Tryptophan oxidation by singlet molecular oxygen [O-2 ((1)Delta(g))]: Mechanistic studies using O-18-labeled hydroperoxides, mass spectrometry, and light emission measurements

Proteins have been considered important targets for reactive oxygen species. Indeed, tryptophan (W) has been shown to be a highly susceptible amino acid to many oxidizing agents, including singlet molecular oxygen [O-2 ((1)Delta(g))]. In this study, two cis- and trans-tryptophan hydroperoxide (WOOH) isomers were completely characterized by HPLC/mass spectrometry and NMR analyses as the major W-oxidation photoproducts. These photoproducts underwent thermal decay into the corresponding alcohols. Additionally, WOOHs were shown to decompose under heating or basification, leading to the formation of N-formylkynurenine (FMK). Using O-18-labeled hydroperoxides ((WOOH)-O-18-O-18), it was possible to confirm the formation of two oxygen-labeled FMK molecules derived from (WOOH)-O-18-O-18 decomposition. This result demonstrates that both oxygen atoms in FMK are derived from the hydroperoxide group. In addition, these reactions are chemiluminescent (CL), indicating a dioxetane cleavage pathway. This mechanism was confirmed since the CL spectrum of the WOOH decomposition matched the FMK fluorescence spectrum, unequivocally identifying FMK as the emitting species.

Preferential location of lidocaine and etidocaine in lecithin bilayers as determined by EPR, fluorescence and H-2 NMR

We have examined the effect of the uncharged species of lidocaine (LDC) and etidocaine (EDC) on the acyl chain moiety of egg phosphatidylcholine liposomes. Changes in membrane organization caused by both anesthetics were detected through the use of EPR spin labels (5, 7 and 12 doxyl stearic acid methyl ester) or fluorescence probes (4, 6, 10, 16 pyrene-fatty acids). The disturbance caused by the LA was greater when the probes were inserted in more external positions of the acyl chain and decreased towards the hydrophobic core of the membrane. The results indicate a preferential insertion of LDC at the polar interface of the bilayer and in the first half of the acyl chain, for EDC. Additionally, 2 H NMR spectra of multilamellar liposomes composed by acyl chain-perdeutero DMPC and EPC (1:4 mol%) allowed the determination of the segmental order (S-mol) and dynamics (T-1) of the acyl chain region. In accordance to the fluorescence and EPR results, changes in molecular orientation and dynamics are more prominent if the LA preferential location is more superficial, as for LDC while EDC seems to organize the acyl chain region between carbons 2-8, which is indicative of its positioning. We propose that the preferential location of LDC and EDC inside the bilayers creates a ""transient site"", which is related to the anesthetic potency since it could modulate the access of these molecules to their binding site(s) in the voltage-gated sodium channel. (C) 2007 Elsevier B.V. All rights reserved.

Synthesis, spectral studies and X-ray crystal structure of N,N `-(+/-)-trans-1,2-cyclohexylenebis(3-ethoxysalicylideneamine) H-2(t-3-EtOsalchxn)

The synthesis, spectra and X-ray crystal structure of N,N`-(+/-)-trans-1,2-cyclohexylenebis(3-ethoxysalicylideneamine) H-2(t-3-EtOsalchxn), a salen-type ligand, are reported. The Schiff base was characterized by elemental analysis, m.p., IR, electronic spectra, H-1 and C-13 NMR spectra. The spectra are discussed and compared with those of N,N`-(+/-)-trans-1,2-cyclohexylenebis(salicylideneamine), H-2(t-salchxn). The electronic and IR spectra were also resolved by deconvolution. The influence of the ethoxy group on the IR, electronic spectrum, H-1 and C-13 NMR spectra is discussed. Strong intramolecular forces are present as supported by the IR and H-1 NMR spectra and the X-ray crystal structure. An intermolecular hydrogen bond is observed and appears twice in a pair of molecules in the unit cell. (c) 2007 Elsevier B.V. All rights reserved.

