ABSTRACT
The crystallization and characterization of a new polymorph of 2-thiouracil by single-crystal X-ray diffraction, Hirshfeld surface analysis and periodic density functional theory (DFT) calculations are described. The previously published polymorph (A) crystallizes in the triclinic space group P\overline{1}, while that described herein (B) crystallizes in the monoclinic space group P21/c. Periodic DFT calculations showed that the energies of polymorphs A and B, compared to the gas-phase geometry, were -108.8 and -29.4â kJâ mol-1, respectively. The two polymorphs have different intermolecular contacts that were analyzed and are discussed in detail. Significant differences in the molecular structure were found only in the bond lengths and angles involving heteroatoms that are involved in hydrogen bonds. Decomposition of the Hirshfeld fingerprint plots revealed that O...H and S...H contacts cover over 50% of the noncovalent contacts in both of the polymorphs; however, they are quite different in strength. Hydrogen bonds of the N-H...O and N-H...S types were found in polymorph A, whereas in polymorph B, only those of the N-H...O type are present, resulting in a different packing in the unit cell. QTAIM (quantum theory of atoms in molecules) computational analysis showed that the interaction energies for these weak-to-medium strength hydrogen bonds with a noncovalent or mixed interaction character were estimated to fall within the ranges 5.4-10.2 and 4.9-9.2â kJâ mol-1 for polymorphs A and B, respectively. Also, the NCI (noncovalent interaction) plots revealed weak stacking interactions. The interaction energies for these interactions were in the ranges 3.5-4.1 and 3.1-5.5â kJâ mol-1 for polymorphs A and B, respectively, as shown by QTAIM analysis.
ABSTRACT
EPR spectroscopy was used to study the complex formed in crystals of 5-methyl-2-thiouracil (2-thiothymine) containing traces of copper. The copper impurities, originally present as Cu(I)-complex of 2-thiothymine in the lattice of 2-thiothymine, are transformed into paramagnetic Cu(II)-complex by ionizing radiation. It was found that the complex is planar, the plane being defined by two pairs of S and N atoms, from two adjacent 2-thiothymine molecules. The structure of the complex suggests that a pair of hydrogen bonds between two neighboring molecules is replaced by a stronger Cu-coordination bonding, with two sulfur and two nitrogen atoms as ligands. The spectroscopic parameters (g-tensor, A(63Cu) and A(14N) tensors) are essentially similar to those earlier observed for copper planar centers in other systems.
Subject(s)
Copper/analysis , Thymine/analogs & derivatives , Copper/chemistry , Crystallization , Electron Spin Resonance Spectroscopy , Molecular Conformation , Thymine/chemistryABSTRACT
Redox behaviour of the iron(III) complex with the antitumour drug hydroxyurea was studied by cyclic voltammetry. The complex underwent a one-electron reduction, followed by an irreversible chemical reaction (EC mechanism) in which a ligand was released. In addition, it was found that the hydroxyurea gave up an electron to iron(III) in solution. Differential-pulse voltammetry revealed an increase in the concentration of the generated iron(II) species. Electron paramagnetic resonance (EPR) spectroscopy studies of the oxidative degradation of hydroxyurea confirmed formation of the radical species H2N-CO-NHO*. Electrochemical data for iron(III) complexes of hydroxyurea and its structural analogue 3-ethylhydroxyurea, which also exhibits antitumour activity, show the same mechanism involved in the electron transfer. The observed redox properties indicate that hydroxyurea may interfere with electron transfer processes in biological systems after binding to iron-containing ribonucleotide reductase.
Subject(s)
Hydroxyurea/chemistry , Iron/chemistry , Antineoplastic Agents/chemistry , Electrochemistry , Electron Spin Resonance Spectroscopy , Oxidation-ReductionABSTRACT
The molecular structure of the title compound, also known as 2-thiothymine [systematic name: 2,3-dihydro-5-methyl-2-thioxopyrimidin-4(1H)-one], C(5)H(6)N(2)OS, is similar to that of thymine, with only small changes in the ring structure, apart from a significant difference at the substitution site [S=C = 1.674 (1) A]. The molecules are connected by hydrogen bonds, with N-H.O = 2.755 (2) A and N-H.S = 3.352 (1) A. The hydrogen-bond network is different from that in thymine, since it involves all the donor and acceptor atoms.