Sugar conformations, assignments

The first three carba-sugars were synthesized by McCasIand and coworkers. Two other carba-sugars were prepared from myoinositol, and the remaining eleven carba-sugars have been synthesized from the Diels-Alder adduct of furan and acrylic acid. Conformational assignments of the carba-sugars were established with the aid of H-n.m.r. spectroscopy. 

This is essentially the same structure published by Crick and Watson (24) and supports their general proposal for the structure of DNA. Although they intended to model B-DNA, once they had assigned the wrong sugar conformation, the other conformationl peculiarities followed. 

The empirical conformational assignments made from the i.r. idence were in good agreement with those reported by Reeves. For those derivatives expected to exist either in a flexible form or in equilibrium between the two chair forms, the i.r. data did not fit the correlations established for the chair conformers. The failure of the method to differentiate between the two possibilities, or to yield any quantitative information, clearly illustrates the limitations of i.r. spectroscopy in the conformational analysis of sugar molecules. 

A conformational assignment from n.m.r. spectroscopy for an acyclic sugar chain in solution was reported in 1965. Analysis of the proton... 

The branched-chain sugar 9 (prepared by intramolecular cycloaddition of a terminal deoxy-iV-hydroxyaminopentofuranose bearing a 3-C-prop-2-enyl branch) on UV irradiation afforded a radical observed by ESR spectroscopy. The information obtained was used for configurational and conformational assignments. ... 

Monte Carlo may be used to study the lateral distribution of lipid molecules in mixed bilayers. This of course is a very challenging problem, and, to date, the only way to obtain relevant information for this is to reduce the problem to a very simplistic two-dimensional lattice model. In this case, the lipid molecules occupy a given site and can be in one of the predefined number of different states. These pre-assigned states (usually about 10 are taken), are representative conformations of lipids in the gel or in the liquid state. Each state interacts in its own way with the neighbouring molecules (sitting on neighbouring sites). Typically, one is interested in the lateral phase behaviour near the gel-to-liquid phase transition of the bilayer [69,70]. For some recent simulations of mixtures of DMPC and DSPC, see the work of Sugar [71]. 

While the broad mission of the National Bureau of Standards was concerned with standard reference materials, Dr. Isbell centered the work of his laboratory on his long interest in the carbohydrates and on the use of physical methods in their characterization. Infrared spectroscopy had shown promise in providing structural and conformational information on carbohydrates and their derivatives, and Isbell invited Tipson to conduct detailed infrared studies on the extensive collection of carbohydrate samples maintained by Isbell. The series of publications that rapidly resulted furnished a basis for assigning conformations to pyranoid sugars and their derivatives. Although this work was later to be overshadowed by application of the much more powerful technique of nuclear magnetic resonance spectroscopy, the Isbell— Tipson work helped to define the molecular shapes involved and the terminology required for their description. 

For molecules with two stereogenic centers, the traditional descriptors are erythro and threo to which, henceforth, a c (short for carbohydrate) is added for a reason that will become apparent (Table 10). They are unambiguously defined for simple sugars. The backbone is given by the carbon chain the reference conformation is the conformation with all carbon atoms eclipsed. This statement is identical with the requirement of a Fischer projection. If two identical substituents X are located on the same side/opposite sides of the plane traced by the backbone, then the c-erythrojc-threo configuration is assigned. 

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