Carbohydrates convention

Models with three or more stereogenic centers create new problems. Again, carbohydrate chemists were first to give efficient and clear, though perhaps to the outsider cumbersome solutions. The following carbohydrate convention (see Table 11) is based on the Fischer projection. i.e., it prescribes an all-eclipsed conformation of the backbone and defines the direction of the backbone by the oxidation numbers of the terminal carbon atoms. 

A serious ambiguity arises for compounds such as the active tartaric acids. If the amino-acid convention is used, (+)-tartaric acid falls in the d series by the sugar convention, it has the l configuration. One way out of this dilemma is to use the subscripts, v and g to denote the amino-acid or carbohydrate conventions, respectively. Then the absolute configuration of (+)-tartaric acid can be designated as either Ds-(+)-tartaric acid of lb-(+)-tartaric acid. 

Two numbering systems are used in this review. According to the first, which is based on the nomenclature of heterocyclic systems, the anomeric carbon atom is denoted as C(2) (number in parentheses). If carbohydrate convention is applied, this carbon atom becomes Cl, and the number is not in parentheses. 

ChemSketch has some special-purpose building functions. The peptide builder creates a line structure from the protein sequence defined with the typical three-letter abbreviations. The carbohydrate builder creates a structure from a text string description of the molecule. The nucleic acid builder creates a structure from the typical one-letter abbreviations. There is a function to clean up the shape of the structure (i.e., make bond lengths equivalent). There is also a three-dimensional optimization routine, which uses a proprietary modification of the CHARMM force field. It is possible to set the molecule line drawing mode to obey the conventions of several different publishers. 

The simplest nutritional role of fats in the diet is that of energy supply. There are differences between members of the same class of food materials, but the accepted convention attributes a value of 9 kcal (37.7 kJ) of energy per gram of fat, and 4 kcal (16.7 kJ) of energy per gram of all carbohydrates and proteins. This is a serious consideration in generating weight loss diets. 

Some of the economic hurdles and process cost centers of this conventional carbohydrate fermentation process, schematically shown in Eigure 1, are in the complex separation steps which are needed to recover and purify the product from the cmde fermentation broths. Eurthermore, approximately a ton of gypsum, CaSO, by-product is produced and needs to be disposed of for every ton of lactic acid produced by the conventional fermentation and recovery process (30). These factors have made large-scale production by this conventional route economically and ecologically unattractive. 

At the present time, use of the Fischer convention is almost entirely restricted to carbohydrates, amino acids, and biologically important molecules of closed related structural types. The problem with more general use is that there are no adequate rules for deciding whether a diiral atom is like D-glyceraldehyde or L-glyceraldehyde when the structures are not closely similar to the reference molecules. This relationship is clear for carbohydrates and amino acids. 

Carbohydrates with more than one chirality center are shown in. Fischer projections by stacking the centers on top of one another. By convention, the carbony] carbon is always placed either at or near the top. Glucose, for... 

Fischer projection, 975-978 carbohydrates and, 977-978 D sugars, 980 i., sugars, 980-981 rotation of, 976 R.S configuration of, 977 conventions for, 975-976 Fishhook arrow, radical reactions and, 139, 240... 

Carb-O. Historical development of carbohydrate nomenclature 0.1. Early approaches 0.2. The contribution of Emil Fischer 0.3. Cyclic forms 0.4. Nomenclature commissions 2-Carb-l. Definitions and conventions... 

Several dozens of aldolases have been identified so far in nature [23,24], and many of these enzymes are commercially available at a scale sufficient for preparative applications. Enzyme catalysis is more attractive for the synthesis and modification of biologically relevant classes of organic compounds that are typically complex, multifunctional, and water soluble. Typical examples are those structurally related to amino acids [5-10] or carbohydrates [25-28], which are difficult to prepare and to handle by conventional methods of chemical synthesis and mandate the laborious manipulation of protective groups. 

