Subramania Ranganathan

Unique assemblies of alternating positively and negatively charged layers, directed by hydrogen bonds, ionic interactions, and Π-stacking in the crystal structures of complexes between mellitic acid (benzenehexacarboxylic acid) and five planar aromatic bases

AbstractBenzenehexacarboxylic acid, mellitic acid (MA), has been used as a core motif to study possible radial self-assembly using complementary aromatic bases. By mixing water solutions of the components, crystals of the salts of MA with 4-aminopyridine (AP), 4-dimethylamino-pyridine (DM), 2,2′-bipyridine (DP), o-phenanthroline (PL), and melamine (ML) have been obtained. The MA−n ions have assembled in either extended sheets for MA−2 or extended ribbons for MA−4 by direct hydrogen bonding between MA and MA and additionally through mediation of hydrogen bonds to water molecules that distribute the negative charges throughout the MA sheet or ribbon. Most of the O atoms in carboxyl groups in the MA ions in the five complexes have been rotated significantly out of the plane of the central benzene ring. There are multiple base molecules, two or four, for each mellitic acid ion in the five complexes. Most of the NH+ moieties in all five bases make direct NH+ ⋅ ⋅ ⋅ O–C hydrogen bonds with MA−n. The planar base ions are generally arranged in stacks in which the components range from being parallel, with interplanar separations of 3.5 Å, to having a considerable tilt with respect to each other with nearest interplanar separation of atoms greater than 3.9 Å. These geometric characteristics are reflected in the color of the crystals. The three-dimensional networking makes some of the crystals very hard. Cell dimensions: 1, C32H30N8O12 ⋅ 2H2O, C2/c, a =13.764(2) Å, b =18.053(3) Å, c =14.876(4) Å, β =105.99(2)° 2, C26H26N4O12 ⋅ 3H2O, P21/n, a =15.891(1) Å, b =10.444(1) Å, c =18.242(1) Å, β =97.00(1); 3, C64H44N8O24⋅7H2O, P21/c, a =23.016(4) Å, b =15.241(2) Å, c =19.124(2) Å, β =100.60(1)° 4, C36H22N4O12, P21/n, a =14.581(1) Å, b =10.472(1) Å, c =20.607(2) Å, β =106.43(1); 5, C18H18N12O12 ⋅ 2H2O,

Highly efficient propane-1,3-dithiol mediated thiol–disulphide interchange: a facile and clean methodology for S–S reduction in peptides

P\bar 1

A robust hybrid peptide crystal formed with weak hydrogen bonds

, a =8.257(2) Å, b =8.986(2) Å, c =9.383(1) Å, α =98.60(1)°, β =96.38(2)°, γ =117.07(1)°.

Solubilization of silica: Synthesis, characterization and study of penta-coordinated pyridine N-oxide silicon complexes #

The design and synthesis of agents that can abstract zinc from their [CCXX] (C=cysteine; X=cysteine/histidine) boxes by thioldisulfide exchange-having as control, the redox parities of the core sulfur ligands of the reagent and the enzyme, has been illustrated, and their efficiency demonstrated by monitoring the inhibition of the transcription of calf thymus DNA by E. coli RNA polymerase, which harbors two zinc atoms in their [CCXX] boxes of which one is exchangeable. Maximum inhibition possible with removal of the exchangeable zinc was seen with redox-sulfanilamide-glutamate composite. In sharp contrast, normal chelating agents (EDTA, phenanthroline) even in a thousand fold excess showed only marginal inhibition, thus supporting an exchange mechanism for the metal removal

Biocompatible Small Molecules that Enhance Silica Solubilization Under Ambient Conditions: Chemical Profile of Such Complexes, Possible Mechanism for Enhancement, and Their Effect on the Growth and Protection from Pests in the Rice Plant (Oryza sativa L.)

Propane-1,3-dithiol, at room temperature and in the absence of any promoter, neatly brings about the S–S → 2SH change, and the latter can be isolated as acrylonitrile adducts; the methodology has been tested with several substrates including c-lysozyme, insulin and oxytocin, and spontaneous imidazole promoted S-deprotection was observed in the case of Z-Cys(S-CH2CH2CN)His-OMe.

The Novel Formation of Ordered and Varied Silica-Imidazole Complexes from Silicic Acid

The structural features of the title compound were determined or examined by three diverse procedures: single crystal X-ray diffraction analysis, solution spectroscopic procedures and Quantum mechanical theoretical calculations. The conformational asymmetry of the macrocycle provides the opportunity to form one strong NH···OC intermolecular hydrogen bond, as well as, a number of weak CH···OC bonds. The interior of the macrocycle has short approaches for NH(...)π and NH···S. The many weak hydrogen bonds cooperate to form a very hard, robust crystal. Crystal parameters: C(18)H(22)N(2)O(6)S(2), P2(1)2(1)2(1), a = 5.108(1) Å, b = 18.948(4) Å, c = 21.029(3) Å, α = β = γ = 90°. Quantum chemical calculations have provided a strong foundation for weak hydrogen bonds. Contrary to popular belief the present work has conclusively proved that the importance of weak hydrogen bonds are perhaps underestimated since calculations show that the energy of duplex are significantly lower then estimated from the identified hydrogen bonding.

Molecular origami: Modular construction of platonic solids as models for reversible assemblies

In the course of our work relating to the design of a bihelical structure (I) from diphenic anhydride by tethering with cystine di‐OMe, stable, hard, and rigid crystals, mp 215–218°C were isolated in low yields (∼2%). The crystal structure established that it was a bis amide (II) arising from diphenic acid and cystine di‐OMe [(II), C22H22N2O6S2 (a = 9.897 (1) Å, b = 12.210 (1) Å, c = 18.192 (1) Å, sp. gr. P212121)]. An authentic sample of (II) was subsequently prepared in 47% yields by condensation of diphenic acid dichloride with cystine di‐OMe. A most surprising feature of II was, despite its high density, rigidity, and hardness, it did not exhibit any normal hydrogen bonds. The nearest approximation to a “usual” hydrogen bond was the single NH···OC linkage that occurred between molecules along a twofold screw axis. In this linkage, N···O = 3.265 Å and H···O = 2.43 Å, values that are at least 10% longer than those usually observed in peptides. The rigidity of the crystals appears to depend upon many weak hydrogen bonds of the type CH···O, CH···π, CH···S, and NH···S working in concert. Even these attractions have separations that are at the high end of the range of previously observed values, although some of the weak hydrogen bonds have been rarely reported and have poorly defined ranges. The attractive effect of each of these weak bonds may be enhanced by the occurrence of a number of them in a parallel fashion like rungs in a ladder.

Jeans and means

In an effort to design agents that could solubilize silica in water, under ambient conditions and pH, as takes place in nature, novel zwitterionic, penta-oxo-coordinated silicon compounds with siliconate cores have been prepared from 4-substituted pyridine N-oxides (H, OMe, morpholino, NO2) as donor ligands, their structures established by1H,13C and MS, and the coordination number of silicon, by29Si NMR. The formation of complexes from pyridine N-oxides is noteworthy since they arise from interaction with a weakly nucleophilic oxygen centre. The ability of the pyridine N-oxides to enhance the solubilization of silica in water has been experimentally demonstrated. Possible rationalization of this observation on the basis of O → Si coordination via the oxygen atom of pyridine N-oxide is suggested