Imino Hydrogen Research Articles

The Cytochrome P450 Catalyzing C−S Bond Formation in S‐Heterocyclization of Chuangxinmycin Biosynthesis

AbstractMicrobial sulfur‐containing secondary metabolites show various biological activities, but the C−S bond‐forming in their biosynthetic metabolism has not been thoroughly understood. Here, we present genetic, biochemical and structural characterization of a cytochrome P450 monooxygenase CxnD exhibiting C−S bond forming activity in S‐heterocyclization of chuangxinmycin biosynthesis. In vivo and in vitro analyses demonstrated that CxnD generated an indole‐fused dihydrothiopyran skeleton from a L‐Trp‐derived thiol intermediate. Furthermore, X‐ray crystal structure of CxnD in complex with a substrate analogue and structure‐based mutagenesis revealed intimate details of the substrate binding mode. A radical mechanism initiated by abstraction of the imino hydrogen atom or an electron from indole group of the substrate was proposed for CxnD, which provided valuable insights into the molecular basis for the intra‐molecular C(sp2)−H thiolation by the P450 in chuangxinmycin biosynthesis.

The Cytochrome P450 Catalyzing C-S Bond Formation in S-Heterocyclization of Chuangxinmycin Biosynthesis.

Microbial sulfur-containing secondary metabolites show various biological activities, but the C-S bond-forming in their biosynthetic metabolism has not been thoroughly understood. Here, we present genetic, biochemical and structural characterization of a cytochrome P450 monooxygenase CxnD exhibiting C-S bond forming activity in S-heterocyclization of chuangxinmycin biosynthesis. In vivo and in vitro analyses demonstrated that CxnD generated an indole-fused dihydrothiopyran skeleton from a L-Trp-derived thiol intermediate. Furthermore, X-ray crystal structure of CxnD in complex with a substrate analogue and structure-based mutagenesis revealed intimate details of the substrate binding mode. A radical mechanism initiated by abstraction of the imino hydrogen atom or an electron from indole group of the substrate was proposed for CxnD, which provided valuable insights into the molecular basis for the intra-molecular C(sp2 )-H thiolation by the P450 in chuangxinmycin biosynthesis.

Zinc complexes supported by pyridine-N-oxide ligands: synthesis, structures and catalytic Michael addition reactions.

New dipyridylpyrrole N-oxide ligands HL1 and HL2 are designed and synthesized via oxidation of 2-(5-(pyridin-2-yl)-1H-pyrrol-2-yl)pyridine (Hdpp) by using 3-chloroperbenzoic acid (m-CPBA) in CH2Cl2. The treatment of ZnEt2 with two equiv. of HL1 and HL2 affords [Zn(L1)2] and [Zn(L2)2] in medium yield, respectively. These ligands and zinc complexes are fully characterized by NMR, IR, UV-vis and ESI-MS spectroscopy and X-ray diffraction analysis. The structure of HL1 and HL2 shows a planar geometry. The intramolecular hydrogen-bond interactions between the imino hydrogen and N-oxide oxygen atom are observed. In [Zn(L1)2] and [Zn(L2)2], two ligands chelate to the zinc metal with a cross perpendicular geometry. The zinc complexes were employed as a highly efficient catalyst for the thiol-Michael addition of thiols to α,β-unsaturated ketones in EtOH at room temperature. The loading of the catalyst is lowered to 0.01 mol%. The catalytic mechanism was proposed based on NMR and ESI-MS experiments.

