   Chapter 22, Problem 130AE

Chapter
Section
Textbook Problem

# a. Use bond energies (see Table 3-3) to estimate ΔH for the reaction of two molecules of glycine to form a peptide linkage.b. Would you predict ΔS to favor the formation of peptide linkages between two molecules of glycine?c. Would you predict the formation of proteins to be a spontaneous process?

(a)

Interpretation Introduction

Interpretation: By using given bond energies the enthalpy ΔH for the reaction of two molecules of glycine to form a peptide linkage is to be estimated. The entropy ΔS to favor the formation of peptide linkages between two molecules of glycine is to be predicted. Also, the formation of proteins is a spontaneous process or not is to be predicted.

Concept introduction: When two or more amino acids are linked in a chain, a compound called peptide is formed in which the carboxyl group of each acid is joined to the amino group of the next forming a -OC-NH- bond, known as peptide bond. Glycine is an amino acid with a single hydrogen atom present in its side chain. The chemical formula of glycine is (C2H5NO2) . When two molecules of glycine joined through a peptide linkage a dipeptide is formed.

To determine: The ΔH for the reaction of two molecules of glycine to form a peptide linkage by using bond energies.

Explanation

Explanation

The enthalpy ΔH for the reaction is 1302 KJ_ . Hence, the reaction is an endothermic reaction.

The reaction between two molecules of glycine to form peptide linkage is,

Figure 1

The enthalpy of the reaction is calculated by using the formula,

ΔHrxnο=D(bonds broken)-D(bonds formed)

In the given reaction shown in Figure 1 bonds are broken and formed during peptide formation and energy is absorbed and released. The bond energies are shown in (Table 3-3) respectively.

Therefore, the energy required to break the bonds is calculated as,

2 moles of 2N-H bonds = 4×391 KJ = 1564 KJ2 moles of C-N bonds = 2×305 KJ = 610 KJ2 moles of 2C-H bonds = 4×413 KJ = 1652 KJ2 moles of C-C bonds = 2×347 KJ = 694 KJ

2 moles of C=O bonds = 2×745 KJ = 1490 KJ2 moles of C-O bonds = 2×358 KJ = 716 KJ2 moles of O-H bonds = 2×467 KJ = 93

(b)

Interpretation Introduction

Interpretation: By using given bond energies the enthalpy ΔH for the reaction of two molecules of glycine to form a peptide linkage is to be estimated. The entropy ΔS to favor the formation of peptide linkages between two molecules of glycine is to be predicted. Also, the formation of proteins is a spontaneous process or not is to be predicted.

Concept introduction: When two or more amino acids are linked in a chain, a compound called peptide is formed in which the carboxyl group of each acid is joined to the amino group of the next forming a -OC-NH- bond, known as peptide bond. Glycine is an amino acid with a single hydrogen atom present in its side chain. The chemical formula of glycine is (C2H5NO2) . When two molecules of glycine joined through a peptide linkage a dipeptide is formed.

To determine: The entropy ΔS to favor the formation of peptide linkages between two molecules of glycine.

(c)

Interpretation Introduction

Interpretation: By using given bond energies the enthalpy ΔH for the reaction of two molecules of glycine to form a peptide linkage is to be estimated. The entropy ΔS to favor the formation of peptide linkages between two molecules of glycine is to be predicted. Also, the formation of proteins is a spontaneous process or not is to be predicted.

Concept introduction: When two or more amino acids are linked in a chain, a compound called peptide is formed in which the carboxyl group of each acid is joined to the amino group of the next forming a -OC-NH- bond, known as peptide bond. Glycine is an amino acid with a single hydrogen atom present in its side chain. The chemical formula of glycine is (C2H5NO2) . When two molecules of glycine joined through a peptide linkage a dipeptide is formed.

To determine: The formation of proteins is a spontaneous process or not.

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