Spectra Of Model Compounds Research Articles

Changes of tyrosine and tryptophan residues in human hemoglobin by oxygen binding: near- and far-UV circular dichroism of isolated chains and recombined hemoglobin

In order to assign the circular dichroism (CD) spectral change in the region between 280 and 300 nm of human adult hemoglobin (Hb A) upon the quaternary structure transition induced by oxygen binding, the near- and far-UV CD spectra of the isolated chains and the recombined hemoglobin were examined. Deoxygenation made the negative CD band at 290 nm of oxy-α chain deeper. On the other hand, positive CD bands of oxy-β chain at the 280∼300 nm became negative upon deoxygenation. These changes were interpreted as being due to environmental alterations of tyrosine (Tyr) and/or tryptophan (Trp) perturbed by tertiary structural changes from the oxy to deoxy form in isolated chains, referring to the CD spectra of model compounds. From the difference between CD bands of the arithmetic mean of deoxy isolated chains and the CD band of deoxyHb tetramer, the contribution of tertiary structural change to the negative CD band of deoxyHb A at 287 nm was estimated to be 50%. This finding has revealed that the net contribution of quaternary structure transition to the negative band is 50%. In far-UV CD spectra, the environmental changes of aromatic residues upon the quaternary structure transition were also detected as a negative band at 225 nm.

Nondestructive Speciation of Solid Mixtures by Multivariate Calibration of X-Ray Absorption Near-Edge Structure Using Artificial Neural Networks and Partial Least-Squares

Two multivariate calibration methods, artificial neural networks (ANN) and partial least-squares (PLS), have been applied to the quantitative determination of iron species in solid mixtures by X-ray absorption near-edge structure (XANES). XANES spectra were successfully resolved by both methods, and the iron compounds in solid mixtures were quantified, even though the spectra of different compounds showed serious overlap. When iron compounds that were not contained in the model mixtures were subjected to the calibration model, ANN recognized the patterns of their XANES spectra as the nearest spectra of model compounds in shape, and gave more robust results than PLS. The self-absorption effect on the calculated values from XANES measured in the fluorescence mode was examined by comparing with transmission mode; it turned out that a spectral distortion by a self-absorption effect is irrelevant to the prediction performance of these multivariate calibration methods. The present study demonstrated that ANN and PLS are applicable to the chemical speciation of elements by XANES measured in the fluorescence mode.

Formation of Layered Single- and Double-Metal Hydroxide Precipitates at the Mineral/Water Interface: A Multiple-Scattering XAFS Analysis

Spectroscopic and microscopic studies have shown that Ni and Co sorption by clay minerals may proceed via formation of surface precipitates. Several studies employing X-ray absorption fine structure (XAFS) spectroscopy suggested the formation of turbostratic, α-type metal hydroxides, of layered double hydroxides (LDH) with Al-for-metal substitution, and of 1:1 or 2:1 phyllosilicates. Distinction of these phases is difficult because they have low crystallinity and/or a small mass compared to the sorbents, and because they have similar metal–metal distances in their hydroxide layers/sheets. Distinction of these phases is crucial, however, because they have substantially differing solubilities. In this paper we show that an XAFS beat pattern at about 8 Å−1 can be used as a fingerprint to unequivocally distinguish LDH from the α-type hydroxides and phyllosilicates. Full multiple-scattering simulations and experimental spectra of model compounds indicate that the beat pattern is due to focused multiple scattering at Me/Al ratios between 1 and 4 (Me=Ni, Co). By applying the fingerprint method to new and to already published XAFS data on Ni and Co surface precipitates, we found that LDH preferentially forms in the presence of the Al-containing sorbents pyrophyllite, illite, kaolinite, gibbsite, and alumina above pH 7.0. However, α-type metal hydroxides form in the presence of the Al-free sorbents talc, silica, and rutile, and in the presence of the Al-containing clay minerals montmorillonite and vermiculite. We believe that the high permanent charge of these latter minerals prevents or retards the release of Al. When Al is available, the formation of LDH seems to be thermodynamically and/or kinetically favored over the formation of α-type hydroxides.

Raman and site-selective fluorescence spectra of model compounds for interchain interactions in poly(phenylenevinylene)

Nonresonant Raman spectra and site-selective fluorescence spectra (polyethylene matrix, 1.5 K) are reported for 1,4-distyrylbenzene, its tert-butyl and paracyclophane substituted derivatives, and the paracyclophane derivative bearing two distyrylbenzene chains in a pseudo- para orientation, a simple model for the interchain interactions in poly(phenylene-vinylene) films. The first three compounds show pronounced fluorescence line-narrowing when excited near the red edge of their absorption spectra. The bichromophoric compound exhibits no narrowing although its Raman spectrum, room-temperature fluorescence spectrum, and lifetime are similar to those of the other three molecules. This may reflect increased low-frequency Franck–Condon activity in the electronically delocalized bichromophoric system.

