Stereochemistry of nucleic acid constituents

摘要: The crystal structure of cytidylic acid b, ‡ cytidine-3′-phosphate, has been refined by the method full matrix least-squares to an R factor 0·045, = Σ | F o − c |/ where and are respectively observed calculated diffraction amplitudes. nucleotide is found exist as a zwitter ion. cytosine ring slightly nonplanar, with N (3) protonated. ribose puckered C (2′) -endo, displaced (on same side (5′) ) about 0·6 A from plane remaining atoms. These latter appear deviate plane. Because base in nucleic acids need not necessarily be planar because (1′) can significantly base, we suggest that definition torsion angle Φ CN defined Donohue & Trueblood (1960) , modified follows. bond −N formed projection −O relative −C (6) (in pyrimidine ring) (9) (8) purine when viewed along −N. This taken zero O anti-planar (2) or (4) ring, positive angles those measured clockwise direction viewing N. For this −42·1° conformation anti . All 14 hydrogen atoms ΔF synthesis their parameters have also refined. Several isotropic thermal anomalously low value. standard deviation atomic co-ordinates refinement P 0·0011 A, 0·0036 0·0042 0·005 H 0·06 A.