Lehninger Principles of Biochemistry
Lehninger Principles of Biochemistry
7th Edition
ISBN: 9781464126116
Author: David L. Nelson, Michael M. Cox
Publisher: W. H. Freeman
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Chapter 3, Problem 14P

(a)

Summary Introduction

To determine: The polypeptide that is more soluble at pH 7 in between (Gly)20 and (Glu)20.

Introduction: Solubility of a polypeptide is its property to dissolve in another substance that depends on its physical and chemical properties. Solubility of polypeptides depends on the polarity of side chains and number of ionized groups.

(a)

Expert Solution
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Explanation of Solution

At pH=7.0, glutamate has more ionizable groups than that of glycine. Glycine belongs to neutral amino acid groups whereas the glutamine has high negative charge at the pH 7. The solubility of a polypeptide increases if more ionisable groups are present in it. This is the reason behind more solubility of (Glu)20 than (Gly)20 at pH=7.0.

Conclusion

The solubility of (Glu)20 would be more than (Gly)20 at pH=7.0.

(b)

Summary Introduction

To determine: The polypeptide that is more soluble at pH 7 in between (LysAla)3 and (PheMet)3.

Introduction: Solubility of a polypeptide is its property to dissolve in another substance. Solubility of a substance depends on the physical and chemical properties of the substance. Solubility of polypeptides depends on the polarity of side chains and number of ionized groups.

(b)

Expert Solution
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Explanation of Solution

(LysAla)3 is a pair of polypeptide that consists of more ionisable groups and have high positively charged groups than (PheMet)3. This is because of larger number of ionisable charged groups present on lysine side chain. (PheMet)3 is less polar than (LysAla)3. So, (LysAla)3 is more soluble than (PheMet)3 at pH=7.0.

Conclusion

(LysAla)3 is more soluble than (PheMet)3 at pH=7.0.

(c)

Summary Introduction

To determine: The polypeptide that is more soluble at pH 6 in between (Ala-Ser-Gly)5 and (AsnSerHis)5.

Introduction: Solubility of a polypeptide is its property to dissolve in another substance. Solubility of a substance depends on the physical and chemical properties of the substance. Solubility of polypeptides depends on the polarity of side chains and number of ionized groups.

(c)

Expert Solution
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Explanation of Solution

(AsnSerHis)5 polypeptides are more soluble than the (Ala-Ser-Gly)5 at pH=6.0. Both polypeptides have the serine side chains and are polymeric molecules. But additionally, (AsnSerHis)5 has polar group asparagine and also are partially protonated. Thus, (AsnSerHis)5 is more soluble than (Ala-Ser-Gly)5 at pH=6.0.

Conclusion

(AsnSerHis)5 is more soluble than (Ala-Ser-Gly)5 at pH=6.0.

(d)

Summary Introduction

To determine: The polypeptide that is more soluble at pH 3 in between (Ala-Asp-Gly)5 and (AsnSerHis)5.

Introduction: Solubility of a polypeptide is its property to dissolve in another substance. Solubility of a substance depends on the physical and chemical properties of the substance. Solubility of polypeptides depends on the polarity of side chains and number of ionized groups.

(d)

Expert Solution
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Explanation of Solution

At pH=3.0, (AsnSerHis)5 shows more solubility than (Ala-Asp-Gly)5. The imidazole groups of the His residues present in the (AsnSerHis)5 polypeptide are positively charged and fully protonated. Whereas the carboxylate groups of the Asp residues are neutral and partially protonated. This is because (AsnSerHis)5 are more soluble at pH=3.0.

Conclusion

(AsnSerHis)5 is more soluble than (Ala-Asp-Gly)5 at pH=3.0.

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