91Ó°ÊÓ

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 localid="1663345833873" $\mathbf{\text{COOH}}$ group and an unprotonated localid="1663345865389" ${\mathbf{\text{NH}}}_{\mathbf{\text{2}}}$group

localid="1663345870225" $\mathbf{(}{\mathbf{\text{K}}}_{\mathbf{\text{X}}}{\mathbf{\text{´Ç´Ú°ä°¿°¿±á²µ°ù´Ç³Ü±è=4.47×10}}}^{\mathbf{\text{-3}}}{\mathbf{\text{;K}}}_{\mathbf{\text{b}}}{\mathbf{\text{ofNH}}}_{\mathbf{\text{2}}}{\mathbf{\text{²µ°ù´Ç³Ü±è=6.03×10}}}^{\mathbf{\text{-3}}}\mathbf{\text{y?}}$

Use glycine, localid="1663345879880" ${\mathbf{\text{NH}}}_{\mathbf{\text{3}}}{\mathbf{\text{CH}}}_{\mathbf{\text{3}}}\mathbf{\text{COOH}}$, to explain why.

(b) Calculate localid="1663345908281" ${\mathbf{\text{[}}}^{\mathbf{\text{+}}}{\mathbf{\text{NH}}}_{\mathbf{\text{3}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{COO}}}^{\mathbf{\text{-}}}{\mathbf{\text{y}}}^{\mathbf{\text{+}}}{\mathbf{\text{NH}}}_{\mathbf{\text{3}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}\mathbf{\text{COOH]atpH5.5.}}$

(c) The R group of lysine is localid="1663345894686" ${\mathbf{\text{-CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{NH}}}_{\mathbf{\text{2}}}{\mathbf{\text{(pK}}}_{\mathbf{\text{b}}}\mathbf{\text{=3.47)}}$Draw the structure of lysine at .localid="1663345916050" $\mathbf{\text{pH}}$physiological localid="1663345902202" $\mathbf{\text{pH(-7),andpH13.}}$

(d) Thelocalid="1663345947279" $\mathbf{R}$group of glutamic acid localid="1663345920800" ${\mathbf{\text{-CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{CH}}}_{\mathbf{\text{2}}}\mathbf{\text{COOH}}\mathbf{(}{\mathbf{\text{pK}}}_{\mathbf{\text{a}}}\mathbf{\text{=}}\mathbf{\text{4.07).}}$of the forms of glutamic acid that are shown below, which predominates at,localid="1663345941655" $\mathbf{\text{(1)}}$localid="1663345993071" $\mathbf{\text{pH1}}$(2) localid="1663345925494" $\mathbf{\text{physaiologicalpHH(-7),}}$

and (3) localid="1663345936358" $\mathbf{\text{pH13?}}$

Step by step solution

01

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,

02

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.

03

Finding the ratio of pH

(b) The value of$\text{pK\_a}$ 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,

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

$\left(\frac{\left[{\text{NH}}_3{\text{CH}}_2{\text{COO}}^{−}\right]}{{\text{NH}}_3{\text{CH}}_2\text{COOH}]}\right)\text{ratioatpH}5.5\text{is}\mathbf{1}×{\mathbf{10}}^3$

Hence, the ratio of pH is $1×{10}^3$

04

Step 3:Draw the structures of lysine

(c)

The required structures of lysine are:

Therefore, the structure shown in solution.

05

Step 4:Draw the Structures

The required structures are:

Therefore, the structure shown in solution.

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