NMR studies of complex carbon-containing materials: Maillard reaction products, soil, nanodiamond, and carbon modified TiO₂

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2008-01-01
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Fang, Xiaowen
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Klaus Schmidt-Rohr
Mei Hong
Victor Lin
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Chemistry

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Advanced solid-state magic angle spinning NMR (SS-MAS NMR) techniques have been applied to study the structures of carbon-containing complex materials, including 13C/15N labeled Maillard reaction products (melanoidins), soil and plant materials, detonation nanodiamond, and glucose-13C modified TiO2 photocatalyst.;Melanoidins, complex high molecular weight mixtures, are formed via the Maillard reactions between sugars and amino acids during heating. Due to their tremendous heterogeneity, the structure of melanoidins has remained poorly characterized. 13C and 15N labeling of the reactants has been combined with various solid-state NMR techniques to elucidate the structures of Maillard reaction products.;(i) The fate of glycine in the model Maillard reaction between 13C/15N-labeled glycine (13C1, 13C2, 13C1,2, 13C2-15N, 15N) and glucose in a 1:1 molar ratio in both dry and solution reaction conditions has been investigated. More than ∼70% of glycine carbon remains in the products, and there is no detectable difference between C1 and C2 of glycine in terms of their individual mass loss. Most glycine C1-C2 and C2-N bonds stay unbroken indicating that the Strecker degradation of amino acids, where the C1 is split off as CO2, is not a major reaction pathway. The C1 of glycine predominantly remains a COO moiety. More than 60% of C2-N bond remains connected while the remaining 40% glycine C2 forms new bonds to either carbon or oxygen of sugar. Novel 13C-detected 15N{lcub}1H{rcub} dipolar dephasing experiments show that the majority of the nitrogen in the melanoidins is not protonated for both dry and solution reactions, implying in particular that nonprotonated N may be a characteristic marker of Maillard reaction products.;(ii) The major structural units of melanoidins have been explored in detail with the products made from glucose of distinct 13C labeling site reacting with 15N-glycine. Quantitative 13C spectra show that alkyl units make up ∼48% of carbon in melanoidins; meanwhile, the spectra also show striking differences between the structures formed by different carbons (C1 through C6) of glucose, strongly suggesting that specific structures are formed during Maillard reaction in dry-reaction condition. The reactivity of the glucose-C1 stands out: more than half of C1 forms additional C-C bonds, a much larger fraction than for the other sites. The C2 carbons are all bonded to heteroatoms and mostly not protonated, while the C3 is predominantly protonated and has a significant fraction of the sites not bonded to heteroatoms as in CCH2 form. Only C4 and C5 remain significantly in alkyl OCH sites. C6 undergoes the least transformation, with most carbons in OCH2; also, 1/5 of C6 is lost from the sample. C1 and C6 form the CH3 end groups in similar quantities (2% of all C). All sites are characterized in more detail by spectral editing.;(iii) N of glycine is a very active site, and most of N form new N-C bonds with sugar molecules. Several complex nitrogen containing structures have been identified using two- and three-dimensional NMR techniques. A significant fraction of N is found in imidazolium, oxazolium and 4-pyridinyl rings. In addition, the N of glycine forms amides, mostly tertiary (nonprotonated), with C=O carbons from glucose, as well as some secondary and tertiary amines.;In another project, different aggregation fractions of four soil samples from two agriculturally managed fields in Iowa have been studied using quantitative 13C direct polarization (DP)/magic angle spin (MAS) and 13 C-1H cross polarization (CP)/total spinning sidebands suppression (TOSS) NMR techniques combined with elemental analysis results. The three fractions of each soil are unfractionated whole soil, particulate organic matter (POM), and the clay size fraction. HCl/HF treatments with or without heating have been applied to the whole soil and clay size fraction. In the untreated whole soil, esters are found to be a minor component. The only detectable effect of washing e.g. HCl/HF with or without heating is the protonation of COO- to COOH. Heating does not produce significant structural changes. 13C spectra were recorded to monitor the transformation of plant matter to POM and then to humic substances in whole soils and clay fractions. The crystalline cellulose in plants is lost and noncrystalline saccharides in POM and soil are formed. Meanwhile, the soil samples exhibit a pronounced signal of polymethylene chains. Oxidized char-coal components resonating around 130 ppm are major structural units in these grassland soils. (Abstract shortened by UMI.)

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Tue Jan 01 00:00:00 UTC 2008