Question

Nylon was designed to be a synthetic silk. (a) The average molar mass of a batch of nylon 66 is \(12,000 \mathrm{~g} / \mathrm{mol}\). How many monomer units are there in this sample? (b) Which part of nylon's structure is similar to a polypeptide's structure? (c) How many different tripeptides (made up of three amino acids) can be formed from the amino acids alanine (Ala), glycine (Gly), and serine (Ser), which account for most of the amino acids in silk?

Short answer

(a) 53 monomer units are present in the sample. (b) The amide link (-CONH-) in the repeating unit of both nylon and a polypeptide is the similar part. (c) 27 different tripeptides can be formed.

Step-by-step solution

Calculate number of monomer units

Given the molar mass of the nylon 66 is 12,000 g/mol. The molecular weight of the nylon 66 monomer (made up of one adipic acid and one hexamethylenediamine molecule) is \(226 \mathrm{~g} / \mathrm{mol}\). You can calculate the number of monomer units by dividing the molar mass of the polymer by that of the monomer. So you have, \( \frac{12000}{226} = 53.1 \), approximately 53 monomer units are in this sample.

Compare nylon's and polypeptide's structure

Nylon has a structure similar to a polypeptide as both of them are constructed by the process of condensation polymerization, which results in the elimination of a small molecule (like water), during the formation of the polymer chain. The repeating unit in both the structures has an amide link (-CONH-). This amide linkage is similar in both nylon (a polyamide) and polypeptides.

Calculate different tripeptides

The three amino acids alanine (Ala), glycine (Gly), and serine (Ser) can form a tripeptide in \(3^3 = 27\) different ways. That's because for each of the three positions in the tripeptide, there are three possible amino acids that can be chosen.

Key concepts

Molar Mass Calculation

When we talk about molar mass calculation, we’re referring to finding the weight of one mole of a substance, which is a fundamental concept in chemistry. It is particularly crucial when dealing with polymers like nylon. To solve the given problem, the molar mass of the entire nylon 66 polymer was provided as 12,000 grams per mole. To find out how many monomer units are present in a sample, you need to understand that the molar mass of the polymer is the sum of the molar masses of all the individual monomer units that make it up.

To calculate the number of monomer units, divide the total molar mass of the polymer by the molar mass of a single monomer unit: \[\frac{{12000 \text{{ g/mol}} }}{{226 \text{{ g/mol}} }} = 53.1\]. This means that there are approximately 53 monomer units in the sample of nylon 66.

Polyamide Structure

Polyamides, such as nylon, feature a distinctive structural characteristic that bears a striking resemblance to the structure of polypeptides: the amide bond. The term polyamide refers to a type of polymer that contains repeating amide groups \(-CONH-\) in its backbone, giving these materials their name. These bonds are formed in a similar way to the bonds found in proteins and peptides, which is why they share structural similarities. In the case of nylon 66, this polyamide is synthesized from adipic acid and hexamethylenediamine, resulting in a robust and durable material with a structure that is comparable to the natural silk used as the benchmark for its development.

Peptide Bond Formation

The formation of a peptide bond is a critical step in creating both proteins and synthetic polymers like nylon. This bond is a type of covalent bond that links amino acids in proteins, and it is formed during a condensation reaction. Specifically, the carboxyl group \(-COOH\) of one amino acid reacts with the amine group \(-NH2\) of another, releasing a molecule of water and creating an amide linkage known as a peptide bond \(-CONH-\). This is exactly the kind of bond found within polyamides and polypeptides, explaining the similarity noted in their structures.

Condensation Polymerization

Condensation polymerization is a process where monomers link together, losing small molecules as byproducts, typically water. This is how nylon is made, with two different types of monomers—acid and amine—joining to form an amide bond and releasing a water molecule. This is different from addition polymerization, where monomers join without the loss of small molecules. The expulsion of these smaller molecules during condensation polymerization helps in creating long, repeating chains that constitute the backbone of materials like nylon and proteins alike.

Amino Acids Combination

The combination of amino acids to form peptides or proteins is akin to a form of biological polymerization. With just three amino acids—alanine (Ala), glycine (Gly), and serine (Ser), we can form numerous unique tripeptides. Each position in the tripeptide can be occupied by any of the three amino acids, leading to \(3 \times 3 \times 3 = 27\) different combinations. This exercise illustrates the incredible variety of structures that can be created from a limited set of building blocks, reflecting the versatility of amino acids and the complexity of protein structures found in nature and mimicked in synthetic materials like nylon.