Furanose rings, conformation

This is one example of the second criterion mentioned previously. However, the other OH substituents, depending upon the furanose ring conformation, are either eclipsed or in the anti conformation. In the former they are disposed to form a strong intramolecular hydrogen bond in the latter they are incapable of such bonding. Further evidence to support the contention that free / -D-fructofuranose is nearly tasteless is seen in the thermal mutarotation (3) of D-fructose. As the temperature of D-fruc-... 

Serianni AS, Chipman DM (1987) Furanose ring conformation The application of ab-initio molecular orbital calculations to the structure and dynamics of erythrofuranose and threofuranose rings. J Am Chem Soc 109 5297 - 5303... 

The furanose ring conformation may be shifted toward C-2 endo as in B using bulky protecting groups at C 5 and C 3 of the purine nucleoside 2 -triflate. It has been suggested that trityl group may force the furanose ring to assume an unfavorable conformation for trans elimination. We therefore synthesized 3, 5 -di-O-trityl-2 -0-triflyl-l-benzylinosine. 

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.

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. 

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... 

Deoxy-3-fluoro- -D-xylofuranosyl)cytosine (821) was prepared by condensation of 2,5-di-0-benzoyl-3-deoxy-3-fluoro-D-xylofuranosyl bromide with bis(trimethylsilyl)cytosine (773). It was proposed that 1 -(3 1eoxy-3-fluoro-)S-D-arabinofuranosyl)uracil (823), prepared from l-(2,3-anhy-dro-)S-D-lyxofuranosyl)uracil (822) with anhydrous HF ( 8% in 1,4-dioxane 116 °, 41 h, 3% yield), has a twist conformation of the furanose ring(°r, acetone- 4-D20 . Jn-vf 13.5, Hz). Reexamination ... 

The conformations of the furanose ring in 250 nucleoside and nucleotide structures were analysed by Bartenev et al. (1987). These authors made the assumption, referred to above, that intermolecular interactions have a random effect on the structure in the crystal, and that the probability JVg of a structure crystallizing in a non-ground-state conformation is the same as the probability of it arising in thermal equilibrium at ambient temperature T in solution (6). (A difficulty arises immediately with the definition of the temperature, because structural parameters for molecules in crystals are... 

Finally, the effect of neomycin-B flexibility on its enzymatic inactivation by Enterococcus faecalis APH(3 ) was tested. This enzyme catalyzes the transfer of a phosphate group from ATP to position 4 of ring I. In this case, the X-ray structure of the enzyme in complex with neomycin-B shows that the antibiotic structure recognized by the enzyme and the target RNA are remarkably similar (see Fig. 7) [40]. In fact, they mainly differ in the conformation adopted by the furanose ring of III (C3-endo and close to C2-endo for the RNA- and enzyme-bound conformations respectively). 

Monosaccharides have many structural variations that correspond to local minima that must be considered. Acyclic carbohydrates can rotate at each carbon, and each of the three staggered conformers is likely to correspond to a local minimum. The shapes of sugar rings also often vary. Furanose rings usually have two major local minima and a path of interconversion. Experimental evidence shows a clear preference for only one chair form for some pyranose rings, but others could exist in several conformers. For exanqple, the and conformers must all be considered as possible structures for L-iduronate, as discussed by Ragazzi et al. in this book. 

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