Furanose conformers

Figure 1. The pseudorotational itinerary 2-14 describing the interconversion of non-planar furanose conformers. Regions of the itinerary are denoted as north, south, east and west as indicated. Envelope and twist conformers are denoted by E and T, respectively.

Figure 2. The two classes of non-planar furanose conformers of 3-deoxy-p-D-giycero-tetrofuranose 10.

Fig. 2.2.6.S Identified acceptor substrates of recombinant sucrose synthase 1 expressed in E. coli. The aldoses are depicted in their furanose conformations based on the conclusion that SuSyl from E. coli accepts D-fructofuranose preferentially.

Since almost no furanose form of D-glucose is observed in solution, it is expected that the energies of the furanose conformational isomers will be much greater than those of the pyranose form. In fact, the four furanose isomers examined are predicted to be about 4-5 kcal mol higher than the lowest pyranose conformers 6a and 6c at HF/6-31G. Quite remarkable, however, was the B3LYP/6-31G ... 

Chattopadhyaya et al. [8] have shown based upon thermodynamic estimates on a set of four isomeric 273 -deoxynucleosides that the net result of the gauche effect and the anomeric effect is of major importance in determining the overall furanose conformation. Many qualitative studies have been conducted [83-88] to understand how the electronic nature, protonation state [89], bulkiness or substitution pattern [90-95], and configuration of the nucleobases [96-115] or Cl substituent [116] modulate the sugar conformation in nucleosides and nucleotides, as well as furan-osides [117-121]. The conformational analysis [122] in solution of a dimer containing a 4 -oxofuran derivative [123], based upon the analysis of vicinal Jhh. has shown that the modified nucleoside adopts exclusively (89 %) the S-type puckered geometries, as a result of the cooperative drive by the 05 -C4 -04 anomeric effect and the [03 -C3 -C4 -04 ] gauche effect. 

Koole LH, Buck HM, Nyilas A, Chattopadhyaya J (1987) Structural properties of modified deoxyadenosine structures in solution. Impact of the gauche and anomeric effects on the furanose conformation. Can J Chem 65 2089-2094... 

Ring dihedral Principal Component Analysis of furanose conformation... 

The overwhelming majority of work on furanose conformation has focused on the N-ribofuranoside structure present in nucleosides. The pioneering work of Altona provided both a useful descriptor of furanose conformation and a reasonable description of the conformation of nucleotides based on two dominant conformational families, the North and South conformations. Somewhat less attention has been paid to 0-furanosides. ... 

Based on this first series of results, the conformational energy profiles of furanose rings seem far from featureless, and the conformation of the p-D-xylosyl derivatives, for example, can be reasonably described as vibrating within the pseudo-rotational cycle around 4E. However, two important caveats remain first, determination of the conformational energy profile required setting an arbitrary constraint (0, = 40°) that may significantly distort the results second, the very description of furanose conformation in terms of two parameters, P and 0n, excludes portions of conformational space from our description by limiting the search to pseudo-symmetrical conformations. 

Furanose conformers differ from each other in their energy only slightly and can interconvert between the different conformations very quickly. The most frequently occurring are envelope (E) and twist (T) conformations. Both of these basic conformations have almost equal energy. In fact, there are ten E conformations and ten T conformations that interconvert. The interconversion between the many conformers is called pseudorotation. Formulae 4-45 and 4-46 are examples of E and T conformers of P-n-glucofuranose. The type of conformer and levels of anomers in furanose solutions and in foods depends on the intramolecular non-bonding interactions. 

---