Alkaloids are nitrogen-containing small molecules which are mainly found in plants.1) To date, approximately 27 thousand alkaloids have been reported in which 21 thousand were of plant origin. Since most of the alkaloids exhibit potent bioactivity, they are the main component of many herbal medicines. Some alkaloids are also used as pharmaceuticals. Alkaloids interact with proteins electrostatically due to the positive charge of the nitrogen atoms which facilitates binding to the negative charge of the proteins. Especially, pharmaceutical alkaloids tend to bind to the receptors in neural systems.
Biosynthetic pathways to alkaloids vary greatly depending on the nitrogen source. Alkaloids are biosynthesized from various kinds of amino acids as depicted in Fig. 1. Further complex alkaloids are also biosynthesized by attaching another alkaloid to the original skeletons.
In the IUPAC Nomenclature Appendix, 67 alkaloid parent structures are shown.2) They can be classified into their basic skeletons based on their biosynthetic origins as shown in Fig. 1. In this section, alkaloids are categorized into nine: eight as shown above and purine alkaloids as the ninth. Polyamines which are occasionally considered as alkaloid-related substances, will be explained in the section “Polyamines”.
In general, alkaloids dissolve in acidic water. Their salts dissolve in neutral water. They are usually insoluble in neutral water as free forms, but soluble in polar organic solvents such as chloroform, methanol or DMSO.
HPLC：In the reversed phase mode using an ODS column, peak tailing is suppressed by addition of acetic acid or trifluoroacetic acid to the mobile phase. Effective analysis would be achieved by a mixed mode column, TCI-Dual, with ion-exchange and reversed phase modes simultaneously. Detection is generally conducted by a UV detector. TLC：In a normal phase silica gel TLC analysis, a solvent system of chloroform-methanol-aqueous ammonia gives a relatively a good separation. NH2-type silica gel is also effective. Dragendorff’s reagent is a selective visualizing reagent for alkaloids. Tertiary amines and quaternary ammonium salts give orange spots.
Alkaloids are relatively stable at room temperature, however, they tend to form carbonate salts due to their basicity by reaction with carbon dioxide in the air. In addition, they are usually colorized by reaction with oxygen. Therefore, opened bottles should be stored in the refrigerator or frozen with inert gas such as nitrogen or argon.
Some of the alkaloids may cause serious physiological actions in very minute amounts. They may cause paralysis, convulsions, and death in the worst case. To avoid inhalation and contacting with skin, wear protective goggles, mask and eyeglasses when handling. Sufficient caution should be taken, when using these compounds, from the opening to the disposal of the reagents.
P. M. Dewick, in Medicinal Natural Products, 3rd ed., John Wiley & Sons, Chichester, 2009, p. 311.