Molecular recognition motifs in cytidinium and 2'-deoxycytidinium salts with composite anions.

In the crystal structures of N3-protonated cytidinium and 2'-deoxycytidinium salts with composite XYn anions capable of accepting hydrogen bonds through their Y atoms, the dominating motif of cytosinium...anion interactions consists of a pair of hydrogen bonds donated from the N3+ -H protonation site and from the exoamino N4-H41 group cis to N3, and accepted by two Y centers of one anion. This multipoint recognition pattern is stable and robust and thus can be classified as a supramolecular synthon. In a broader group of N3-protonated, N1-substituted cytosinium salts with composite anions it occurs with 70% frequency. The C5 side of the cytosine ring mimics the N3+ -H type synthon and shows a propensity to form an analogous motif in which a C5-H5...Y hydrogen bond replaces the strong N3+ -H...Y interaction. Since the C-H...Y bond is much weaker, the secondary motif shows higher deformability and is less frequent (44%).

Crystal structure of 2'-deoxycytidine hemidihydrogenphosphate reveals C+.C base pairs and tight, hydrogen-bonded (H2PO4-)infinity columns (1).

2'-Deoxycytidine hemidihydrogenphosphate has been crystallized in the hexagonal space group P6(2) with a = 25.839(3), c = 12.529(1) A. The structure has been solved using the Patterson search method. The asymmetric unit contains two protonated, base-paired 2'-deoxycytidine dimers and two H2PO4- anions. The C+.C base pairs are composed of a protonated and a neutral species each and are triple H-bonded, the central N(3) ... N(3) bonds being 2.850(7) and 2.884(5) A. The conformations of the four nucleosides fall in the same category (sugar puckers 2'-endo, glycosidic links anti) but in one of them the glycosidic torsion angle is quite low with consequences in other geometrical parameters. The H2PO4- anions are located on twofold axes and form two types of tight columns with P ... P separations about 4.18 A. The neighboring units along a column are linked via two very short O ... H ... O hydrogen bonds (O ... O about 2.49 A) leading to effective equalization of the P-O bonds. The base pairs of the two dC+.dC cations are coplanar and form layers perpendicular to the phosphate columns repeating every c/3. Within the layers, the dimers form a network through O(5') ... O(2) hydrogen bonds but their primary intermolecular interactions have the form of H-bond anchors [N(4)-H ... O-P and O(3')-H ... O-P] to the phosphate groups.