Comparison of the Structural and Dynamic Effects of 5-Methylcytosine and 5-Chlorocytosine in a CpG Dinucleotide Sequence

Inflammation-mediated reactive molecules can result in an array of oxidized and halogenated DNA-damage products, including 5-chlorocytosine ((Cl)C). Previous studies have shown that (Cl)C can mimic 5-methylcytosine ((m)C) and act as a fraudulent epigenetic signal, promoting the methylation of previously unmethylated DNA sequences. Although the 5-halouracils are good substrates for base-excision repair, no repair activity has yet been identified for (Cl)C. Because of the apparent biochemical similarities of (m)C and (Cl)C, we have investigated the effects of (m)C and (Cl)C substitution on oligonucleotide structure and dynamics. In this study, we have constructed oligonucleotide duplexes containing C, (Cl)C, and (m)C within a CpG dinucleotide. The thermal and thermodynamic stability of these duplexes were found to be experimentally indistinguishable. Crystallographic structures of duplex oligonucleotides containing (m)C or (Cl)C were determined to 1.2 and 1.9 Å resolution, respectively. Both duplexes are B-form and are superimposable on a previously determined structure of a cytosine-containing duplex with a rmsd of approximately 0.25 Å. NMR solution studies indicate that all duplexes containing cytosine or the cytosine analogues are normal B-form and that no structural perturbations are observed surrounding the site of each substitution. The magnitude of the base-stacking-induced upfield shifts for nonexchangeable base proton resonances are similar for each of the duplexes examined, indicating that neither (m)C nor (Cl)C significantly alter base-stacking interactions. The (Cl)C analogue is paired with G in an apparently normal geometry; however, the G-imino proton of the (Cl)C-G base pair resonates to higher field relative to (m)C-G or C-G, indicating a weaker imino hydrogen bond. Using selective ¹⁵N-enrichment and isotope-edited NMR, we observe that the amino group of (Cl)C rotates at roughly half of the rate of the corresponding amino groups of the C-G and (m)C-G base pairs. The altered chemical shifts of hydrogen-bonding proton resonances for the (Cl)C-G base pair as well as the slower rotation of the (Cl)C amino group can be attributed to the electron-withdrawing inductive property of the 5-chloro substituent. The apparent similarity of duplexes containing (m)C and (Cl)C demonstrated here is in accord with results of previous biochemical studies and further suggests that (Cl)C is likely to be an unusually persistent form of DNA damage.

Stable Radical Trianions from Reversibly Formed Sigma-Dimers of Selenadiazoloquinolones Studied by In Situ EPR/UV–vis Spectroelectrochemistry and Quantum Chemical Calculations

The redox behavior of the series of 7-substituted 6-oxo-6,9-dihydro[1,2,5]selenadiazolo[3,4-h]quinolines and 8-substituted 9-oxo-6,9-dihydro[1,2,5]selenadiazolo[3,4-f]quinolines with R(7), R(8) = H, COOC(2)H(5), COOCH(3), COOH, COCH(3), and CN has been studied by in situ EPR and EPR/UV-vis spectroelectrochemistry in dimethylsulfoxide. All selenadiazoloquinolones undergo a one-electron reduction process to form the corresponding radical anions. Their stability strongly depends on substitution at the nitrogen atom of the 4-pyridone ring. The primary generated radical anions from N-ethyl-substituted quinolones are stable, whereas for the quinolones with imino hydrogen, the initial radical anions rapidly dimerize to produce unusually stable sigma-dimer (σ-dimer) dianions. These are reversibly oxidized to the initial compounds at potentials considerably less negative than the original reduction process in the back voltammetric scan. The dimer dianion can be further reduced to the stable paramagnetic dimer radical trianion in the region of the second reversible reduction step. The proposed complex reaction mechanism was confirmed by in situ EPR/UV-vis cyclovoltammetric experiments. The site of the dimerization in the σ-dimer and the mapping of the unpaired spin density both for radical anions and σ-dimer radical trianions with unusual unpaired spin distribution have been assigned by means of density functional theory calculations.

