Question

Amino acids are the building blocks of proteins. The simplest amino acid is glycine \(\left(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{COOH}\right)\). Draw a Lewis structure for glycine. (Hint: The central atoms in the skeletal structure are nitrogen and the two carbon atoms. Each oxygen atom is bonded directly to the right-most carbon atom.)

Short answer

The Lewis structure for glycine has NH2 bonded to the left carbon, followed by CH2, which is bonded to a right carbon. The right carbon is bonded to two oxygens (one in a hydroxyl group and another as a carbonyl group). Each oxygen atom has two lone pairs, and nitrogen has one lone pair.

Step-by-step solution

Identify the atoms and their valences

First, write down the atoms present in glycine: nitrogen (N), hydrogen (H), carbon (C), and oxygen (O). Their valences are: N (3), H (1), C (4), and O (2). Hydrogen will form one bond, oxygen will form two, nitrogen will form three, and carbon will form four bonds.

Arrange the Skeleton of the Molecule

As the hint suggests, place nitrogen and the two carbon atoms as the central atoms in a chain with nitrogen on the left, followed by the first carbon atom which is directly bonded to the second carbon atom on the right. Connect oxygen atoms to the right-most carbon.

Add Hydrogen Atoms

Place the remaining hydrogen atoms. There are two hydrogens bonded to nitrogen (forming NH2), two hydrogens bonded to the first (middle) carbon atom, and one hydrogen bonded to the second (right-most) carbon atom. The other bonds of the right-most carbon are occupied by the oxygen atoms.

Complete the Octets for Oxygen and Carbon

Oxygen needs to complete its octet by forming double bonds. Therefore, the two oxygen atoms will form a double bond with the right-most (carbonyl) carbon atom, which is also bonded to a hydroxyl group (OH) and one hydrogen atom. The middle carbon has its four bonds completed with the adjacent hydrogen and nitrogen atoms.

Check the Formal Charges

Make sure all atoms have the lowest possible formal charge. In the final structure, all atoms should have a formal charge of zero, following the octet rule.

Add Lone Pairs on Oxygen and Nitrogen

Include the unshared pairs of electrons (lone pairs) on nitrogen and oxygen atoms to complete the Lewis structure. Nitrogen has one lone pair, while each oxygen has two lone pairs.

Key concepts

Amino Acids

Amino acids are often referred to as the building blocks of proteins, playing a critical role in various biological processes. Each amino acid consists of a central carbon atom, known as the alpha carbon, to which an amino group (NH₂), a carboxyl group (COOH), a hydrogen atom, and a distinctive side chain are attached.

These molecules are unique in that they contain both an amino group and a carboxyl group, characterized as amphoteric — they can act as either acids or bases in chemical reactions. The variation in side chains among the different amino acids affects their properties and functions within proteins.

For example, glycine, the simplest amino acid, has a single hydrogen atom as its side chain, making it non-polar and yielding it quite flexible in protein structures. Understanding the basic structure of amino acids like glycine helps in comprehending how they connect to form peptides and larger protein molecules through peptide bonds — a key type of chemical bonding in biochemistry.

Chemical Bonding

Chemical bonding is the physical process that enables atoms to be held together in chemical compounds. There are several types of bonds including ionic, covalent, hydrogen bonds, and van der Waals forces, with covalent bonding being paramount in organic compounds such as amino acids.

In the context of glycine, the Lewis structure illustrates the covalent bonding within the molecule. Covalent bonds occur when valence electrons are shared between atoms, enabling each to fill its valence shell, achieving stability. For instance, the nitrogen in glycine shares its three valence electrons with hydrogen and carbon atoms to form stable bonds. Similarly, the carbon atoms in glycine form four covalent bonds, adhering to the octet rule for maximum stability.

Glycine and Peptide Bonding

Furthermore, when amino acids like glycine combine to form proteins, they establish peptide bonds. These are special covalent bonds that link the carboxyl group of one amino acid to the amino group of another, releasing a molecule of water in the process. This condensation reaction is fundamental to protein synthesis.

Valence Electrons

Valence electrons play a crucial role in understanding chemical bonding, as they are the outermost electrons involved in forming bonds. The number of valence electrons an atom possesses determines its bonding capacity, which is critical when predicting how atoms will combine to form molecules.

For example, in the Lewis structure of glycine, nitrogen has five electrons in its outer shell but only needs three more to complete the octet, which is why it forms three covalent bonds. Carbon, having four valence electrons, can make four bonds, as seen in both carbon atoms in glycine. Oxygen, with six valence electrons, needs two additional electrons to reach an octet, leading it to form two bonds — typically one or two covalent bonds as seen in the carboxyl group of glycine.

When drawing the Lewis structure, one begins by counting the valence electrons available and then pairing them up to fulfill the octet rule (or duet rule for hydrogen). This is crucial for forming stable molecules. In amino acids like glycine, correctly pairing up these electrons using the step-by-step approach ensures an accurate representation of the molecule's electron configuration.