Problem 99 - Chemistry The Central Science

Question

The standard free energy of formation of solid glycine is \(-369 \mathrm{~kJ} / \mathrm{mol},\) whereas that of solid glycylglycine is \(-488 \mathrm{~kJ} / \mathrm{mol}\). What is \(\Delta G^{\circ}\) for the condensation of glycine to form glycylglycine?

Step-by-Step Solution

Answer

The standard free energy change (ΔG°) for the condensation reaction of glycine to form glycylglycine can be calculated using the formula \(\Delta G^{\circ}= \sum (\Delta G_{\mathrm{products}}^{\circ}) - \sum (\Delta G_{\mathrm{reactants}}^{\circ})\). Considering only non-zero values for glycine and glycylglycine, the calculation is: \(\Delta G^{\circ} = (-488 \mathrm{~kJ/mol}) - (2 * -369 \mathrm{~kJ/mol}) = 250 \mathrm{~kJ/mol}\). Thus, ΔG° for the reaction is 250 kJ/mol.

Step 1: (Step 1: Write down the given information)

We are given the standard free energy of formation of: - Solid glycine: -369 kJ/mol - Solid glycylglycine: -488 kJ/mol

Step 2: (Step 2: Write down the reaction)

To find the ΔG° for the condensation reaction of glycine to form glycylglycine, we need to first write down the reaction: \(2 \mathrm{Glycine} \rightarrow \mathrm{Glycylglycine} + \mathrm{Water}\)

Step 3: (Step 3: Use the formula to find ΔG° for the reaction)

The formula to find ΔG° for a reaction, using the standard free energy of formation is: \(\Delta G^{\circ}= \sum (\Delta G_{\mathrm{products}}^{\circ}) - \sum (\Delta G_{\mathrm{reactants}}^{\circ})\) In our reaction, since water is very stable and its standard free energy of formation is virtually zero, we can consider only the non-zero values for glycine and glycylglycine. ΔG° = (ΔG° (glycylglycine)) - (2 * ΔG°(glycine))

Step 4: (Step 4: Substitute the given values and find ΔG°)

Now, we can plug in the given values into the formula: \(\Delta G^{\circ} = (-488 \mathrm{~kJ/mol}) - (2 * -369 \mathrm{~kJ/mol})\) \(\Delta G^{\circ} = -488 \mathrm{~kJ/mol} + 738 \mathrm{~kJ/mol}\) \(\Delta G^{\circ} = 250 \mathrm{~kJ/mol}\) The standard free energy change (ΔG°) for the condensation reaction of glycine to form glycylglycine is 250 kJ/mol.

Key Concepts

Condensation Reaction

A condensation reaction is a type of chemical reaction where two molecules combine to form a larger molecule, with the loss of a small molecule such as water. In the case of forming glycylglycine from glycine, it is an example of a condensation reaction. Here’s how the reaction occurs:

• Two glycine molecules are involved, and they combine to create a single glycylglycine molecule.
• During this process, a molecule of water (H₂O) is released as a byproduct.
• This type of reaction is fundamental in forming peptide bonds, which are crucial in building proteins.

Thus, understanding condensation reactions helps in grasping how many complex biological molecules are synthesized.

Glycine

Glycine is the simplest amino acid, and it's important in biochemistry. Its structure contains:

• An amino group (-NH₂)
• A carboxylic acid group (-COOH)
• A hydrogen atom as its side chain, making it unique because it is non-chiral.

These characteristics make glycine versatile and essential in several biological processes, including:

• Protein synthesis: As a building block, glycine is crucial for creating proteins that carry out numerous cell functions.
• Neurotransmitter: It functions as a neurotransmitter in the central nervous system, where it can either stimulate or inhibit neuronal activity.

Within the context of the exercise, two molecules of glycine react to initiate the formation of a larger molecule, glycylglycine, through a condensation reaction.

Glycylglycine

Glycylglycine is a dipeptide, which means it's composed of two amino acid residues linked by a peptide bond. Here’s a breakdown:

• The peptide bond forms between the carboxyl group of the first glycine molecule and the amino group of the second glycine molecule.
• This bond formation results in glycylglycine, which can act as a simple model for studying peptide and protein behavior.
• It's often used in research and laboratory settings to understand more complex protein structures and functions.

By forming glycylglycine from glycine, one can better grasp how amino acids link to construct peptides, an essential process for protein synthesis and cellular functions.