Individual Basepair Stability of DNA and RNA Studied by NMR-Detected Solvent Exchange

In this study, we have optimized NMR methodology to determine the thermodynamic parameters of basepair opening in DNA and RNA duplexes by characterizing the temperature dependence of imino proton exchange rates of individual basepairs. Contributions of the nuclear Overhauser effect to exchange rates measured with inversion recovery experiments are quantified, and the influence of intrinsic and external catalysis exchange mechanisms on the imino proton exchange rates is analyzed. Basepairs in DNA and RNA have an approximately equal stability, and the enthalpy and entropy values of their basepair dissociation are correlated linearly. Furthermore, the compensation temperature, Tc, which is derived from the slope of the correlation, coincides with the melting temperature, and duplex unfolding occurs at that temperature where all basepairs are equally thermodynamically stable. The impact of protium-deuterium exchange of the imino hydrogen on the free energy of RNA basepair opening is investigated, and it is found that two A·U basepairs show distinct fractionation factors.

Imino hydrogen positions in nucleic acids from density functional theory validated by NMR residual dipolar couplings.

Hydrogen atom positions of nucleotide bases in RNA structures solved by X-ray crystallography are commonly derived from heavy-atom coordinates by assuming idealized geometries. In particular, N1–H1 vectors in G and N3–H3 vectors in U are commonly positioned to coincide with the bisectors of their respective heavy-atom angles. We demonstrate that quantum-mechanical optimization of the hydrogen positions relative to their heavy-atom frames considerably improves the fit of experimental residual dipolar couplings to structural coordinates. The calculations indicate that deviations of the imino N–H vectors in RNA U and G bases result from H-bonding within the base pair and are dominated by the attractive interaction between the H atom and the electron density surrounding the H-bond-acceptor atom. DFT optimization of H atom positions is impractical in structural biology studies. We therefore have developed an empirical relation that predicts imino N–H vector orientations from the heavy-atom coordinates of the base pair. This relation agrees very closely with the DFT results, permitting its routine application in structural studies.

Photoinduced decarboxylation of 9‐oxo‐6,9‐dihydro[1,2,5]selenadiazolo[3,4‐f]quinoline‐8‐carboxylic acid

Photoinduced reactions of 9‐oxo‐6,9‐dihydro[1,2,5]selenadiazolo[3,4‐f]quinoline‐8‐carboxylic acid (SeQCA) were investigated in alkaline media (aqueous NaOH solutions) by electron paramagnetic resonance (EPR) spectroscopy, following the in situ formation of paramagnetic species. According to UV–Vis and nuclear magnetic resonance investigations, protonation (pH ≈ 11) and deprotonation (pH ≈ 13) of the imino hydrogen of the 4‐pyridone moiety has to be considered, reflected also in the different EPR spectra observed upon irradiation. Photoinduced generation of radicals was found only for carboxylate substituted SeQCA; other studied selenadiazoloquinolone derivatives, together with those substituted at the C(8) position (R = H, COOCH2CH3, COOCH3, COCH3 or CN), did not generate paramagnetic species during exposure. Consequently, photodecarboxylation was suggested as the decisive step, accompanied by the decomposition of the selenadiazole ring, resulting in the formation of ortho‐hydroxylate anions. EPR parameters elucidated from experimental EPR spectra obtained at pH ≈ 11 and pH ≈ 13 indicate the formation of oxygen‐centered radicals at the decarboxylated 4‐pyridone ring. EPR spin trapping experiments with nitromethane confirmed a very effective photoinduced electron transfer from all the selenadiazoloquinolones investigated. Copyright © 2011 John Wiley & Sons, Ltd.

Nitrothienylporphyrins: Synthesis, crystal structure and, the effect of position and number of nitro groups on the spectral and electrochemical properties

This article describes the investigation on the effect of orientation of the meso thienyl groups of porphyrins in deciding the site of nitration. The thienyl rings present at the meso position is found to be more susceptible for electrophilic nitration reaction than the pyrrole β-position in the molecules where there is a better conjugation between the thienyl rings and the porphyrin π-system. Signal corresponding to the imino hydrogens in the proton NMR spectrum of meso nitrothienylporphyrins get shifted to upfield with increase in the number of nitro groups on the porphyrin. This is also due to the extended conjugation of the porphyrins π-system with the meso thienyl rings. The above observations are also supported by the redox potentials of those compounds.

