Plant structural characterization

The isolation and structural characterization of plant constituents are often difficult and time-consuming. Consequently, at the present time, only a small percentage of the total number of natural plant products has been identified. Early in the development of natural product chemistry it was apparent to many botanists and chemists that plants might be characterized and classified on the basis of their chemical constituents. The distribution of chemical constituents among plant families is presented by Swain (139) and Hegnauer (66). 

Mosses and liverworts (Bryophyta) are more complex than algae. Some of the larger species have structures that superficially appear similar to roots, stems and leaves, but they lack the internal conducting systems present in the vascular plants (Tracheophyta). Internal transport systems (vascular systems) make possible the large sizes of terrestrial plants where the soil is the source of some requisites (water, mineral nutrients) and the air is the source of others (CO2, sunlight). The different groups of vascular plants are characterized primarily by their methods of reproduction. Vascular plants are the source of all wood. 

The identification and structural characterization of biological materials, obtained for example from plants, was traditionally carried out via the classical sequence involving extraction, separation, isolation and characterization, a sequence which requires large amounts of substance and a great deal of time. Industrial problems, for example the search for small amounts of contaminants in industrial products or in waste water, also require intensive analytical studies. 

With the advent of modern techniques, instrumentation and automation in isolation and structural characterization, numerous antidiabetic compounds have been isolated, purified and identified from different natural sources, especially medicinal plants. This part summarizes promising natural chemical entities with diverse structures reported for their therapeutic activities against diabetes. These compounds are broadly categorized into alkaloids, terpenoids, flavonoids, and phenolics, including compounds from other groups. 

Frederick SE, Newcomb EH, Vigil EL, Wergin WP (1968) Fine-structural characterization of plant microbodies. Planta 81 229-252 (Berlin)... 

The largest number of indole derivatives which have been structurally characterized are the indole alkaloids these mainly arise from plant sources. Not unexpectedly, in view of tryptophan s status as one of the essential amino acids, there are also diverse derivatives found in microorganisms. Indole-3-acetic acid plays a major role in plant metabolism, being a growth regulator (74MI30600). 

As indicated at the outset of this broad-based review, although of the order of ten thousand plant defensive compounds have been isolated and structurally characterized, only of the order of a thousand have so far been shown to interact with specific proteins and most of these biochemical sites of interaction are signal transduction components. The elucidation of such targets now permits rapid biochemical screening of complex ecosystems for plant-derived ligands for such proteins. 

T ignin is one of the most abundant natural products constituting about one-fourth of the woody tissue in plants. Nature has chosen a unique synthetic technique to prepare this cross-linked polymeric material from coniferyl alcohol and related substances. The mechanism of lignin formation is not completely known yet, and the structural characterization of lignin has been only partially successful despite considerable research. 

Marcone, M.F., Kakuda, Y., and Yada, R.Y. 1998. Salt-soluble seed globulins of dicotyleonous and monocotyledonous plants. II. Structural characterization. Food Chem. 63, 265-274. 

Kogel-Knabner, I., Zech, W., Hatcher, P. G., and deLeeuw, J. W. (1991). Fate of plant components during biodegradation and humification in forest soils Evidence from structural characterization of individual biomacromolecules. In Advances in Soil and Organic Matter Research The Impact of Agriculture and the Environment, vol. 90, Wilson, W. S., eds., Royal Society of Chemistry, London, pp. 61-70. 

Since many of the plant growth substances now have been structurally characterized, it is no longer necessary to rely on bioassays for PGS identification. In fact, physico-chemical techniques have become the methods of choice. There have been a number of reviews discussing some of the aspects of PGS analysis by physico-chemical procedures (1, 2, A, 5). This report will... 

Unlike most structurally characterized methyltransferases of non-plant origin that are monomeric, ChOMT forms a symmetric homodimer.21 Dimerization appears to be critical for activity in vitro as well as in vivo. The presence of a dimerization interface appears to be common to plant OMTs, and in this family of OMTs this homotypic protein-protein interface intimately contributes to substrate binding. Because of the broad structural diversity of phenylpropanoid compounds, ChOMT, like many plant OMTs, possesses highly selective substrate and positional specificity. Efficient substrate discrimination and binding is achieved through shape... 

Higher plants are sessile and are consumed by motile organisms, namely other eukaryotes and prokaryotes. Plants defend themselves by physical barriers including cell walls at the cellular level, by the waxy cuticle of leaves and by bark and thorns at the macroscopic level. Plants also defend themselves from fungal and bacterial pathogens and animal herbivores by elaborating a variety of bioactive secondary metabolites and defensive proteins. There may be as many as 100,000 different kinds of plant defensive compounds of which about 30,000 have been isolated and structurally characterized. Biochemical targets have been determined in vitro or in vivo for some thousands of the defensive compounds isolated to date. 

Determining the molecular sites of action of bioactive medicinal plant constituents is clearly important for establishing the chemical and physiological basis for herbal medicinal efficacy, for quality control of commercial herbal preparations and for the discovery of lead compounds for synthetic (or semi-synthetic) pharmaceutical development. Of course, it must be recognized that medicinal plant efficacy may derive from complex synergistic effects or even from quasi-placebo effects connected with the taste, mild effects and appearance of the preparation. While recognizing these possible holistic complications, in order to find out how such preparations work, it is clearly important to initially isolate, structurally characterize and define the biochemical targets of plant bioactive substances. 

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