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The Use of Model Compounds in a Theoretical Study of Carbohydrate C-13 Chemical Shift Effects.

Durran, David Michael. (1996) The Use of Model Compounds in a Theoretical Study of Carbohydrate C-13 Chemical Shift Effects. Doctoral thesis, University of Surrey (United Kingdom)..

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Due to the local nature of nuclear shielding (which increases as chemical shift decreases on the same ppm scale), model glycosidic systems are suggested as a way of studying the potentially useful relationship that exists between solid state CP-MAS NMR chemical shifts and glycosidic conformation as described by the &phis; and psi torsion angles. Theoretical ab initio nuclear shielding calculations have been performed using both the gauge included atomic orbital (GIAO) and individual gauge for localised orbital (IGLO) methods. The reliability of these methods is demonstrated from a comparison with literature data. The rationale behind the use of model compounds is then introduced along with a comparative study of the alpha- and beta-(1→4)-glucan model systems and the parent disaccharides which takes into account the total shielding, principal component values and their directions, which are illustrated graphically. Good agreement is found between the calculated shielding tensors of the model systems and disaccharides. These model alpha- and beta-(1→4)-glucans are then used in a study of how the Cl and C4' chemical shielding varies versus glycosidic torsion angles. In particular, the correlation suggested by Gidley between Cl shielding and ps torsion angle. It is found that calculations are able to predict this shielding dependence and demonstrate its local nature as there is a much smaller variation in the nuclear shielding of the remote carbon atoms in the model-alpha-(1→4)-glucan. These models are then used to build nuclear shielding surfaces over a reasonably allowed conformational region in ph,ps space. Good agreements with experimental data are found for both alpha-(1→4)-glucan and beta-(1→4)-glucan surfaces and they provide an illustration of how the alpha- and beta- anomers differ in this respect. Shielding surfaces are also calculated for model mannan and galactan systems. The latter is chosen as a representative model of pectin. Finally, a study of the direct dependence on Cl and C4' shielding with intra- and inter-molecular hydrogen bonds in models of cellulose I and alpha-cyclodextrin is presented. It is concluded that the hydrogen bonds have a secondary effect through influencing the ph,ps torsion angle conformation.

Item Type: Thesis (Doctoral)
Divisions : Theses
Authors : Durran, David Michael.
Date : 1996
Additional Information : Thesis (Ph.D.)--University of Surrey (United Kingdom), 1996.
Depositing User : EPrints Services
Date Deposited : 30 Apr 2019 08:08
Last Modified : 20 Aug 2019 15:33

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