In this decade, all chemistry research fields have adopted and/or applied the dendrimers and/or dendrimer methodologies. The table of contents of this series. Topics in Current Chemistry Dendrimers Volumes I-IV, clearly indicates this situation [1-4], that is, the concept of dendritic compounds has already been introduced in host-guest and/or supramolecular chemistry (Vol. I/Chap. 2, Vol. Il/Chaps. 3,4, Vol. IV/Chap. 3), chiral chemistry (Vol. I/Chap. 4), electrochemistry (Vol. I/Chap. 6, Vol. Ill/Chap. 3), heteroatom and/or organometalHc chemistries (Vol. I/Chap. 3, Vol. Il/Chaps. 2,5, and Vol. IV/Chap. 4), and carbohydrate chemistry (Vol. IV/Chap. 6), as well as applied in the field of medicine (Vol. Il/Chap. 6) and nanoscience (Vol. Ill/Chap. 4). The dendrimer methodology is expected to be used in future novel science as a conventional chemistry concept. 

Conventionally, central and special metabolic pathways are distinguished. Central pathways are common to the decomposition and synthesis of major macromolecules. Actually, they are much alike in all representatives of the living world. Special cycles are characteristic of the synthesis and decomposition of individual monomers, macromolecules, cofactors, etc. Special cycles are extremely diversified, especially in the plant kingdom. For this reason, the plant metabolism is conventionally classified into primary and secondary metabolisms. The primary metabolism includes the classical processes of synthesis and deeradation of major macromolecules (proteins, carbohydrates, lipids, nucleic acids, etc.), while the secondary metabolism ensuing from the primary one includes the conversions of special biomolecules (for example, alkaloids, terpenes, etc.) that perform regulatory or other functions, or simply are metabolic end byproducts. 

The microwave-assisted PTC transesterification of several carbohydrates in basic medium with methyl benzoate or laurate has been studied [96], Small amounts of DMF were necessary to provide good yields within 15 min at 160 °C. Rate enhancements were compared to conventional heating (A) under the same conditions and specific microwave activation was mostly seen when the less reactive fatty compounds were involved (Eq. (44) and Tab. 3.19)... 

This study was next extended to the synthesis of benzoyl and dodecanoyl derivatives from protected carbohydrates [67]. Microwave-assisted PTC transesterifications with methyl benzoate or dodecanoate were studied for several carbohydrates. Small amounts of dimethylformamide (DMF) were shown to be necessary to provide good yields (76-96%) within 15 min. Rate enhancements when compared to conventional heating (A) and specific microwave activation were especially noticeable when less reactive fatty compounds were involved (Eq. 48). 

A method for the determination of formaldehyde in the presence of acetaldehyde was developed by Nicolet and Shinn.88,100 103 After the excess periodate had been destroyed, these workers swept the acetaldehyde (from the neutral reaction mixture) into a sodium bisulfite solution by means of a stream of carbon dioxide. The acetaldehyde was measured by conventional, bisulfite methods, and the residual formaldehyde was precipitated with Dimedon. This procedure was applied to protein hydroly-zates and to terminal deoxy structures of carbohydrates.88,280 ... 

Fischer s Convention. Initially, the absolute configurations of optical isomers were unknown to chemists working with optically active compounds. Emil Fischer, the father of carbohydrate chemistry, decided to relate the possible configurations of compounds to that of glyceraldehyde of which the absolute configuration was yet unknown but was defined arbitrarily. 

The merit of Fischer s convention is that it enables the systematic stereochemical presentation of a large number of natural products, and this convention is still useful for carbohydrates or amino acids today. Its limitations, however, become obvious with compounds that do not resemble the model reference compound glyceraldehyde. For example, it is very difficult to correlate the terpene compounds with glyceraldehyde. Furthermore, selection of the correct orientation of the main chain may also be ambiguous. Sometimes different configurations may even be assigned to the same compound when the main chain is arranged in a different way. 

Blattner, R., Ferrier, R. J., Fumeaux, R. H., and Ludewig, M., Abstracts, 11th Royal Australian Chemical Institute Convention (2000) and Abstracts, 20th International Carbohydrate Symposium, Hamburg, Germany (2000), B-341, p. 228. 

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