13C-1H spin-coupling constants in carbohydrates: magnitude and sign determinations via 2D NMR methods

✍ Scribed by Anthony S. Serianni; Carol A. Podlasek

• Publisher: Elsevier Science
• Year: 1994
• Tongue: English
• Weight: 700 KB
• Volume: 259
• Category: Article
• ISSN: 0008-6215
• DOI: 10.1016/0008-6215(94)84062-8

✦ Synopsis

Conformational analysis of carbohydrates has depended, in large part, on the use of three-bond (vicinal) IH_1H spin-coupling constants (31HH)' whose magnitudes can be correlated with molecular torsion angles, as first enunciated by Karplus [1]. In the ensuing years, more sophisticated Karplus relationships have been advanced that account for substitution and/or electronegativity effects along H-C-C-H coupling pathways [2]. The relative ease of measurement of 3 1HH compared to other potentially useful coupling constants within carbohydrates (e.g., lCH and lee) is largely responsible for their dominant role in structure determination. However, in some situations, 3 1HH are not available to probe the conformational features of these molecules. A well-known example of this problem involves the assessment of O-glycoside linkage conformation in oligosaccharides where only 13C_l H and 13C_ 13C spin-couplings are available for evaluating the phi (4)) and psi (l/1) torsion angles [3a-e]. Other situations, such as the presence of conformational averaging in solution, demand access to as large a parameter pool as possible in order to provide sufficient data on which to confirm or discard potential conformational models. An example of the latter is found in structural studies of furanosyl rings [4], either as free entities or as components of more complex biologically important structures (e.g., polysaccharides, RNA, and DNA). Seminal work by Perlin and co-workers [5] and Pedersen and co-workers [6] has attempted to define the effect of carbohydrate structure on one-(llCH)' two- elCH) and three-elCH) bond 13C_1H spin-couplings. Karplus relationships have been proposed for C-C-C-H and C-O-C-H coupling pathways [5a,?]. In contrast, 2 1CCH have been used less frequently as conformational probes, although