Compound, Natural

a substance occurring as an intermediate or final product in the life processes of organisms. This term is arbitrary since it does not include a number of simple metabolic products (methane, acetic acid, ethyl alcohol), certain components of coals and petroleum, and inorganic compounds that are either formed in metabolic processes (O₂, CO₂, H₂O) or are present in the inorganic world (minerals, gases). A distinction is made between high-molecular-weight natural compounds, for example, biopolymers, and low-molecular-weight natural compounds; the arbitrary boundary between the two lies in the vicinity of a molecular weight of 5,000 daltons. High-molecular-weight natural compounds include proteins, nucleic acids, and polysaccharides, as well as such composite biopolymers as glycoproteins, nucleoproteins, and lipoproteins. These substances are cells, and they carry out extremely important biological functions (biological catalysis, storage and transmission of genetic information, transport of substances, immunity). Polyisoprenoids, another type of biopolymer, are found in some plants, such as plants yielding rubber and gutta-percha. Low-molecular-weight natural compounds include a large number of organic compounds of different chemical nature. This category encompasses monomeric components of biopolymers (amino acids, nucleotides, monosaccharides), compounds constructed of a small number of monomeric units (oligonucleotides, oligosaccharides), and lipids, as well as a large number of aliphatic, alicyclic, aromatic, and heterocyclic organic compounds (natural pigments, steroids, iso-prenoids, alkaloids). In organisms, low-molecular-weight natural compounds serve as structural materials in the synthesis of biopolymers, agents having specific biological regulatory effects (hormones, mediators, vitamins), and protective agents (toxins, antibiotics). They also figure in the communication between organisms through such chemical substances as pheromones.

The study of natural compounds, a major area of modern biology and chemistry, is laying the basis for an understanding of biological processes on the molecular level. Although many natural compounds, for example, the natural dye Tyrian purple and several poisons, were used in antiquity, the modern history of the study of natural compounds began in the late 18th and early 19th centuries. This study was the logical consequence of the developments in iatrochemistry and the interest of investigators in the composition of living organisms, the chemical nature of biologically active compounds, and the biochemical basis of physiological processes. The first advances in the isolation and purification of natural compounds were achieved by C. Scheele in the period between 1769 and 1785. In the 1830’s and 1840’s, J. von Liebig and his school established that proteins, fats, and carbohydrates are constituents of food products. Great contributions to the study of natural compounds in the 19th and early 20th centuries were made by P. Berthelot, L. Pasteur, and E. Fischer and by the Russian scientists A. M. Butlerov, A. Ia. Danilevskii, M. V. Nentskii, and V. S. Gulevich. In Russia, the departments of medical and physiological chemistry founded in the 1860’s were centers for the study of natural compounds. (A. I. Khodnev published the first textbook of physiological chemistry in Kharkov in 1847.)

In the mid-20th century, with the development of new methods for the isolation, purification, and analysis of the structure of complex compounds (chromatography, electrophoresis, isotope tracers, ion exchange, X-ray diffraction, and optical, mass, and radio-frequency spectroscopy), there was a rapid development in many areas of the study of natural compounds. Spatial structures were established for many proteins, including such complex proteins as myoglobin (J. Kendrew, 1957) and hemoglobin (M. Perutz, 1959). The synthesis of the enzyme ribonuclease was carried out by R. Merrifield in 1969, and in the period between 1949 and 1955, A. Todd developed methods for the synthesis of nucleotides. Then methods for the synthesis of nucleic acids were developed, which led to the synthesis of the gene of alanine transfer ribonucleic acid by H. Khorana in 1970. Owing to the work of scientists of the schools of R. Robinson in Great Britain and A. P. Orekhov in the USSR, the structures of many alkaloids were established, and the alkaloids themselves were synthesized. Significant advances were achieved in the study of the structure and mechanism of enzymes, antibiotics (A. Fleming, H. Florey, and E. Chain in Great Britain; S. Waksman in the United States; and M. M. Shemiakin in the USSR), and vitamins (synthesis of vitamin B₁₂ by R. Woodward in 1970).

Natural compounds are studied in classical biochemistry and in molecular biology. The latter field is generally considered to have begun with the establishment of the three-dimensional structure of deoxyribonucleic acid by J. Watson and F. Crick in 1953. The study of the chemical structures of natural compounds is the province of a separate division of organic chemistry called the chemistry of natural compounds. Bio-organic chemistry, which developed as an independent discipline in the 1960’s, seeks to establish the relationships between the structures of natural compounds and the compounds’ functions in organisms. There is increasing scientific interest in the comparative study of the structure and function of given classes of natural compounds at different levels of biological evolution. Thus, the distribution of natural compounds in the plant and animal world, the structure and biosynthetic pathways of the compounds, and the effect of the compounds both on the organism as a whole and on individual biochemical processes form the subject of an integrated study by various disciplines using mathematical, physical, chemical, and biological methods. Methods have been developed and introduced for the industrial production of vitamins, hormones, amino acids, antibiotics, and other natural compounds.

Studies of natural compounds are conducted at many scientific institutions in the USSR and other countries. In the USSR, these studies are coordinated by the department of biochemistry, biophysics, and the chemistry of physiologically active compounds of the Academy of Sciences of the USSR. Regional and international symposia and conferences on various aspects of the study of natural compounds are held regularly. The results of studies on the chemistry of natural compounds are published in the USSR in the journals Khimiia prirodnykh soedinenii (The Chemistry of Natural Compounds; Tashkent, since 1965), Bioor-ganicheskaia khimiia (Bio-organic Chemistry; 1975), Zhurnal or-ganicheskoi khimii (Journal of Organic Chemistry; 1965), and Antibiotiki (Antibiotics; 1956). Advances in the field are also published in the international annual review Fortschritte der Chemie Organischer Naturstoffe (Vienna-New York-Berlin-Heidelberg; 1938).