Question

The following is a simplified (skeletal) structure of the amino acid tryptophan. Draw a complete Lewis structure of the molecule.

Short answer

Draw tryptophan with a central amino acid backbone, an indole ring, complete octets, and proper hydrogen placements.

Step-by-step solution

Identify Key Atoms and Bonds

Begin by identifying all atoms within the tryptophan molecule. Tryptophan is comprised of carbon (C), hydrogen (H), nitrogen (N), and oxygen (O). Determine the primary bonding arrangement: carbon atoms typically form four bonds, nitrogen three bonds (with a lone pair), oxygen two bonds (with two lone pairs), and hydrogen one bond.

Arrange Atoms to Form Carbon Backbone

Place the carbon atoms in a typical arrangement seen in amino acids, which is a central carbon with a nitrogen on one side (forming the amino group), a carboxyl group (COOH) on the other end, and a side chain (R group) characteristic of tryptophan, which is an indole ring.

Complete the Indole Ring

Tryptophan has an indole side chain. Draw the indole ring by connecting five carbon atoms in a hexagon shape, then attach a five-membered ring containing a nitrogen atom. Make sure to complete the rings with alternating double bonds (conjugated system).

Add Remaining Bonds and Lone Pairs

After forming the backbone and the side chain, add hydrogen atoms to complete the tetravalency of carbon atoms. Add lone pairs to nitrogen and oxygen to complete their octet. Oxygen will have two lone pairs, nitrogen will have one lone pair. Double bonds may be used in the carboxyl group and within the indole ring.

Check Octet Rule and Formal Charges

Ensure all atoms (except hydrogen) follow the octet rule. Adjust any atoms if necessary to maintain stability. Check for formal charges and ensure that they are minimized where possible, ensuring all carbon, nitrogen, and oxygen atoms are in their optimal bonding states.

Key concepts

Amino Acid Tryptophan

Amino acids are the building blocks of proteins, and tryptophan is one of the essential amino acids required by the body. It is characterized by having a central carbon atom to which an amino group (NH\(_2\)) and a carboxyl group (COOH) are attached. These groups are typical for all amino acids, but what makes tryptophan unique is its indole side chain. The indole ring contributes to tryptophan's aromatic properties and is composed of a six-membered benzene ring fused with a five-membered nitrogen-containing pyrrole ring.

This molecular complexity plays a vital role in the protein construction and biological processes, such as serotonin synthesis. Understanding how to sketch its Lewis structure helps in grasping these biochemical interactions.

Carbon Bonding

Carbon atoms are versatile in their bonding capabilities, which is why they are the backbone of organic molecules like tryptophan. Each carbon atom can form four covalent bonds with other atoms, which allows the formation of complex structures.

In tryptophan, carbon forms bonds with nitrogen, oxygen, hydrogen, and other carbon atoms. This includes a single bond with hydrogen and various single, double, or resonant bonds in the indole structure. Typically:

• Carbon to carbon bonds can be single (C-C) or double (C=C) depending on the molecule's needs.
• The carbon bonds in the backbone and rings must adhere to stability principles, balancing strength and flexibility.
• The indole ring requires specific carbon bonding arrangements for resonance.

Proper understanding of these bonds in tryptophan is crucial as they define its three-dimensional shape and reactivity.

Octet Rule

The octet rule is a principle in chemistry that reflects how atoms strive to have eight electrons in their valence shell, similar to noble gases. This rule is applied to ensure the stability of molecules like tryptophan.

In practice, you'll find that:

• Carbon atoms achieve a full octet by forming four bonds.
• Nitrogen atoms, found in the amino and indole group, typically form three bonds and carry a lone pair.
• Oxygen fulfills its octet by forming two bonds and retaining two lone pairs.

When drawing Lewis structures, you should always ensure each non-hydrogen atom fulfills the octet rule, if possible, to predict the most stable structure. In situations where molecules form resonance structures, such as in aromatic systems, the octet rule guides how electrons are shared among atoms.

Formal Charges

Formal charge calculation involves a comparison between the number of valence electrons in an isolated atom and the electrons it "owns" in a molecule. This concept is important in determining the most plausible Lewis structure for tryptophan.

The steps to calculate formal charges include:

• Assigning half the shared electrons to each bonded atom.
• Calculating the formal charge using the formula: \[ \, \text{Formal Charge} = \text{Valence Electrons} - \text{Non-bonding Electrons} - \frac{1}{2} \times \text{Bonding Electrons} \, \]
• Ensuring the sum of all formal charges in the molecule equals the total charge of the molecule.
• Optimizing structures by having formal charges close to zero.

Minimizing formal charges helps in developing a stable structure for tryptophan, aiding its role in structural formation and chemical reactions in biological systems.