99/03018 Catalytic reforming method of hydrocarbons and coking inhibition

High resolution (≤ 0.2 eV) S L-edge X-ray absorption near edge structure (XANES) spectra of several inorganic and organic sulphur model compounds are presented. The spectra were recorded in both total electron yield (TEY) and fluorescence yield (FY) modes. A qualitative interpretation is used to explain the origin of the fine features in the XANES spectra of certain model compounds. The XANES spectra of several coals such as the Rasa coal (Cretaceous, Yugoslavia), P-803 coal (Cretaceous, Canada) and fossilized Latex (Eocene, Germany) have also been recorded. Using the spectra of model compounds as fingerprints, several organic functional groups were identified in untreated coal. These include alkyl and aryl sulphides, alkyl and aryl disulphides, and heterocyclic sulphurs. Sulphonic acid and sulphate were observed for aerially oxidized coal. The S K-edge of several model compounds and coals were also recorded to compare with the L-edge. Advantages and drawbacks of the L-edge and K-edge techniques are described.

Photolysis of bisphenol-based polyurethanes in solution

Photolysis of bisphenol-based polyurethanes, using bisphenols A, S, and AF with methylene diphenyl diisocyanate (MDI), has been shown by the observed changes in fluorescence spectra, reduced viscosity, and UV absorbance. Analysis of the fluorescence spectra of model compounds and polymers showed that para-photo-Fries and cleavage-type products were the major components formed during photolysis. The reduced viscosity and UV absorbance changes are consistent with a two-step photodegradation process. The reduced viscosity changes indicate that oxygen inhibition on the cleavage process is more significant for bisphenol S-based polyurethanes than for bisphenol A- and bisphenol AF-based polyurethanes. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1331–1339, 1999

Photolysis of bisphenol‐based polyurethanes in solution

Photolysis of bisphenol-based polyurethanes, using bisphenols A, S, and AF with methylene diphenyl diisocyanate (MDI), has been shown by the observed changes in fluorescence spectra, reduced viscosity, and UV absorbance. Analysis of the fluorescence spectra of model compounds and polymers showed that para-photo-Fries and cleavage-type products were the major components formed during photolysis. The reduced viscosity and UV absorbance changes are consistent with a two-step photodegradation process. The reduced viscosity changes indicate that oxygen inhibition on the cleavage process is more significant for bisphenol S-based polyurethanes than for bisphenol A- and bisphenol AF-based polyurethanes. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1331–1339, 1999

Quantitative Speciation of Lead in Selected Mine Tailings from Leadville, CO

We have characterized Pb speciation in selected tailings from the Leadville, CO, area using a variety of analytical techniques, including X-ray absorption fine structure (XAFS) spectroscopy. Samples from three locations were analyzed, including two chemically distinct tailings piles located within the city limits [Apache (pyrite-rich, low pH) and Hamms (carbonate-rich, near-neutral pH)] and tailings material deposited as overbank sediments along the Arkansas River approximately 13 km downstream from Leadville (Arkansas River tailings). Extended XAFS (EXAFS) spectra of these multicomponent samples were fit using linear combinations of model compound spectra. In accordance with pH differences among the samples, adsorbed Pb accounts for ∼50% of total Pb (PbT) in fine fractions of the near-neutral pH Hamms tailings, whereas Pb-bearing jarosites account for the majority of PbT in the fine fractions of the low pH Apache and Arkansas River tailings. EXAFS analyses following sequential extraction by MgCl2 and EDTA show evidence of significant redistribution (readsorption) of Pb during the MgCl2 extraction and for removal of adsorbed Pb and dissolution of Pb-carbonates during the EDTA extraction. Changes in Pb speciation with water extraction (dissolution of anglesite and precipitation of plumbojarosite) are observed in one sample of Arkansas River tailings. These molecular-scale results show that Pb speciation varies dramatically among environments in the Leadville area and that Pb occurs in a number of phases not amenable to definitive characterization by conventional microanalytical and/or chemical extraction techniques.

