Amines

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Plant amines can be considered simply as the products of decarboxylation of amino acids, although they are mainly synthesised in vivo by transamination of the corresponding aldehydes. The most widespread plant amines are conveniently divided into three groups: aliphatic monoamines, aliphatic polyamines (including diamines) and aromatic amines.

Aliphatic monoamines are volatile compounds, ranging from simple compounds like methylamine, CH₃NH₂, to n-hexylamine, CH₃(CH₂)5NH₂. They are widely distributed in higher plants and fungi and, when present in any concentration, have an unpleasant fish-like smell. They function in flowers (e.g., in the cow parsley, Heracleum sphondylium) as insect attractants by simulating the smell of carrion.

By contrast to the monoamines, diamines and other polyamines are less volatile, although they still possess offensive odours. Widespread polyamines include putrescine, agmatine, spermidine and spermine. There are several others of more limited occurrence, such as cadaverine (Smith, 1981). Polyamines are of research interest because of their growth-stimulating activity in relation to their effect on ribosomal RNA.

Polyamines can occur in plants conjugated with hydroxycinnamic acids (q.v.). There are simple ester conjugates (e.g., caffeoylputrescine) and also more complex derivatives. The antifungal hordatines A and B of barley are derived from the condensation of agmatine with p-coumaric and ferulic acids. Polyamine fatty acid conjugates have also been found (Smith, 1977).

The best known aromatic amine from plants is probably mescaline, the active principle of the flowering heads (peyote) of the cactus Lophophora williamsii, and a powerful natural hallucinogenic compound. Indeed, many of the known aromatic amines are physiologically active and, for this reason, they are sometimes classified with the alkaloids. Three substances which are very important in animal physiology (e.g., in brain metabolism) are noradrenalin, histamine, and serotonin; all three occur in plants, noradrenalin, for example, being present in the banana and the potato. Perhaps the most widespread aromatic amine is tyramine, which has been detected in 15 of 77 plant families surveyed.

All the amines so far mentioned are primary amines, i.e., have the general formula RNH₂. Secondary amines, general formula R₂NH, and tertiary amines, R₃N, are known in plants but they are not very common. A typical secondary amine found in plants is dimethylamine; a typical tertiary amine is hordenine or N,N-dimethyltyramine, which is the typical alkaloid of barley.

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