18-Crown-6 as a linker for (imino ester)2Pt centers providing their assembly into 1D arrays via hydrogen bonding

Abstract The platinum(II- and IV) complexes trans-[PtCln{HN C(OMe)Et}2] (n = 2, 4) were assembled with 18-crown-6 into novel 1D arrays by hydrogen bonding thus forming 1:1 associates. The crystal structure determinations revealed the presence of the N–H⋯O and the non-conventional weak CH⋯O interactions between the imino hydrogen and the methyl hydrogens, respectively, of trans-[PtCln{HN C(OMe)Et}2] and the oxygens of 18-crown-6.

Organotin meclofenamic complexes: Synthesis, crystal structures and antiproliferative activity of the first complexes of meclofenamic acid – Novel anti-tuberculosis agents

The complexes [Me(2)(Meclo)SnOSn(Meclo)Me(2)](2) (2) and [Ph(3)Sn(Meclo)] (3) where HMeclo is meclofenamic acid, N-(2,6-dichloro-m-tolylanthranilic acid)], have been prepared and structurally characterized by means of vibrational, (1)H and (13)C NMR spectroscopies. The crystal structure of complexes (2) and (3) have been determined by X-ray crystallography. Three distannoxane rings are present to the dimeric tetraorganodistannoxane of planar ladder arrangement of (2). The structure is centro symmetric and features a central rhombus Sn(2)O(2) unit two additional tin atoms linked at the oxygen atoms. Five- and six-coordinated tin centers are present in the dimer distannoxane. X-ray analysis of (3) revealed a penta-coordinated structure containing Ph(3)Sn coordinated to the chelated carboxylato group. The polar imino hydrogen atom participates in intra-molecular hydrogen bonds. Complexes (2) and (3) are self-assembled via pi-->pi, C-H-pi, stacking interactions and intra-molecular hydrogen bonds. Meclofenamic acid and [Ph(3)Sn(Meclo)] have been evaluated for antiproliferative activity in vitro against three human cancer cell lines: MCF-7 (human breast cancer cell line), T24 (bladder cancer cell line), A-549 (non-small cell lung carcinoma) and a mouse L-929 (a fibroblast-like cell line cloned from strain L). The [Ph(3)Sn(Meclo)] complex exhibited high cytotoxic activity against all the cancer cell lines. Meclofenamic and [Ph(3)Sn(Meclo)] were tested for anti-mycobacterial activity against Mycobacterium tuberculosis H37Rv. The [Ph(3)Sn(Meclo)] complex was found to be a promising anti-mycobacterial lead compound, displaying high activity against M. tuberculosis H37Rv.

Synthesis, characterization and catalytic properties of diorganotin derivatives. Crystal and molecular structure of the first complex of 2-(2-methyl-3-nitroanilino)benzoic acid of 1,2:3,4-di-μ2-2-(2-methyl-3-nitroanilino)benzoato- O, O-1,3-bis-2-(2-methyl-3-nitroanilino)benzoato- O-1,2,4:2,3,4-di-μ3-oxo-tetrakis[di-methyltin(IV)

The complexes [Me 2(MNAB)SnOSn(MNAB)Me 2] 2 ( 2) and [Me 2Sn(MNAB) 2] ( 3), where HMNAB is 2-(2-methyl-3-nitroanilino)benzoic acid, ( 1), have been prepared and structurally characterized by means of vibrational, 1H and 13C NMR spectroscopies. The crystal structure of [Me 2(MNAB)SnOSn(MNAB)Me 2] 2 has been determined by X-ray crystallography. Three distannoxane rings are present to the dimeric tetraorganodistannoxane of planar ladder arrangement. The structure is centro-symmetric and features a central rhombus Sn 2O 2 unit with two additional tin atoms linked at the O atoms. Six-coordinated tin centers are present in the dimmer distannoxane. This structure is self-assembled in one dimensional infinite chain via intermolecular hydrogen bonds, π → π and C–H → π stacking interactions. The polar imino hydrogen atom participates in intramolecular hydrogen bonds. The catalytic activity of the prepared complexes in transesterification reactions has been studied. The diorganotin complexes efficiently catalyze the transesterification reaction of 2-phenylethanol without addition of free ligand or any promoting additive.