Photophysics of photochromic spiroperimidinecyclohexadienone

Fragment interaction in photochromic spiroperimidinecyclohexadienones I was studied by absorption spectroscopy and semiempirical calculations. Absorption spectra of spirans I can be presented as an approximate superposition of the spectra for compounds III and IV modelling the spiran fragments. The weak, long-wavelength absorption band of I at ca. 24 600 cm −1 is absent in the spectra of model compounds and cannot be attributed to the individual absorption of molecular fragments. The CNDO/S calculation of optical transitions for the spiran structures optimized by AM1 and PM3 has shown that the long-wavelength band is formed by forbidden transition S 0 → S 1 with mixed nπ *-CT nature. All other singlet states observed in the spiran spectrum are of the LE nature. The S 2 and S 4 states are localized on the perimidine fragment and the S 3 state is the ππ * state of the cyclohexadienone fragment. Investigations of all the spirosystems in polar and nonpolar solutions, polymeric films and crystal phase at various temperatures (77–300 K) have shown the absence of fluorescence and phosphorescence for compounds I. This phenomenon is explained by the formation of an intermediate for photoconversion of the spiran structure to the quinonimine one.

Identification of Major Species in Industrial Metal-Refining Solutions with Raman Spectroscopy

Raman and resonance Raman spectroscopies were used to characterize industrial metal-refining solutions containing nitrogen, sulfur, and Ni, Co, Zn, and Cu ammine complex species. To identify the species present in these solutions, we also recorded Raman spectra of model compounds in similar matrices. The results indicate that clearly resolved diagnostic bands in the Raman spectrum can be used to identify and discriminate free ammonia, metal-bound ammonia, sulfate, sulfamate, and metal species in industrially relevant plant solutions. We roughly estimate that Raman can determine cobalt at the 0.3 g Co/L level, nickel at the ∼5 g Ni/L level, and copper at the 2–3 g Cu/L level in these solutions. Also, sulfate and sulfamate can be detected at the 1 g/L level with Raman spectroscopy. Finally, the Raman spectrum can be used to determine bound ammonia with detection limits of about 20 g/L. The detection of free ammonia depends on the concentration ratio of free to bound ammonia because of their overlapping vibrational bands and the weaker intensity of free ammonia Raman scattering.

Proton spectroscopy without water suppression: the oversampled J-resolved experiment.

A method is introduced for obtaining proton spectra in vivo with all the advantages of a full water signal. The method, based on F1 oversampled J-resolved spectroscopy, makes it possible to separate metabolite signals from unwanted baseline artifacts. The dominant water resonance is used as a 2D reference signal for the phase-sensitive reconstruction of the 2D J-resolved metabolite spectra. The powerful specificity of this method is demonstrated with model compound spectra, phantoms, and in vivo examples.

Chain‐coupling reaction of amine‐terminated oligomers by bis(4‐monosubstituted‐5(4 H )oxazolinones)

Bis(5(4H)oxazolinones) derived from naturally occurring α-amino acids were reacted with amine-terminated polyethers and polyamides in the bulk at 175-200°C. Model reactions were also carried out using primary alkylamines. The reactions were studied by means of SEC, and 1 H and 13 C NMR, and the resulting polymers were characterized by DSC and TGA. The chain-coupling reaction is extremely fast and yields high molar mass copolymers containing peptide linkages in less than 5 min. The NMR spectra of model compounds and polymers were fully assigned, showing that the oxazolinone/amine polyaddition reaction proceeds in the expected way, without any noticeable side reaction. The polymers exhibit lower crystallinity, higher T g and a melting temperature close to or lower than that of the starting oligomers.

Fluorescence of the diterpenoid alkaloids lappaconitine andN-deacetyllappaconitine in acetonitrile

The absorption, excitation, and fluorescence spectra of the diterpenoid alkaloids, lappaconitine andN-deacetyllappaconitine, in acetonitrile were studied. On the basis of the coincidence of the spectra with the analogous spectra of model compounds, methyl esters of anthranilic andN-acetylanthranilic acids, the conclusion was drawn that the—OOC(Ph)NRH groups are fluorochromes in the alkaloids studied. The high quantum yield of fluorescence and the bathofluoric shift in the luminescence and absorption spectra ofN-deacetyllappaconitine were explained by an increase in the electron-releasing ability of the —NRH group in the deacetylation of lappaconitine.

Chemical characterization of tribochemical and thermal films generated from neutral and basic ZDDPs using X-ray absorption spectroscopy

X-ray absorption near edge structure (XANES) spectroscopy at the phosphorus L-edge and sulphur L-edge has been used to characterize the chemical nature of tribochemical and thermally generated films from several ZDDP antiwear agents in the neutral and basic forms. Using the P and S L-edge XANES spectra of model compounds with known structure as fingerprints, the chemical structures of P and S species in the films have been identified. P appears in all the films as polyphosphates in different proportions of short and long chain polyphosphates. In some films, polyphosphates are accompanied by unchanged ZDDP. Generally films generated from neutral and basic ZDDPs show similar P and S chemistry (polyphosphates and sulphides) but contain different proportions of unchanged ZDDP. However, the aryl ZDDP films have different polyphosphate structure compared to the alkyl ZDDP films. The sulphur proportion in the tribochemical films is decreased a great deal, but remains in the reduced form. However, S in the thermo-oxidatively generated films, appears both in the reduced and oxidized form, depending on the ZDDP and the temperature.

