253 



ALKALOIDS 



Mayer's reagent (potassium mercuric iodide) is most commonly used for detection 

 of alkaloids since it gives a precipitate with nearly all of them. As it also precipitates 

 other plant components, a preliminary purification is advisable before applying the test. 

 Other reagents such as 5% silicotungstic acid, 5% tannic acid, Dragendorff's reagent 

 (potassium tetraiodobismuthate) and saturated picric acid are also frequently used. For 

 rapid testing plant juice can be squeezed onto paper impregnated with Dragendorff's rea- 

 gent (17). Some alkaloids contain specific functional groups which may be determined by 

 special reagents. For example, morphine is phenolic so that phenol reagents can be used 

 to distinguish it. Systematic application of such reagents can be used for classification 

 of alkaloids, and procedures have been given by Fulton (18) for dividing the alkaloids into 

 large groups on the basis of orderly use of reagents. 



If a more complete characterization of alkaloids is desired, the techniques of paper 

 chromatography and spectrophotometry can be used to provide the most information with 

 the least effort. Macek et al. (19) have proposed a classification scheme based on paper 

 chromatographic behavior. By using several different solvents and reagents, an alkaloid 

 may be placed into one of 6 major categories. The application of special reagents and 

 comparison with knowns can then be used for a more nearly complete identification. 

 Another paper chromatographic classification based on polarity is described by Waldi(20), 



Resplandy (21) has claimed that the use of electrolytes for the chromatography of 

 alkaloids is especially useful for providing insights into chemical structure. The litera- 

 ture on paper chromatography of alkaloids has been periodically reviewed by Br'Auniger 

 (22-28). The most useful solvents seem to have been formamide-chloroform mixtures 

 and butanol-acetic acid mixtures. The most useful detection reagents have been iodine 

 vapor on still moist paper and Dragendorff's reagent (potassium tetraiodobismuthate). 

 Improvements in the Dragendorff's reagent have been suggested (29, 30). A modification 

 of the Mayer reaction for use on chromatograms has been described by Pan and Wagman 

 (31), Resplandy (32) has devised a method for indicating the structures of alkaloids on 

 a micro scale by pyrolyzing either isolated alkaloid or intact plant tissue with calcium 

 oxide and identifying the volatile products by paper chromatography. 



Paper electrophoresis is also useful for separating many alkaloids. It is often 

 faster and more reproducible than paper chromatography (33). Thin layer chromatography 

 is also a rapid technique which can be applied to alkaloid separations. Waldi et al. (34) 

 have devised a routine procedure for alkaloid identification using silica gel G as the ad- 

 sorbent with 8 different solvent systems. In spite of the low volatility of many alkaloids 

 it has also been possible to use gas chromatography for separating them (35). 



If a pure alkaloid can be obtained, determination of its absorption spectrum can 

 provide a valuable means of identification. Elution of a spot from a paper chromatogram 

 may provide enough material for this purpose. Ultraviolet absorption spectra for a great 

 many alkaloids have been presented by Elvidge (36) and by Oestreicher et al. (37). Infra 

 red spectra are given by Levi et al. (38) and by Marion et al. (39). 



METABOLIC PATHWAYS 



The schemes of alkaloid biosynthesis shown in the accompanying diagrams are 

 largely based on the proposals made by Sir Robert Robinson in 1917 (40). These ideas 

 have since been elaborated both by him and others (cf. 41,42). They constitute a com- 

 prehensive and reasonable picture of alkaloid biosynthesis but are based primarily on 

 analogy with reactions of organic chemistry and by considerations of structural similarity 

 rather than direct, biochemical evidence. In the diagrams only carbon skeletons are 

 shown with no indication of detailed structures. Names in parentheses are specific alka- 

 loids having the accompanying carbon skeleton. Other names refer to classes of alka- 

 loids. The assumptions of Robinson's scheme of biosynthesis may be summarized as 

 follows: 



