature.

The three letter symbol for aspartic acid is Asp and the one letter symbol for aspartic acid is D.