Chapter 21, Problem 49P

Glycine has pK₂ values of 2.34 and 9.60. At what pH does glycine exist in the forms shown?

Interpretation Introduction

Interpretation:

The pH at which glycine exists at the given form has to be calculated.

Concept introduction:

The isoelectric point $\text{(pI)}$ is the $\text{pH}$ at which the amount of positive charge and negative charge on an amino acid becomes balanced.

The value of $pI$ is depends upon the $p{K}_a$ value of protonated amino group and $p{K}_a$ value of carboxyl group of an amino acid.

Protons are released when the $\text{pH}$ becomes $p{K}_a$ of species.

Answer to Problem 49P

The pH of the glycine for the given form is $\text{9}\text{.60}\text{.}$

Explanation of Solution

In the given form of glycine,

The $\text{50\%}$ of amino group of the glycine are released their protons, so the pH of the system becomes $p{K}_a$.

The $p{K}_a$ of the amino group of glycine is $\text{9}\text{.60}\text{.}$

Therefore,

The pH of the glycine for the given form is $\text{9}\text{.60}\text{.}$

Interpretation Introduction

Interpretation:

The pH at which glycine exists at the given form has to be calculated.

Concept introduction:

The isoelectric point $\text{(pI)}$ is the $\text{pH}$ at which the amount of positive charge and negative charge on an amino acid becomes balanced.

The value of $pI$ is depends upon the $p{K}_a$ value of protonated amino group and $p{K}_a$ value of carboxyl group of an amino acid.

Protons are released when the $\text{pH}$ becomes $p{K}_a$ of species.

Explanation of Solution

In the given form of glycine,

The amount of positive charge and negative charge are equal or balanced for 100% system.

The isoelectric point ($pI$) is the $\text{pH}$ at which the amount of positive charge and negative charge on an amino acid becomes balanced.

Therefore,

The pH of the glycine for the given form is $\text{6}\text{.02}\text{.}$

Interpretation Introduction

Interpretation:

The $\text{pH}$ at which glycine exists at the given form has to be calculated.

Concept introduction:

The isoelectric point $\text{(pI)}$ is the $\text{pH}$ at which the amount of positive charge and negative charge on an amino acid becomes balanced.

The value of $pI$ is depends upon the $p{K}_a$ value of protonated amino group and $p{K}_a$ value of carboxyl group of an amino acid.

Protons are released when the $\text{pH}$ becomes $p{K}_a$ of species.

Answer to Problem 49P

The pH of the glycine for the given form is $\text{2}\text{.34}\text{.}$

Explanation of Solution

In the glycine form of glycine,

The $\text{50\%}$ of $\text{–OH}$ group of the glycine are released their protons, so the pH of the system becomes $p{K}_a$ of the $\text{–OH}$ of glycine.

The $p{K}_a$ of the $\text{–OH}$ group of glycine is $\text{2}\text{.34}\text{.}$

Therefore,

The pH of the glycine for the given form is $\text{2}\text{.34}\text{.}$