An insight into the beginning stage of the aqueous ring opening metathesis polymerization of exo,exo-7-oxabicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid with Ru(III) compounds: the consumption of Ru(III) species in the presence of carboxylic acids monitored by ESR spectroscopy

The presence of paramagnetic species in the aqueous ring opening metathesis polymerizations of the exo,exo-7-oxabicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid monomer with RuCl(3) and K(2)[RuCl(5)H(2)O] compounds was studied using ESR techniques. It was observed that the intensities of the Ru(III) signals in the ESR spectra decrease on the time scale of the induction period so that the ROMP can take place. The intensity of the Ru(III) signal almost disappeared 50 min after reacting with K(2)[RuCl(5)H(2)O] and after 100 mm in the case of RuCl(3). Reactions of the cis-[Ru(NH(3))(4)(H(2)O)(2)](tfms)(3) and [Ru(NH(3))(5)H(2)O](tfms)(3) complexes with the monomer and different organic compounds representing the organic functions in the monomer (furan, norbornene, but-2-ene-1,4-diol and formic, acetic, oxalic and maleic acids) were also monitored by ESR and UV/vis spectra. It was deduced that the organic acids provide the disappearance of the Ru(III) signal. The proton NMR relaxation times of the residual water in D(2)O for reactions with oxalic acid suggested that the presence of paramagnetic ions in the solution decreases along with

Synthesis and structure of tris(triphenyltin)benzene-1,3,5-tricarboxylate and its tri-DMSO adduct

The molecular structure of (Ph₃SnO₂C)₃C₆H₃ reveals distorted tetrahedral C₃O geometries for each tin atom. No intermolecular Sn...O associations occur between the molecules so that the trinuclear molecule may be considered monomeric. An increase in coordination number at tin is indicated by ¹¹⁹Sn NMR measurements conducted in d₆-DMSO solution and this has been confirmed by a structure analysis of (DMSO·Ph₃SnO₂C)₃C₆H₃·2 DMSO. In this structure, the tin atom geometries are distorted trigonal bipyramidal, existing within trans-C₃O₂ donor sets. Thus, while is it possible for the tin atoms to increase their coordination numbers in (Ph₃SnO₂C)₃C₆H₃, molecular aggregation to form polymeric arrays is precluded, most likely due to steric constraints.

Secondary bonding in para-substituted diphenyltellurium dichlorides (p-XC6H4)2TeCl2 (X = H, Me, MeO) probed by 125Te MAS NMR spectroscopy. Crystal and molecular structure of (p-MeC6H4)2TeCl2

The solid-state structures of the previously known para-substituted diphenyltellurium dichlorides, (p-XC₆H₄)₂TeCl₂ (X=H (1), Me (2), MeO (3)) were investigated by ¹²⁵Te MAS NMR spectroscopy and in case of 2 by single crystal X-ray diffraction. The ¹²⁵Te-NMR shielding anisotropy (SA) was studied by tensor analyses based on relative intensities of the observed spinning sidebands. Solid-state NMR parameters, namely the isotropic chemical shift (δ_iso), anisotropy (ζ) and asymmetry (η), were discussed in relation to the molecular structures established by X-ray crystallography. The asymmetry (η) was found to be particularly sensitive to structural differences stemming mostly from the diverse secondary Te...Cl interactions, but no correlation with geometric parameters could be established.

Studies on the mechanism of the peroxyoxalate chemiluminescence reaction Part 1. Confirmation of 1,2-dioxetanedione as an intermediate using 13C nuclear magnetic resonance spectroscopy

A simple model peroxyoxalate chemiluminescence system was monitored directly across a range of temperatures (from −80 to +20 °C) using ¹³C nuclear magnetic resonance spectroscopy. These experiments were made possible by the utilisation of ¹³C doubly labelled oxalyl chloride, which was reacted with anhydrous hydrogen peroxide in dry tetrahydrofuran. Ab initio quantum calculations were also performed to estimate the ¹³C nuclear magnetic resonance (NMR) shift of the most commonly postulated key intermediate 1,2-dioxetanedione and this data, in concert with the spectroscopic evidence, confirmed its presence during the reaction.

