The term alkaloids (or alkali-like) was first and foremost proposed by the pharmacist, W. Meissner, in 1819, for the basic nitrogen-containing compounds of plant origin. Ladenburg defined alkaloids,—‘as naturally occurring plant compounds having a basic character and containing at least one nitrogen in a heterocyclic ring.’ With the advent of recent advanced knowledge in the chemistry of various alkaloids two more inevitable characteristic features were logically and justifiably added to the definition of alkaloids, namely:
(a) Complex molecular structure, and
(b) Significant pharmacological activity.
Furthermore, it was broadly observed that the basic properties of the alkaloids is solely by virtue of the presence of N-atom embedded into the five-or six- membered ring.
Therefore, the alkaloids are now generally defined as,—‘physiologically active basic compounds of plant origin, in which at least one nitrogen atom forms part of a cyclic system.’ Even this definition has a few anomalies as stated below, namely:
(i) Cholines and Betaines: These two substances have the N-atom in the side chain and not in the aromatic ring as shown below:

Cholines and Betaines

The cholines and betaines are regarded as simple alkylamines and not classified as alkaloids. They are designated by some school of thoughts as ‘biological-amines’ or ‘protoalkaloids’.
(ii) Ephedrine: It has the N-atom only in the side chain and not embedded in the aromatic ring as given below:

(iii) Piperidine: It is obtained Piper nigrum (Black Pepper) and does not possess any pharmacological activity, but has a N-atom in a heterocyclic ring as given below:

(iv) Colchicine: It is found to be neither basic nor it contains the N-atom in a heterocyclic ring, whereas it is considered as an alkaloid due to the fact it possesses distinct pharmacological activity as shown below:

(v) Thiamine (Vitamin B1): It confines to the definition of alkaloids but is not regarded as an ‘alkaloid’ because of its almost universal distribution in living matter.

Thiamine (Vitamin B1)
Interestingly, alkaloids represent one of the most important group of chemical constituents occurring in the entire plant kingdom which exert extremely potent and vital physiological and pharmacological activities in the human beings. Therefore, it will be worthwhile to study the alkaloids with regard to the following aspects, namely:
These various aspects of alkaloids shall now be discussed adequately in a sequential manner so as to have a better in-depth of knowledge.

Source:Pharmacognosy And Pharmacobiotechnology By Ashutosh Kar

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