Metal Derivatives of 1,3‐Imidazolidine‐2‐thione with Divalent d10 Metal Ions (Zn–Hg) : Synthesis and Structures

AbstractReactions of divalent Zn‐Hg metal ions with 1,3‐imidazolidine‐2‐thione (imdtH2) in 1 : 2 molar ratio have formed monomeric complexes, [Zn(η1‐S‐imdtH2)2(OAc)2] (1), [Cd((η1‐SimdtH2)2I2] (2), [Cd(η1‐S‐imdtH2)2Br2] (3), and [Hg(η1‐S‐imdtH2)2I2] (4). Complexes 1–4, have been characterized by elemental analysis (C, H, N), spectroscopy (IR, 1H, NMR) and x‐ray crystallography (1‐4). Hydrogen bonding between oxygen of acetate and imino hydrogen of ligand, {N(2)–H(2C)···O(2)#} in 1, ring CH and imino hydrogen, {C(2A)–H(2A)···Br(2)#} in 3 have formed H‐bonded dimers. Similarly, the interactions between molecular units of complexes 2 and 4 have yielded 2D polymers. The polymerization occurs via intermolecular interactions between thione sulfur and imino hydrogen, {N(2)–H(2)···S(1)#}, imino hydrogen and the iodine atom, {NH(1)···I(2)#} in 2 and imino hydrogen – iodine atom {N(2A)–H(2A)···I(2)} and I···I interaction in 4. Crystal data: [Zn(η1‐S‐imdtH2)2(OAc)2] (1), C10H18N4O4S2Zn, orthorhombic, Pbcn, a = 9.3854(7) Å, b = 12.4647(10) Å, c = 13.2263(11) Å; V = 1547.3(2) Å3, Z = 4, R = 0.0280 [Cd((η1‐S‐imdtH2)2I2] (2), C6H12CdI2N4S2, orthorhombic, Pnma, a = 13.8487(10) Å, b = 14.4232(11) Å, c = 7.0659(5) Å; Z = 4, V = 1411.36(18) Å3, R = 0.0186.

The Influence of the Methyl Substituents at C2 Carbon of Thiosemicarbazones {R1R2C2=N3‐N2H‐C1(=S)N1H2} on Bonding and Structures of Copper(I) Complexes

AbstractReaction of copper(I) halides with acetone thiosemicarbazone (Hactsc) and acetaldehyde thiosemicarbazone (Hmtsc) in CH3CN in the presence of PPh3 has formed dimeric complexes of type, [Cu2(μ‐X)2(η1‐S‐Htsc)2(Ph3P)2] (Htsc: X; Hmtsc, Br, 1; I, 2; Hactsc, I, 3; Cl, 5), and a monomer, [CuBr((η1‐S‐Hactsc)(Ph3P)2 (4). The complexes have been characterized using analytical data, spectroscopy and X‐ray crystallography (1–4). The presence of methyl substituents at C2 carbon of thiosemicarbazone seems to alter inter‐ and intra‐molecular hydrogen bonding. Intermolecular and intramolecular interactions by amino and imino hydrogen atoms respectively with bridging halogen atom shorten X–Cu–X bond angle, and open up Cu–X–Cu bond angle, resulting in longer Cu···Cu separation (3.50–3.61Å) in complexes 1–3.

