CHEMISTRY (LL) W/CNCT   >BI<
CHEMISTRY (LL) W/CNCT >BI<
13th Edition
ISBN: 9781260572384
Author: Chang
Publisher: MCG
Question
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Chapter 25, Problem 25.48QP

(a)

Interpretation Introduction

Interpretation:

From the given the pKa values of the two groups, +NH3 and COOH of valine, the more acidic one has to be determined.

Concept Introduction:

Greater the pKa value of a substance lower will be its acidic property.

According to equation given in chapter 16 in the text book (equation 16.4),

pH and pKa are related as,

pH = pKa + log [conjugate base][acid]

(a)

Expert Solution
Check Mark

Answer to Problem 25.48QP

COOH group is more acidic.

Explanation of Solution

Given that pKa of +NH3 and COOH groups of valine are 9.62 and 2.32 respectively.

Since COOH has lower pKa value of 2.32, it is more acidic.

(b)

Interpretation Introduction

Interpretation:

The predominant form of valine at pH 1.0,7.0 and 12 has to be predicted.

Concept Introduction:

Greater the pKa value of a substance lower will be its acidic property.

According to equation given in chapter 16 in the text book (equation 16.4),

pH and pKa are related as,

pH = pKa + log [conjugate base][acid]

(b)

Expert Solution
Check Mark

Explanation of Solution

Given data:

pKa of carboxyl group, COOH = 2.3pKa of ammonium group, NH3+ = 9.6

At pH = 1,

Concentration of COOH

pH = pKa + log [conjugate base][acid]1 = 2.32+ log [COO][COOH][COOH][COO] = 21 

Concentration of NH3+

pH = pKa + log [conjugate base][acid]1 = 9.62+ log [NH2][NH3+][NH3+][NH2] = 4.2×108

Concentration of NH3+ is greater than COOH.  Hence, the predominant species is CH(CH3)2CH(+NH3)COOH.

At pH = 7,

Concentration of COOH

pH = pKa + log [conjugate base][acid]7 = 2.32+ log [COO][COOH][COO][COOH] = 4.8×104 

Concentration of NH3+

pH = pKa + log [conjugate base][acid]7 = 9.62+ log [NH2][NH3+][NH3+][NH2] = 4.2×102

Concentration of COO is greater than NH3+.  Hence, the predominant species is CH(CH3)2CH(+NH3)COO.

At pH = 12,

Concentration of COOH

pH = pKa + log [conjugate base][acid]12 = 2.32+ log [COO][COOH][COO][COOH] = 4.8×109 

Concentration of NH3+

pH = pKa + log [conjugate base][acid]12 = 9.62+ log [NH2][NH3+][NH2][NH3+] = 2.4×102

Concentration of COO is greater than NH3+.  Hence, the predominant species is CH(CH3)2CH(NH2)COO.

(c)

Interpretation Introduction

Interpretation:

Isoelectric point of valine has to be calculated.

Concept Introduction:

Isoelectric point (pI) of an amino acid refers to the pH at which the amino acid remains neutral and does not migrate in an electric field

It is calculated as,

pI = pKa1+pKa22

Where ,

pKa1and pKa2 are two pKa values of amino acid.

(c)

Expert Solution
Check Mark

Answer to Problem 25.48QP

Isoelectric point of valine is calculated as 5.97

Explanation of Solution

Given data:

pKa1 of carboxyl group, COOH, = 2.32pKa2 of ammonium group, NH3+ = 9.62

Therefore isoelectronic point of valine is,

pI = pKa1+pKa22 = 2.32 + 9.622 = 5.97

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