Oligomethylene-bridged dinuclear triorganotin triflates and diphenylphosphinates. Ion pairing in the solid state and electrolytic dissociation in solution of [Ph2Sn(CH2)nSnPh2X](O3SCF3) (X = OH, O2PPh2; n = 1-3)

The condensation of [Ph₂(OH)Sn(CH₂)_nSn(OH)Ph₂] (1-3; n = 1-3) with HO₃SCF₃ and HO₂PPh₂ provided [Ph₂Sn(CH₂)_nSnPh₂(OH)](O₃SCF₃) (4-6; n = 1-3) and [Ph₂(O₂PPh₂)Sn(CH₂)_nSn(O₂PPh₂)Ph₂] (10-12; n = 1-3), respectively. The reaction of [Ph₂Sn(CH₂)_nSnPh₂(OH)](O₃SCF₃) (4-6; n = 1-3) with HO₂PPh₂ and NaO₂PPh₂ gave rise to the formation of [Ph₂Sn(CH₂)_nSnPh₂(O₂PPh₂)](O₃SCF₃) (7-9; n = 1-3) and [Ph₂(OH)Sn(CH₂)_nSn(O₂PPh₂)Ph₂] (13-15; _n = 1-3), respectively. In the solid state, compounds 4-9 comprise ion pairs of cationic cyclo-[Ph₂SnCH₂SnPh₂(OH)]₂²⁺, cyclo-[Ph₂Sn(CH₂)_nSnPh₂(OH)]⁺ (n = 2, 3), and cyclo-[Ph₂Sn(CH₂)_nSnPh₂(O₂PPh₂)]⁺ (n = 1-3) and triflate anions. In MeCN, the eight-membered-ring system cyclo-[Ph₂SnCH₂SnPh₂(OH)]₂²⁺ appears to be in equilibrium with the four-membered-ring system cyclo-[Ph₂SnCH₂SnPh₂(OH)]⁺. In contrast, compounds 10-15 show no ionic character. Compounds 1-15 were characterized by multinuclear NMR spectroscopy in solution and in the solid state, IR spectroscopy, conductivity measurements, electrospray mass spectrometry, osmometric molecular weight determinations, and X-ray crystallography (4, 5, 7, and 12).

4-Amino-1,8-naphthalimide-based anion receptors: employing the naphthalimide N-H moiety in the cooperative binding of dihydrogenphosphate

The 4-amino-1,8-naphthalimide-based anion receptor 3 binds dihydrogenphosphate with 1:1 stoichiometry through cooperative hydrogen bonding to a naphthalimide N–H and thiourea N–H groups. This was clearly established from ¹H NMR titration experiments in DMSO-d₆ where a substantial shift in the resonance for the naphthalimide N–H was observed concomitant with the expected thiourea N–H chemical shift migration upon successive additions of H₂PO₄⁻. However, whilst ¹H NMR titration experiments indicate that 3 was capable of binding other anions such as acetate, the naphthalimide N–H does not participate and the N–H resonance was essentially invariant during the titration. The lack of cooperative binding in this instance was justifiable on steric grounds.

Fluorescent anion sensors based on 4-amino-1,8-naphthalimide that employ the 4-amino N-H

The new charge neutral 4-amino-1,8-naphthalimide based anion sensors 2 and 3 bind to both acetate and dihydrogenphosphate with 1:1 stoichiometry through hydrogen bonding to both thiourea N–H atoms and in the case of dihydrogenphosphate, the naphthalimide 4 amino N–H group as well. This was clearly established from ¹H NMR titration experiments with H₂PO₄^- in DMSO-d₆ where a substantial shift in the resonance for the naphthalimide N–H was observed concomitant with the expected migration of the thiourea N–H chemical shifts. The binding constants determined from the titration studies indicate that the new sensors bind H₂PO₄^- more strongly than AcO⁻. Fluorescence titrations with sensor 3 indicate quenching of 59% and 36% upon addition of acetate and dihydrogenphosphate, respectively.