Electronic structure and spectra of NH-isomers of isobacteriochlorin

Using the restricted and unrestricted Hartree-Fock method (RHF and UHF) with the AM1 Hamiltonian, we have calculated the geometric structure of the NH-isomers of isobacteriochlorin and its meso-cyano derivative. We have used the CNDO/S method for all the considered structures to calculate the transitions to excited electronic states. We have analyzed the effect of taking into account electron correlation on the results of the calculations of the chemical and geometric structure and the electronic spectra. For the a isomer with the imino hydrogens opposite each other, we obtained a planar structure corresponding to literature x-ray diffraction data. For the aromatic c isomer and other isomers with the imino hydrogens adjacent to each other, the calculations lead to a substantial nonplanarity of the macrocycle (defined mainly by the tilt of the pyrrole rings) and also deviation of the NH bond from the plane of the pyrrole ring. The ground-state energies of the a and c isomers have close values, which generally explains the observed NH tautomerism. We calculated the dipole moments of the isomers in the ground state and the excited state. The results of the calculation of the characteristics of the excited states with UHF geometry on the whole seem to be preferred over those with RHF geometry. We show that only for the aromatic c isomer can we describe the electronic absorption spectrum within a four-orbital model.

TAUTOMERISM OF 4-HYDROXY-4(1H)QUINOLON

The tautomerism of 4-Hydroxy-4(1H) quinolon I was studied using infrared spectroscopy, 1 H, 13 C NMR spectroscopy and X-ray crystallography. The keto-form of I is favored in the crystal form and at room temperature in polar solutions like water and dimethylsulfoxide. Introduction. Quinolons produced by different bacteria inhibit the respiratory (1) as well as photosynthetic electron transport chains (2). They were used as lead compounds for finding new strong inhibitors. One of the most important physical and chemical properties of this class of heteroaromatic compounds is their involvement in a prototropic tautomerism (Figure 1). In order to understand the biological activity of I we investigated its crystal structure and studied the tautomerism in water and DMSO solution by 1 H and 13 C NMR spectroscopy. To further investigate the tautomerism of compound I in solution we synthesized the O-methylated derivate II, which mimics the enol-tautomeric form (Figure 1). Single crystals of I for X-ray analysis were obtained from dichloromethane. The crystal structure exists as a dimer. The monomers are connected by an intermolecular hydrogen bond (O...HN distance of 1.795 A) between the imino hydrogen and the carbonyl group (Figure 2 and Figure 3). The carbon oxygen bonds length of 1.246 A for C1A-O1A and 1.262 A for C1-O1 are in the range of the C=O length. Also the C3N4 and C5N4 bonds of 1.352 A and 1.383 A indicate a nitrogen carbon single bond. This results show that the keto tautomer is favored in the crystal structure.

Condensation Reaction of Dimethyltin(IV) Oxide with 2-(2,3-Dichloroanilino)benzoic Acid Synthesis and Study of a Novel Dimeric Tetraorganodistannoxane

The novel symmetric dimeric tetraorganodistannoxane [(Me2Sn)4(DCPA)2)O2(OH)2] (2) and [(Me2Sn)4(DCPA)2)O2(OC2H5)2] (3) where HDCPA is 2-(2,3-dichloroanilino)benzoic acid (1) have been prepared. The crystal structure of 3 has been determined by X-ray crystallography. Three distannoxane rings are present to the centrosymmetric dimeric tetraorganodistannoxane by virtue of μ3-oxo form the central R4Sn2O2 core with a planar Sn2O2 ring, resulting in a ladder type structural motif. Five-coordinated tin atoms are present in the distannoxane dimer. The ligands act as monodentate agents, thus rendering the tin atom five-coordinated. Significant π → π stacking interactions and intramolecular hydrogen bonds stabilize the structure 3. The polar imino hydrogen atom participates in intramolecular hydrogen bonds. The formation of the dimeric distannoxanes 2 and 3 represent a ladder-type carboxylates in which the insertion of a μ2-OH or a μ2-OC2H5 group occurs. This unusual result can be interpreted in terms of a competition between the strength different donors, in which the –OH or the –OC2H5 groups show higher donor capacity than the carboxylato group of DCPA.

