Carbohydrates determining structure

O-Isopropylidene derivatives of carbohydrates form structural isomers from carbohydrates which themselves are epimers. Since structural isomers often fragment differently whereas epimers do not, mass spectra of these derivatives may permit interpretation in terms of stereochemistry. Although molecular-ion peaks are not observed, the molecular weight can be determined readily from a relatively intense M-CH/ peak, resulting from loss of a methyl radical from a 1, 3-dioxolane ring (12). 

L. C. Methods for Determining Structure or Sequence or Both in Carbohydrates... 

BIOCHEMICAL NOMENCLATURE Carbohydrates, determining the cyclic structures of,... 

It is to be emphasized that, in the absence of elements of symmetry, as is the case for carbohydrates, determination of the molecular structure should be based on both the experimental, vibrational spectra and the calculated frequencies. In order to minimize the differences between experimental and calculated results, the structure factors utilized in the calculation should take into account the previous conformational studies. The peculiarities of carbohydrate structures, such as anomeric and exo-anomeric effects, are revealed by bond shortening and torsion-angle modifications. These modifications are accompanied by a change in the vibrational-energy level, and hence, by the corresponding information in their infrared or Raman spectra. 

These three questions will be examined in relation to the conformational data which have been given by recent carbohydrate crystal structure determinations. Because of the added reliability and accuracy arising from the use of automatic diffractometers only those numerical data will be quoted where these instruments were used. 

Although studies on potato structure had been carried out previously using conventional SEM, van Marie et al [70] used cryo-SEM to advantage in this high moisture material. The fracture planes of cooked and uncooked samples were used to help characterize cell wall adhesion in the four potato cultivars. In particular, differences in cell wall contact area and surface detail were used to explain the mealy versus firm textural attributes in the cultivars. By determining the parameters which contributed to the texture of potatoes, processing conditions and selection of suitable raw materials could be facilitated. Such information would be difficult to obtain with conventional, chemically fixed material due to the high moisture content and the inability of standard chemical fixation to retain carbohydrate-based structures. 

Thble 6.1 shows some comparisons of values determined by X-ray and neutron diffraction studies for O-H and H- -O bond lengths for some carbohydrate crystal structures that have been analyzed by both methods. Application of the normalization reduces the discrepancy between the H - - O distances obtained by X-ray and neutron diffraction experiments from 0.102 A to 0.033 A. Similar results have been reported for NH -0=C bonds [387], where the random errors in the X-ray N-H and H 0 distances varied between 0.02 and 0.17 A with a mean of 0.065 A when compared with the same neutron diffraction values. Cor-... 

The reactions in parts (b) and (c) are used to determine structural features of carbohydrates like maltose We will learn much more about maltose and similar carbohydrates in Chapter 27. 

Chemists have devoted much effort to exploring this natural world of chemistry as well as to determining structures the natural world has stimulated the extension of the chemical world into models and analogs of the natural chemicals. The field of organic chemistry was influenced heavily by the types of chemical structures found in natural products many medicinal compounds are still invented by using natural products as models for analogs. Chemists have also invented important polymers once nature showed us the natural polymeric carbohydrates, polypeptides, nucleic acids, and the polymers such as rubber that are produced from natural materials. 

As stated by Kenner, it is unfortunate that alkaline degradations were not scrutinized further, for it has recently been demonstrated that the process of alkaline degradation in the absence of oxygen can be used in determining structures of carbohydrates. Had it been realized earlier that such degradation may be employed as a tool in structural determinations, the structures of many carbohydrates could have been determined before the development of the procedures of methylation analysis. 

Solution structures from CIDNP experiments were compared with crystallo-graphically determined structural parameters, and CIDNP, in conjunction with other methods, was used to characterize several unmodified ° ° and chemically modified peptides and proteins in their native states. Structural changes during carbohydrate binding " or binding of other ligands " were investigated by CIDNP. 

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