New insights in asymmetric tetraorganodistannoxane ladder formation. A NMR-spectroscopic and crystallographic study

The syntheses of the asymmetrically substituted tetraorganodistannoxanes [t-Bu₂(X)SnOSn(Y)(CH₂SiMe₃)₂1₂ (1, X = Y = OH; 2, X = Cl, Y = OH; 3, X = Y = Cl) are reported and their structures in solution and in the solid state are characterized by multinuclear NMR spectroscopy and single crystal X-ray analyses. In toluene, the tetrahydroxy-substituted derivative 1 is in equilibrium with the organotin oxides cyclo-[t-Bu₂Sn{OSn(CH₂SiMe₃)₂}₂O] (4), cyclo[(Me₃SiCH₂)₂Sn(OSnt-Bu₂)₂O] (5), and cyclo-(t-Bu₂SnO)₃, and some additional, undefined species containing pentacoordinated tin atoms. In contrast, the dihydroxydichloro-substituted derivative 2 is inert in solution.

Prevalence of the thioamide {···H-N-C=S}2 synthon-solid-state (X-ray crystallography), solution (NMR) and gas-phase (theoretical) structures of O-methyl-N-aryl-thiocarbamides

Structural investigations, i.e. solid-state (X-ray), solution (¹H NMR) and gas-phase (theoretical), on molecules with the general formula MeOC(S)N(H)C₆H₄-4-Y: Y = H (1), NO₂ (2), C(O)Me (3), Cl (4) have shown a general preference for the adoption of an E-conformation about the central C–N bond. Such a conformation allows for the formation of a dimeric hydrogen-bonded {H–N–C=S}₂ synthon as the building block. In the cases of 1–3, additional C–H^...O interactions give rise to the formation of tapes of varying topology. A theoretical analysis shows that the preference for the E-conformation is about the same as the crystal packing stabilisation energy and consistent with this, the compound with Y = C(O)OMe, (5), adopts a Z-conformation in the solid-state that facilitates the formation of N–H^...O, C–H^...O and C–H^...S interactions, leading to a layer structure. Global crystal packing considerations are shown to be imperative in dictating the conformational form of molecules 1–5.

1-Naphthyl- and mesityltellurium(IV,II) derivatives of small bite chelating organic ligands: effect of steric bulk and intramolecular secondary bonding interaction on molecular geometry and supramolecular association

The synthesis and characterization of unsymmetric diorganotellurium compounds containing a sterically demanding I-naphthyl or
mesitylligand and a small bite chelating organic ligand capable of 1,4-Te···N(O) intramolecular interaction is described. The reaction
of ArTeCl₃ (Ar = I-C_lOH₇, Np; 2,4,6-Me₃C₆H_2' Mes) with (SB)HgCI [SB = the Schiff base, 2-(4,4'-N0₂C₆H₄CH=NC₆H₃-Me)] or a methyl ketone (RCOCH₃) afforded the corresponding dichlorides (SB)ArTeCI₂ (Ar = Np, 1Aa; Mes, 1Ba) or (RCOCH₂)ArTeCl₂ (Ar = Np; R = Ph (2Aa), Me (3Aa), Np (4Aa); Ar = Mes, R = Ph (2Ba)). Reduction of 1Aa and 1Ba by Na₂S₂0₅ readily gave the tellurides (SB)ArTe (Ar = Np (1A), Mes, (1B) but that of dichlorides derived from methylketones was complicated due to partial decomposition to tellurium powder and diarylditelluride (Ar₂Te₂), resulting in poor yields of the corresponding tellurides 2A, 2B and 3A. Oxidation of the isolated tellurides with S0₂Cl₂, Br₂ and I₂ yielded the corresponding dihalides. All the synthesized compounds have been characterized with the help of IR, ¹H, ^l3C, and ¹²⁵Te NMR and in the case of 2Aa, and 2Ba by X-ray crystallography. Appearance of only one ¹²⁵Te signal indicated that the unsymmetric derivatives were stable to disproportionation to symmetric species. Intramolecular 1,4-Te· . ·0 secondary bonding interactions (SBIs) are exhibited in the crystal structures of both the tellurium(IV) dichlorides, 2Aa, and 2Ba. Steric repulsion of the mesityl group in the latter dominates over lone pair-bond pair repulsion, resulting in significant widening of the equatorial C-Te-C angle. This appears to be responsible for the lack of Te· . ·CI involved supramolecular associations in the crystal structure of 2Ba.

