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Figure 3
Conformational interconversions. According to IUPAC carbohydrate nomenclature (McNaught, 1997BB63), the different conformations are identified by an italic capital letter, chair (C), envelope (E), boat (B), skew-boat (S), half-chair (H) and twist (T), with the atoms on the upper or lower side of the main ring plane in superscript and subscript lettering, respectively. Wavy lines identify those atoms that are roughly coplanar (i.e. forming the main plane) in that particular conformation. Here, the different conformations are drawn as a function of the Cremer–Pople puckering parameters (Cremer & Pople, 1975BB22). (a, b) Pseudo-rotational itinerary for furanoses and possible conformations. Furanoses are able to adopt twist and envelope conformations, with a very small energy barrier separating them. O atoms, which are assumed to be located at the top vertex in the pentagons, have been omitted from this diagram for reasons of clarity. In addition, the diagram does not show the total puckering amplitude (Q). (c, d) Cremer–Pople sphere describing the conformational itineraries for pyranoses and possible conformations. In order to convert the chair conformation of a pyranose ring to a boat conformation, both of which typically sit at energy minima, with the chair being the more favourable, the ring must pass through envelope or half-chair conformations which, having eclipsed substituents and considerable angle strain, require a considerable energetic investment. In context, these energy barriers are usually proportional to the cost of breaking three or four hydrogen bonds in peptides (Sheu et al., 2003BB85; Davies et al., 2012BB27).

Journal logoSTRUCTURAL
ISSN: 2059-7983
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