Generally an alkaloid is a nitrogen containing plant compound (though some synthetic compounds are also dubbed alkaloids), always composed of nitrogen, carbon, and hydrogen, but may also have other atoms as part of their structure, such as oxygen, sulphur or phosphorus. These compounds are generally basic, but some acidic compounds are also included.
Quote:The boundary between alkaloids and other nitrogen-containing natural compounds is not clear-cut.[14] Compounds like amino acid peptides, proteins, nucleotides, nucleic acid, amines, and antibiotics are usually not called alkaloids.[2] Natural compounds containing nitrogen in the exocyclic position (mescaline, serotonin, dopamine, etc.) are usually classified as amines rather than as alkaloids.[15] Some authors, however, consider alkaloids a special case of amines. -Wikipedia
The compounds in saint John's wort, such as Hypericin, contain no nitrogen and thus are not alkaloids, they are naphthodianthrones, which are anthraquinone-derivatives...
-eg