Structural characterization of rare intramolecularly (1,4-Te· · ·N) bonded diorganotellurides and their monomeric complexes with mercury(II) halides: metal assisted C-H· · ·X (Hg) interactions leading to supramolecular architecture

Monomeric tellurides 4-RC₆H₄(SB)Te [SB = 2-(4,4'-N0₂C₆H₄CH=NC₆H₃-Me); R = H, 1a; Me,1b; OMe, 1c], which incidentally represent the first example of a telluride with 1,4-Te···N intramolecular interaction, have been prepared and characterized by solution and solid-state ¹²⁵Te NMR, ¹³C NMR and X-ray crystallography. Interplay of weak C-H···O and C-H-··π{ interactions in the crystal lattice of 1b and1c are responsible for the formation of supramolecular motifs. These tellurides undergo expected oxidative addition reactions with halogens and interhalogens and also interact coordinatively with mercury(II) halides to give 1:2 complexes, HgX₂[4-RC₆H₄(SB)Te]₂ (X = CI, R = H, 2a; Me, 2b; OMe, 2c and X = Br, R = H, 3a; Me, 3b; and OMe, 3c) with no sign of Te-C bond cleavage, as has been reported for some 1,5-Te·· ·N(O) intramolecularly bonded tellurides. The complexes 2a and 3c are the first structurally characterized monomeric 1:2 adducts of mercury(II) halides with Te ligands. The 1,4-Te···N intramolecular interactions in the solid-state are retained in the complexes highlighting simultaneously the Lewis acid and base character of the Te(lI) atom. Packing of molecules in the crystal lattice of 2a
and 3c reveals that non-covalent C-H· . ·Cl/Br interactions involving metal-bound halogen atoms possess significant directionality and in
combination with coordinative covalent interactions may be of potential use in creating inorganic supramolecular synthons.

Immunostimulatory polysaccharides from Chlorella pyrenoidosa. A new galactofuranan. Measurement of molecular weight and molecular weight dispersion by DOSY NMR.

Fractionation of the hot water extract of Chlorella pyrenoidosa was performed using a combination of ethanol precipitation, size exclusion chromatography, and anion exchange chromatography. One fraction contained a new polysaccharide, and this compound was shown to be a 1→2-linked β-d-galactofuranan from its 1D and 2D 1H and 13C NMR spectra, with a molecular weight of 15 kDa from DOSY NMR measurements. A number of other fractions were shown to have the same repeating unit as the previously identified arabinogalactan. However, arabinogalactans from different fractions were shown by DOSY NMR to have different molecular weights, which ranged from 27 to 1020 kDa. Agreement with molecular weights measured for some of these fractions by SEC-MALS was very good, further confirming the relationship established by Viel et al. between molecular weights of neutral polysaccharides and self-diffusion coefficients. The smaller molecular weight polysaccharides, the galactofuranan and the 27 and 50 kDa arabinogalactans, were shown to be close to monodisperse by analysis of the distributions of the self-diffusion coefficients for the polymers. The larger arabinogalactans had considerable variation in their molecular weights (188 ± 109 kDa and 1020 ± 370 kDa). Only the two larger arabinogalactans showed immunostimulatory activity.