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1980 (1)
1Author    G. Klimke, I. Cuno, H.-D Lüdemann, R. Mengel, M. J. RobinsRequires cookie*
 Title    Xylose Conformations of 9-/?-D-Xylofuranosyladenine Analogues Modified at the 2'3' or 5'-Positions and Lyxose Conformations of 9-^-D-Lyxofuranosyladenine  
 Abstract    Proton magnetic resonance studies o f 9-/?-D-xylofuranosyladenine (xyloA), its 2'-amino-2'-deoxy, 2'-azido-2'-deoxy, 2'-bromo-2'-deoxy, 3'-thio-3'-deoxy, 3'-amino-3'-deoxy, 3'-azido-3'-deoxy, 3'-chloro-3'-deoxy, 3'-fluoro-3'-deoxy, 3'-0-methyl, 3',5'-diazido-3',5'-dideoxy analogues and 9-/?-D-lyxofuranosyladenine (lyxoA) have been carried out to study the effect o f chemical modifications at the sugar moiety on the solution conformational equilibria in these classes of nucleosides. Analogously to previous studies the xylose pucker can be described in the two-state N <=> 5 model o f Altona and Sundaralingam. For the xylosides, however, a somewhat different N state (C3'-endo-C4'-exo) has to be used than for the ribosides and arabinosides (C2'-exo-C3'-endo). The results o f the conformational analysis are that xyloA exists almost exclusively as an N conformer. The effect o f the substitutions in the modified compounds is to destabilize the N state. This decrease in the mole fraction o f N is accompanied by an increase in the population of the g + rotamer o f the exocylic 5'-CH2OD group. Thus for the xylosides a correlation N-t/g~ or S-g +, respectively, can be derived. Proton relaxation rate measurements on 2'-azido-2'-deoxyxylo-furanosyladenine indicate that in the xylosides the standard syn or anti ranges do not represent stable positions for the adenine base, but that a glycosyl torsion angle (* ~ 160°, T ~ 80°) be­ tween syn and anti is preferred. LyxoA behaves similar to the xylosides and also favours the N state o f the sugar pucker. In a summarizing discussion the conformational equilibria o f the different modified pentosides -ribose, arabinose, xylose and lyxose -are compared. It is shown that generally intramolecular hydrogen bonding does not yield an important contribution to the stabilization o f conformational equilibria in solution. It is also not possible to derive a quantitative relationship between such parameters as Van-der-Waals' radii or electronegativity o f the substituents and the mole fractions of the different conformers. It can, however, be seen that in all cases, where the hydroxyl groups at C2' or C3' are substituted by a more voluminous atom or group, steric effects become dominant and allow a qualitative explanation o f the changes of the conformational equilibria. Only for the smallest and most electronegative substituents, like fluorine, other interactions (e. g. electrostatic) may become important. It is thus suggested that the purine(/7)nucleoside conforma­ tion is essentially determined by steric interactions between the different parts o f the molecule. 
  Reference    Z. Naturforsch. 35c, 865—877 (1980); received June 251980 
  Published    1980 
  Keywords    Adenosine Analogues, Conformation, NMR 
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 TEI-XML for    default:Reihe_C/35/ZNC-1980-35c-0865.pdf 
 Identifier    ZNC-1980-35c-0865 
 Volume    35