Opium Alkaloids: Structure and Functional Groups

Whether or not morphine is a white powder, it derives its properties from its shape at the molecular level. The production of morphine requires the use opium, a latex obtained from Papaver Somniferum, and opium itself is comprised of many different chemicals which have different structures. All of the intricacies of the molecular structures of these chemicals are what create the chemical and some of the physical properties of the analgesic drug morphine. Let’s take a closer look.

 The Structure and Properties of Morphine

To begin with, the chemical formula of morphine is C17H19NO3, and these components are arranged in such a way that the molecule follows what is called the “morphine rule”. All opioid analgesics follow the morphine rule, and their ability to relieve pain is derived from their fulfillment of the morphine rule. To follow the morphine rule, a molecule must have the following structures: an aromatic ring; a quaternary carbon atom; and two carbon atoms that connect the quaternary carbon atom to a tertiary amine group. A ring, in general, is when a group of atoms bond to each other and form a closed figure with their bonds. An aromatic ring is a type of ring in which the bonds between atoms alternate between double bonds and single bonds. Additionally, the aromatic ring is flat, which means that the atoms that make up the ring lie on a single plane. The quaternary carbon atom is one that is bonded to four other carbon atoms, and there exists only one in the structure of morphine. This quaternary carbon is attached to the aromatic ring, and it is bonded to a chain of 2 CH2 groups and the tertiary amine group. Whereas a primary amine group is made of a nitrogen atom with a lone pair of electrons that’s bonded to 2 hydrogen atoms and a variable group, a tertiary amine group is not bonded to any hydrogen atoms at all. This bonding results in the nitrogen atom having a slightly positive charge, which contributes to its chemistry in the brain. On the Lewis structure for morphine included below, this chain goes out of the plane that the aromatic ring lies on. This fact is also a contributor to how morphine binds to receptor sites in nerve cells in the brain.

                                            Structure of the morphine molecule

Pictured here is the chemical structure of morphine, an alkaloid found in opium, in two different forms. On the 3D rendering, the red portions represent a bonding oxygen and the blue portion represents the amine group. Note the functional groups present in its structure.

The morphine molecule itself is polar because the nitrogen in the amine group and the various oxygen atoms in the molecule form polar covalent bonds with the hydrogen atoms and the carbons in the molecule. However, these oxygen and nitrogen atoms are only four of the atoms of the morphine molecule, which is quite large in relation to these atoms. Because of this, water cannot surround the whole molecule and it only dissolves a little when placed into water. When administering morphine via injection, the insolubility of morphine in water can be a problem. To help remedy this, morphine is made into the salts morphine hydrochloride or morphine sulfate, which are much more soluble than pure morphine. However, since dissolving these salts in water results in a pH of 5, sodium hydroxide must be added to make the solution’s pH more basic and suitable for injection into the body.

 Relations to Other Substances

At the end of our previous blog post, we mentioned that morphine can be used to make heroin and codeine, which are also opioids. In fact, the molecular structures of heroin and codeine are similar to the molecular of morphine. Heroin has the chemical formula C21H23NO5, and it possesses 4 more carbon atoms, 4 more hydrogen atoms, and 2 more oxygen atoms than morphine. These differences can be seen in the picture [below / above / to the side], with the additions to the morphine structure depicted in red. As it can be seen from the structure, heroin still follows the morphine rule, so it has similar effects on receptor sites in nerve cells.


Pictured here is the molecular structure of heroin. Its structure is the morphine molecule, shown in black, with two acetyl groups attached, which are shown in red. An acetyl group is simply a methyl group single-bonded to a carbonyl group.

Codeine can be made by replacing the hydroxyl group, OH, on the aromatic ring of morphine with a methoxy group, OCH3, as it is depicted on the diagram to the right in red. Like heroin, codeine also still obeys the morphine rule and can block receptor sites in the brain 

Recent studies involving the chemical structure of morphine have found that there are compounds in the brain that show remarkably intricate similarities between their structures, and that of morphine. These compounds, called enkephalins, are found in two forms in the brain.  These compounds are similar to morphine’s structure, but they are peptides made from five amino acids. An amino acid is a compound composed of a carbon bonded to a hydrogen atom, an amine group, and a carboxyl group, which is comprised of a carbon atom double bonded to an oxygen and single bonded to an OH. Between the two forms of enkephalins, the only difference is the final amino acid.  Their similarity in structure to morphine is demonstrated by the initial “tyr” peptide. The enkephalins act to bind to analgesic receptor sites, which can be seen in the diagram below, in the brain, thus blocking the transfer of pain signals.in the same way that morphine can. 

Analgesic Receptor Site

In this picture, a diagram of the analgesic receptor site is shown.  This site can receive morphine, as portrayed in this particular picture.  There are three main parts to this site: a flat area which accommodates a flat aromatic ring, a cavity to receive other rings perpendicular to it, and an anionic site amine group interaction.

The function of enkephalins, morphine, and other similar substances in the nervous system will be discussed more thoroughly in our next blog post. If you’re interested in the biological aspects of opium and its alkaloids, be on the look out for the next post.


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