Alkaloid Isolation & Purification

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

The primary focal point of this lesson will be on a specific type of natural product known as alkaloids. Our topics of discussion will include how they are isolated and purified from their natural sources.

Where Does Medicine Come From?

Have you ever stopped to consider all of the drugs that are on the market today for the wide range of illnesses and ailments we suffer from? Sinus infections? There's a drug for that. You suffer from diabetes? Luckily, there's medicine for that too. But where did those drugs come from? Although it's true that a lot of prescription drugs are made by synthetic organic chemists in the lab, what you may not realize is that a lot of the molecular architectures we use today in the clinical setting were either derived from or inspired by molecules found in nature.

It turns out nature is extremely good at using biological machinery to construct compounds that are not only structurally complex but also very useful to humans for medicinal applications. Today we're going to be talking about a specific class of compounds found in nature known as the alkaloids. Our primary focal points will be on how they are isolated and purified from their natural sources. Let's get started!

What Is an Alkaloid?

Let's start by briefly talking about how alkaloids are defined. An alkaloid is simply a compound derived from natural sources, like plants or bacteria, that contains a basic nitrogen atom. A basic nitrogen atom means that the nitrogen itself contains a lone pair of electrons that can be used to react with an acid of some sort.

You may already be familiar with a very popular alkaloid without knowing it. The compound known as morphine is a very important alkaloid that was isolated from the poppy seeds of opium poppy plants back in 1804 and is heralded for its pain-relieving properties. It's also received a lot of negative press due to its highly addictive nature. Notice that lone pair of electrons on the nitrogen atom though, which is what makes it an alkaloid.

Morphine is an example of an alkaloid with a basic nitrogen atom.

Isolation of Alkaloids

Suppose you know for a fact that a sea sponge sample you collected while scuba diving in Fiji contains alkaloids that are active against cancer cells. The job you have in front of you now is to extract and isolate the alkaloids from among the other thousands of organic compounds present in the sponge sample. To put things in perspective, it would be like trying to find a tiny needle among millions of pieces of straw in a barn. Fortunately there are some steps we can follow that will make things a bit easier.

Step 1

The first thing we need to do is take our sponge sample and crush it up into a very fine powder. The reason this is necessary is the finer the particles, the greater the surface area becomes and the better the extraction goes.

Step 2

The next thing we need to do is get rid of all the fats, terpenes, and other oils from the sample, because we aren't really interested in those types of compounds. This can be easily done by extracting them from the powder with hexanes or petroleum ether.

Solvents like hexanes and petroleum ether work best because they are very good at solvating non-polar compounds like fats, oils, and terpenes alike. These can then be discarded since we don't need them.

Step 3

Next, we need to extract the alkaloids that are left with a solvent. Methanol or ethanol tend to be the best solvents for extracting alkaloids. The methanol or ethanol solvent will pull all of the alkaloids out of the crude sponge material. This extraction needs to be done multiple times to make sure all of the alkaloids get isolated. An instrument called a rotary evaporator can then be used to evaporate off the solvent. At this point, we have our crude alkaloid mixture (natural sources rarely contain only one alkaloid).

Purification of Alkaloids

Now that we have our crude alkaloid mixture, we need to separate and purify each individual alkaloid from one another. This is most commonly accomplished by using an analytical separation technique called high-performance liquid chromatography (HPLC). HPLC is an excellent way to fractionate the mixture repeatedly until we have only pure individual alkaloids left.

HPLC is an analytical technique that allows for the separation of individual alkaloids from a mixture

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