Electronic Spectra of Jet-Cooled 3-Methyl-7-azaindole Dimer. Symmetry of the Lowest Excited Electronic State and Double-Proton Transfer

The fluorescence excitation spectra are recorded for jet-cooled dual hydrogen-bonded 3-methyl-7-azaindole dimer (3MAI)2-hh and deuterated dimers (3MAI)2-hd and (3MAI)2-dd near the electronic origin region of the S1−S0 transition, where hd and dd indicate the deuteration of an imino hydrogen and two imino hydrogens, respectively. A single origin is detected in the spectra of (3MAI)2-hh and (3MAI)2-dd, whereas two electronic origins separated by 13 cm-1 are detected in the spectrum of (3MAI)2-hd. The excited-state double-proton transfer (ESDPT) occurs in (3MAI)2-hh, while (3MAI)2-hd and (3MAI)2-dd undergo excited-state proton/deuteron transfer and excited-state double deuteron transfer, respectively. In (3MAI)2-hd, the excitation is localized on either the 3MAI-h or 3MAI-d moiety. The localization of the excitation is explained by a weak coupling of the excitonic states of (3MAI)2-hh and (3MAI)2-dd. The effective symmetry of the lowest excited state of (3MAI)2-hh and (3MAI)2-dd belongs to the C2h point grou...

Trans-hydrogen bond deuterium isotope effects of A:T base pairs in DNA.

The chemical shifts of (13)C2 of adenosine residues of DNA were observed to experience a through-space or trans-hydrogen bond isotope effect as a result of deuterium substitution at the imino hydrogen site of base-paired thymidine residues. NMR measurements of several self-complementary DNA duplexes at natural abundance (13)C in 50% H(2)O, 50% D(2)O solvent mixtures yielded an average trans-hydrogen bond isotope effect, (2h)Delta(13)C2, of -47 ppb. The data suggest that stronger hydrogen bonds have more negative (2h)Delta(13)C2 values, which means that A:T N1.H3 hydrogen bonds increase the anharmonicity of the effective vibrational potential of H3. However, (2h)Delta(13)C2 values do not correlate with intra-residue (2)Delta(13)C4 values of thymidine observed here and earlier (Vakonakis et al., 2003), which suggests that (2h)Delta(13)C2 is not determined entirely by hydrogen bond strength. Instead, the variations observed in (2h)Delta(13)C2 values suggest that they may also be sensitive to base pair geometry.

Synthesis and spectroscopic studies of diorganotin derivatives with 2-[(2,6-dimethylphenyl)amino]benzoic acid. Crystal and molecular structure of the first complexes of 2-[(2,6-dimethylphenyl)amino]benzoic acid. Crystal and molecular structures of 1,2:3,4-di- μ2-2-[(2,6-dimethylphenyl)amino]-benzoato- O,O-1,3-bis-2-[(2,6-dimethylphenyl)amino]benzoato- O-1,2,4:2,3,4-di- μ3-oxo-tetrakis[di-butyltin(IV)] and bis-2-[(2,6-dimethylphenyl)amino]benzoato-di- n-butyltin(IV)

The complexes [Bu 2(DMPA)SnOSn(DMPA)Bu 2] 2 ( 2) and [Bu 2Sn(DMPA) 2] ( 3), where HDMPA is 2-[bis(2,6-dimethylphenyl)amino]benzoic acid, have been prepared and structurally characterized by means of, vibrational, ultra-violet, 1H and 13C NMR spectroscopies. The crystal structure of complexes 2 and 3 have been determined by X-ray crystallography. Three distannoxane rings are present to the dimeric tetraorganodistannoxane of planar ladder arrangement of 2. The structure is centro-symmetric and features a central rhombus Sn 2O 2 unit with two additional tin atoms linked at the O atoms. Five- and six-coordinated tin centers are present in the dimmer distannoxane. The crystal structure of 3 consists of discrete molecular units and the two modeprononated ligands are co-ordinated to the SnBu 2 fragment. The ligands act as anisobidentate chelating agents, thus rendering the tin atom six-co-ordinated. Significant π→π, C–H-π stacking interactions and intramolecular hydrogen bonds stabilize the structures 2 and 3. The polar imino hydrogen atom participates in intramolecular hydrogen bonds. Complexes 2 and 3 are self-assembled via π→π C–H-π and stacking interactions.