Alumina oxide is the sorbent of
choice for the separation of basic compounds which
include alkaloids, amines, steroids, terpenes,
aromatic and aliphatic hydrocarbons. Surprisingly,
most separation scientists remain unaware of the
benefits provided by alumina and continue to use
silica gel for these separations. To that end this
paper addresses the role of alumina oxide in the
purification of natural plant alkaloids.
There is great structural and biological diversity
in the natural plant alkaloids, with more than 100
alkaloids introduced into medical practice.
Medicinal and aromatic plants are playing an
increasingly important role in the pharmaceutical
world in addition to their usage in the cosmetic,
fragrance, specialty food and nutraceutical sectors.
There are more than 420,000 distinct plant species,
yet less than 10% of them have been fully analyzed.
Almost all alkaloids are toxic and most display
pharmacologic activity. The isolation and
purification of these distinct alkaloid species is a
major goal of the biotechnology and pharmaceutical
industries with screening procedures for
phytochemical analysis beyond the scope of this
paper. However, each of the sundry classes of plant
alkaloids has been successfully isolated and
purified using activated alumina as the sorbent of
choice in TLC and flash chromatographic systems.
Given that fact that most botanical agents are plant
alkaloids TLC plates coated with alumina are the
preferred screening tool of choice, as botanicals
have species specific fingerprints.
HPLC is an excellent tool for the quantitative
analysis of marker compounds in botanical samples.
Selecting a desired phytochemical is an appropriate
method of establishing a quantitative analysis for a
marker compound. The crystallization of alkaloids,
both their bases and their salts from different
solvents has been phased out as a separation method
for isolating and purifying natural plant products.
An alkaloid is a cyclic organic
compound in a negative oxidation state found in
living organisms. The majority of alkaloids are
found in plant species, although there are alkaloids
which have also been isolated from animals.
The process of isolating plant alkaloids follows the
following sequence of events:
-
Extraction of the raw
material from the plant species
-
Separation of the total
alkaloids from the other extracted substances
-
Separation of the alkaloids
-
Purification of each of the
individual alkaloids
Plant species may contain many
unique alkaloids, making the separation process
complex. For example the plant species Catharanthus
roseus contains more than 90 unique alkaloid
substances. The location of these alkaloids within
plants varies widely, with the highest
concentrations of alkaloid compounds being found in
the root system, seeds and bark. Of interest, during
the beginning of the vegetation period in the
springtime these alkaloid compounds pass from these
depot sites into the shoots and then into the plant
leaves.
Due to the complex taxonomy of plant species many
types of separation techniques have been utilized
for isolating and purifying plant alkaloids.
Separation may be achieved through a combination of
extraction, crystallization, chromatography,
molecular distillation and other purification
processes.
Extraction is the first step in
the process. Examples of extraction solvents
currently used to disrupt plant species to yield
alkaloids are hexane, methyl acetate, acetone,
methanol and hydrofluorocarbons such as
1,1,1,2-tetrafluoroethane. Some of these solvents
have high boiling points and the elevated
temperatures used in the distillation process can
degrade some of the desired plant alkaloids.
The goal in using liquid chromatography for the
purification of plant alkaloids is clear. Once
alkaloids are extracted and ready for purification,
they are ready to be eluted. The crude alkaloid
extract is mixed with an eluent known as the mobile
phase and placed in contact with a solid adsorbent
known as the stationary phase to progressively and
selectively remove various compounds held on the
adsorbent. The particular alkaloid of interest can
then be collected in an essentially pure form by
monitoring and collecting the solvent eluate as it
is recovered from the adsorbent. As the solvent
flows through the column, it carries the crude
material along with it, but the various components
of the crude material are carried along at different
rates owing to the different affinities that those
components have for the adsorbent/solvent pair.
The development of thin layer chromatography began
with a desire to isolate unique plant alkaloids. TLC
began in 1938 when Izmailov and Shraiber separated
plant extracts using aluminum oxide spread on a
glass plate.
For the extraction and
purification of alkaloids alumina oxide is the
sorbent of choice. Alumina possesses both Lewis acid
and basic sites and is excellent at adsorbing plant
alkaloids, possibly through strained Al-O bonds. The
sorbent activity of alumina is modified by altering
both the conditions of temperature and hydration.
Alumina oxide becomes activated by heating to a
specified temperature for a defined time. The
optimal activation temperature for alumina is
unclear but fully activated alumina for coating of
TLC plates can be produced by heating at 150-200 C
for a few hours. The activity of alumina can be
lowered by adding precise amounts of water. ````
The activity of alumina may be defined according to
the Brockmann scale. Using this scale the addition
of 3% water is characterized as Brockmann II, the
addition of 6% water is Brockman III and the
addition of 10% and 15% water respectively leads to
Brockmann IV and Brockman V. Therefore, the lower
the amount of water mixed in with the alumina oxide,
the lower the Brockman level and the greater the
power for alumina to act as a sorbent agent.
Working with plant alkaloids the
solid adsorbent is usually a polar material,
preferably in particulate form and normally packed
in a column to form a bed. The solvent entrains or
dissolves the composition to be treated and
transports it through the column, specifically
through the adsorbent material packing the column.
Although the solvent may move passively through the
packed column using gravity, most extraction
processes of plant alkaloids today use the technique
of flash chromatography in which the solvent is
driven through the column using a pump or some other
means to create a positive (superatmospheric)
pressure at the inlet end of the column.
The type of activated alumina used as the adsorbent
will be selected based on pore size, particle size
and pH. In turn, the decision on which activated
alumina to use will be based on the characteristics
of the plant alkaloids, such as the compound
polarity, solubility, molecular size and shape.
Active aluminas are quite sensitive to the differing
shapes of various aromatic hydrocarbons and some of
their derivatives, permitting an excellent
separation of many aromatic isomers. The weight
ratio of the composition to be treated to the
adsorbent is typically in the range from 20:1 to
1:2, and is preferably in the range of 1:1 to 4:1.
At this stage unique alkaloids
can be collected and made available for further
testing regarding biological activity, structure
analysis and determination of therapeutic utility.
