Conformations of furanose rings

Fig. 7.2 Torison angles of deoxyribonucleotide and conformations of furanose ring in DNA. a) Torsion angles of nucleotide and deoxyribose b) Conformation of endo furanose c) Conformation of exo furanose.

The practice of including the conformation after the name of the parent monosaccharide should be used only with caution because not all conformations are known with certainty. In the case of furanose rings especially, conformations might differ between the crystalline and solution states. 

The conformational properties of furanose rings have received considerable attention in recent years because of the impact these properties may have in mediating biological processes . Most notable in this respect are the p-D-ribofuranose 1 and 2-deoxy-p-D-erythro-pentose 2 (Scheme 1) components of ribo- (RNA) and deoxyribonucleic (DNA) acids. It is well known that the furanose ring adopts specific shapes depending on its local structural environment in a biopolymer. For example, in tRNA, the... 

The conformational dynamics of furanose rings may be described by the mechanisms of pseudorotation and inversion. The former mechanism describes a continuous pathway of interconversion between twenty idealized non-planar (envelope, twist) conformers (Figures 1 and 2) that does not involve the planar form (e.g., - E4 — ... 

These spectral markers make it possible to determine the relative content of each geometry in mixtures of A and B conformations. The ratio of the integrated intensities of the IR absorptions around 860 and 835 cm, A835/(/4835-i-.4860) for example, provides an estimate of the amount of B form in a sample (Liquier et ah, 1990). Similarly, the percentage of furanose rings in the north type geometry may be measured by Raman spectroscopy using the relative intensities of the 807 cm line and the band at 1100 cm (Brown et ah, 1972 Thomas and Hartman, 1973). 

FIGURE 12.40. The conformations of furanose sugars, (a) Conformations of five-membered rings. When a line is drawn through C4 -01 -C1, the pucker is described by which atom in the five-membered ring is most out of this plane of the five-membered ring, (b) CZ -endo with C3 most out of the plane on the C5 side, (c) C2 -exo with C2 most out of the plane on the opposite side from C5, (d) C2 -endo with C2 most out of the plane on the C5 side, and (e) C3 -exo with C3 most out of the plane on the opposite side from C5. ... 

In the case of furanose rings, where a mixture of conformers is equilibrating rapidly on the NMR time scale, conformational analysis becomes complicated because averaging of coupling constants is observed. To facilitate analysis, a commer-... 

The conformations of furanose and pyranose rings are described qualitatively by italicised letters T, E, C, H, S and B for twist, envelope, chair, halfchair, skew and boat, respectively T refers only to furanose rings and C, H, S and B only to pyranose rings, but E, describing conformations with just one ring atom out of the plane of the remainder, can refer to both pyranoses and furanoses. The particular conformation is specified by superscripts and subscripts referring to atoms above and below a defined plane thus (the... 

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. 

The historic development of the understanding of carbohydrate stereochemistry has been briefly reviewed, and a lecture with four references on the energetics and geometry of furanoid tems has been published. Two complementary descriptions of the conformational behaviour of furanose rings have been presented (i) quantum-mechanical energy calculations and (ii) by a geometrical model of pseudorotation in five-membered rings. ... 

The furanose rings of the deoxyribose units of DNA are conformationally labile. All flexible forms of cyclopentane and related rings are of nearly constant strain and pseudorotations take place by a fast wave-like motion around the ring The flexibility of the furanose rings (M, Levitt, 1978) is presumably responsible for the partial unraveling of the DNA double helix in biological processes. 

Notice that the eclipsed conformation of d ribose derived directly from the Fischer pro jection does not have its C 4 hydroxyl group properly oriented for furanose ring forma tion We must redraw it m a conformation that permits the five membered cyclic hemi acetal to form This is accomplished by rotation about the C(3)—C(4) bond taking care that the configuration at C 4 is not changed... 

Conformation of D ribose suitable for furanose ring formation... 

Figure 3 The underlying tree of a furanose ring in nucleic acids. Atoms are numbered 1,. . . , 5 corresponding to the natural tree ordering. All bond lengths are fixed. Arrows illustrate five internal coordinates that determine the ring conformation.

Jh-3. f 20.2 Hz in D2O), concluded that the furanose ring adopts the envelope form with C-3 endo, which differs greatly from the conformation of... 

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