Compounds that contain nitrogen as part of a chain rather than being incorporated into a ring structure generally belong to this category of natural products. For the amines, this by definition imparts a basic nature to the molecule. For the amino acids this results in molecules that may be zwitterionic (forming a dipolar ion) depending on the pH of the environment.
The common plant amines can be subdivided into aliphatic monoamines, aliphatic polyamines, and aromatic amines. Occasionally these materials are classified as alkaloids rather than amines.
Simple aliphatic amines exist as low-boiling liquids and include most of the primary amines from methylamine, CH3NH2, through hexylamine, CH3(CH2)5NH2. These molecules typically have strong, fish-like aromas. In the case of cow parsley (Heracleum sphondylium), they are believed to act as insect attractants by simulating the smell of carrion.
Common polyamines include putrescine, NH2(CH2)4NH2; agmatine, NH2(CH2)4NHC(*NH)NH2; spermidine, NH2(CH2)3NH(CH2)4NH2; and spermine, NH2(CH2)3NH(CH2)4NH(CH2)3NH2. Both putrescine and s-adenosylme-thionine are used for the formation of spermine and spermidine. These polyamines are thought to have many functions and are invariably found com-plexed with nucleic acids, including both DNA and RNA.
Many of the known aromatic amines are physiologically active. Perhaps the most well known member of this class is mescaline. It is the active principle of the peyote cactus, Lophophora williamsii, and is a potent hallucinogen. Similarly, three compounds critical to brain metabolism in animals are noradrenaline, histamine, and serotonin. All three occur in common plants (Figure 1.26).
Much of the genetic information contained within each cell of plants and animals is expressed as proteins. Proteins are made up individually from large chains of amino acids and small oligomers comprise peptides. Proteins play a variety of roles. Some carry out the transport and storage of small molecules, while others make up a large part of the structural framework of cells and tissues. Perhaps the most important class of proteins are the enzymes, the catalysts that promote the variety of reactions that channel metabolism into essential pathways (see Chapter 2). Individual types of cells may contain several thousand kinds of proteins.
The protein amino acids are normally considered to be 20 in number for plants. The amino acids are high-melting, water soluble, zwitterionic colorless solids. Since they have both basic (amine) and acidic (acid) functionalities, the amino acids have specific pKa's unique to each amino acid. The 20 principal amino acids are shown in Figure 1.27.
A nonprotein amino acid that is regularly found in plants is d-aminobutyric acid. Several hundred others are known, though no others have been found to be more or less ubiquitous. Additionally, atypical amino acids, peptides, and proteins exist which are constructed from nonribosomal processes that are also essential to the life of a plant. This is a more recent field that is currently popular with natural-product chemists.
Proteins are high-molecular-weight polymers of amino acids. They are synthesized based on the triplet base code of DNA in the nucleus of a cell. Because the individual amino acids that make up proteins in plants and animals exist each as a single enantiomer, the polypeptide has a nonrandom form which gives rise to a particular three-dimensional shape, flexibility, and conformational lability. This is an active area of research.
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