Q19.121CP - Chemistry: Molecular Nature Of Matter And Change

Question

Amino acids [general formula  ${\mathbf{\text{NH}}}_{\mathbf{\text{2}}}\mathbf{\text{CH(R)COOH]}}$can be considered polypro tic acids. In many cases, the  R group contains additional amine and carboxyl groups.

(a) Can an amino acid dissolved in pure water have a protonated $\mathbf{\text{COOH}}$ group and an unprotonated ${\mathbf{\text{NH}}}_{\mathbf{\text{2}}}$group  

$\mathbf{(}{\mathbf{\text{K}}}_{\mathbf{\text{X}}}{\mathbf{\text{of COOH group =4.47×10}}}^{\mathbf{\text{-3}}}{\mathbf{\text{;K}}}_{\mathbf{\text{b}}}{\mathbf{\text{ of NH}}}_{\mathbf{\text{2}}}{\mathbf{\text{ group =6.03×10}}}^{\mathbf{\text{-3}}}\mathbf{\text{y?}}$

Use glycine, ${\mathbf{\text{NH}}}_{\mathbf{\text{3}}}{\mathbf{\text{CH}}}_{\mathbf{\text{3}}}\mathbf{\text{COOH}}$ , to explain why.

(b) Calculate  ${\mathbf{\text{[}}}^{\mathbf{\text{+}}}{\mathbf{\text{NH}}}_{\mathbf{\text{3}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{COO}}}^{\mathbf{\text{-}}}{\mathbf{\text{y}}}^{\mathbf{\text{+}}}{\mathbf{\text{NH}}}_{\mathbf{\text{3}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}\mathbf{\text{COOH] at pH 5.5.}}$

(c) The R group of lysine is ${\mathbf{\text{-CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{NH}}}_{\mathbf{\text{2}}}{\mathbf{\text{ (p K}}}_{\mathbf{\text{b}}}\mathbf{\text{= 3.47)}}$Draw the structure of lysine at  .$\mathbf{\text{pH}}$ physiological  $\mathbf{\text{pH (-7), and pH 13.}}$

(d) The$\mathbf{R}$ group of glutamic acid ${\mathbf{\text{-CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}\mathbf{\text{COOH}}\mathbf{(}{\mathbf{\text{pK}}}_{\mathbf{\text{a}}}\mathbf{\text{=}}\mathbf{\text{ 4.07).}}$of the forms of glutamic acid that are shown below, which predominates at  ,$\mathbf{\text{(1)}}$$\mathbf{\text{pH1}}$(2) $\mathbf{\text{physaiological pHH(-7),}}$

 and (3) $\mathbf{\text{pH13?}}$

Step-by-Step Solution

Answer

(a) No, amino acid dissolved in pure water have a protonated $\text{COOH }$  group and an unprotonated  ${\text{NH}}_{\text{2}}$ group

(b)  The ratio of pH is .$1\times {10}^3$

(c) The structure shown in solution.

(d) The structure shown in solution.

Step 1: Concept introduction

Polypro tic Acid is a chemical capable of giving more than one proton. Diprotic and triprotic polypro tic acids are two forms of polyprotic acid that can donate two and three protons,

Step 2:protonated and unprotonated group

(a)No. A protonated $\text{COOH}$ group and an unprotonated  group will not exist in glycine dissolved in pure water. It will instead have a protonated  group and an unprotonated $\text{COOH}$ group.

As a result,No, amino aciddissolved in pure water have a protonated $\text{COOH}$group and an unprotonated group.

Step 3: Finding the ratio of pH

(b) The value of$\text{p K\_a}$  can be computed as follows: begin gathered  

$\begin{array}{c}{\text{pK}}_{\text{a}}{\text{=-logK}}_{\text{a}}\\ {\text{pK}}_{\text{a}}\text{=-log}\left({\text{4.47×10}}^{\text{-3}}\right)\\ {\text{pK}}_{\text{a}}\text{=2.34969}\end{array}$

Obtained by plugging the values of  and pH into the Henderson-Hasselbalch Equation.

The  $p{K}_a$  $\frac{\left[{\text{NH}}_3{\text{CH}}_2{\text{COO}}^{-}\right]}{{\text{NH}}_3{\text{CH}}_2{\text{COOH}}^{-1}}$ratio can be calculated as below,

$\begin{array}{c}\text{5.5=2.34969+}\left(\frac{\left[{\text{NH}}_{\text{3}}{\text{C}}_{\text{2}}{\text{COO}}^{\text{-}}\right]}{\left[{\text{NH}}_{\text{3}}{\text{CH}}_{\text{2}}\text{COOH}\right]}\right)\\ \left(\frac{\left[{\text{NH}}_{\text{3}}{\text{CHH}}_{\text{2}}{\text{COO}}^{\text{-}}\right]}{\left[{\text{NH}}_{\text{3}}{\text{CH}}_{\text{2}}\text{COOH}\right]}\right){\text{=1×10}}^{\text{3}}\end{array}$

$\left(\frac{\left[{\text{NH}}_3{\text{CH}}_2{\text{COO}}^{-}\right]}{{\text{NH}}_3{\text{CH}}_2\text{COOH}]}\right)\text{ ratio at pH }5.5\text{ is }\mathbf{1}\times {\mathbf{10}}^3$

 

 Hence, the ratio of pH is  $1\times {10}^3$

Step 3:Draw the structures of lysine

(c)

The required structures of lysine are:

Therefore, the structure shown in solution.

Step 4:Draw the Structures

The required structures are:

Therefore, the structure shown in solution.