Sulphur characterization in coal from X-ray absorption near edge spectroscopy

High resolution (≤ 0.2 eV) S L-edge X-ray absorption near edge structure (XANES) spectra of several inorganic and organic sulphur model compounds are presented. The spectra were recorded in both total electron yield (TEY) and fluorescence yield (FY) modes. A qualitative interpretation is used to explain the origin of the fine features in the XANES spectra of certain model compounds. The XANES spectra of several coals such as the Rasa coal (Cretaceous, Yugoslavia), P-803 coal (Cretaceous, Canada) and fossilized Latex (Eocene, Germany) have also been recorded. Using the spectra of model compounds as fingerprints, several organic functional groups were identified in untreated coal. These include alkyl and aryl sulphides, alkyl and aryl disulphides, and heterocyclic sulphurs. Sulphonic acid and sulphate were observed for aerially oxidized coal. The S K-edge of several model compounds and coals were also recorded to compare with the L-edge. Advantages and drawbacks of the L-edge and K-edge techniques are described.

13C NMR of Lignins in Aqueous Alkali. I. Ionisation Effect on the Spectra of Model Compounds and Milled Wood Lignins

Article 13C NMR of Lignins in Aqueous Alkali. I. Ionisation Effect on the Spectra of Model Compounds and Milled Wood Lignins was published on January 1, 1996 in the journal Holzforschung (volume 50, issue 3).

Manganese L-Edge X-ray Absorption Spectroscopy of Manganese Catalase from Lactobacillus plantarum and Mixed Valence Manganese Complexes

The first Mn L-edge absorption spectra of a Mn metalloprotein are presented in this paper. Both reduced and superoxidized Mn catalase have been examined by fluorescence-detected soft X-ray absorption spectroscopy, and their Mn L-edge spectra are dramatically different. The spectrum of reduced Mn(II)Mn(II) catalase has been interpreted by ligand field atomic multiplet calculations and by comparison to model compound spectra. The analysis finds a 10 Dq value of ∼1.1 eV, consistent with coordination by predominately nitrogen and oxygen donor ligands. For interpretation of mixed valence Mn spectra, an empirical simulation procedure based on the addition of homovalent model compound spectra has been developed and was tested on a variety of Mn complexes and superoxidized Mn catalase. This routine was also used to determine the oxidation state composition of the Mn in [Ba8Na2ClMn16(OH)8(CO3)4L8]·53H2O (L = 1,3-diamino-2-hydroxypropane-N,N,N‘,N‘-tetraacetic acid).

Studies in organic mass spectrometry. Part 20: a hidden ortho effect in the electron ionisation mass spectra of some 2′-alkyl substituted 2-and 3-thiophenecarboxanilides

The electron-ionisation-induced amide-bond cleavage of some 2′-methyl- and 2′-ethyl-substituted 2- and 3-thiophenecarbox-anilides, which yields formally anilylium ions having relative intensities apparently in contrast with the Stevenson–Audier rule, has been investigated by mass-analysed ion kinetic energy (MIKE) spectrometry and compared to that of the 3′- and 4′-isomers. It has been shown that, in the case of the 2′-methyl and 2′-ethyl derivatives, the amide-bond cleavage is anchimerically assisted through the hidden migration of a benzyl hydrogen to the nitrogen. Analysis of the MIKE and collision-induced decomposition (CID) MIKE spectra of model compounds indicates that this cryptic ortho effect produces a stable ortho quinoide or aminotropylium structure.

Investigation of germination and aging in Moravian III barley grain by nuclear magnetic resonance

High-resolution, solid-state 1H nuclear magnetic resonance (NMR) techniques are used for the first time to study germination in imbibed Moravian III barley grains. Whereas magic-angle spinning 1H NMR spectra reveal the water and lipid components in barley grains, combined rotation and multiple-pulse spectroscopy techniques provide 1H NMR spectra of grains that reveal the protein and carbohydrate as well as the water and lipid components. Spectra of grains are compared with spectra of model compounds to verify assignments. 1H T1 and T2 measurements using magic-angle spinning only and combined rotation and multiple-pulse spectroscopy techniques provide information about molecular mobility within the grains during inhibition. Some grains were subjected to artificial aging conditions. 1H NMR spectral comparisons are made between normal, viable grains and artificially aged grains.

Electronic structure and spectra of model compound I complexes

Electronic structure and spectra of model compound I complexes