Y. Rubinsztain

Investigations on the structural units of melanoidins

A series of sugar-amino acid derived melanoidins were synthesized and their chemical and physical properties were evaluated using spectral, chemical and thermal analyses. It is suggested that the melanoidin “back-bone” is probably a sugar-moiety polymer in which various nitrogen containing groups are grafted. Some resemblance between marine humics and synthetic melanoidins was found.

Carbon-13 cross polarized magic-angle samples spinning nuclear magnetic resonance of melanoidins

Abstract Melanoidins synthesized from amino-acids and sugars in various ratios and humic substances from Hula Basin (Israel) have been investigated by 13 C-CP/MAS NMR spectroscopy. Most spectra indicated a significant similarity between melanoidins and humics. It is suggested that the Maillard reaction plays a more significant role in the formation of humic substances than the “traditional” theory based on a lignin source. Several heterocyclic and heteroaromatic moieties (mainly furanoid, hydroxy-alkyl-furanone and hydroxy-alkyl-cyclopentenone) have been considered as major “building blocks” of humic substances and melanoidins. Aromaticities calculated from NMR spectral data of SM (“sugar” enriched melanoidins) and AAM (amino acid enriched melanoidins) coincide remarkably with the aromaticities of terrestrial and marine humics, respectively.

Thermal evaluation of some model melanoidins by Curie point pyrolysis-mass spectrometry and gas chromatography-mass spectrometry

Abstract Melanoidins of glucose or galactose and the amino acids tyrosine, methionine and lysine were analysed by Curie point pyrolysis-mass spectrometry and pyrolysis-gas chromatography-mass spectrometry. Specific chain characteristics of the reactant amino acids were visible in the pyrolysis data, whereas pyrolysis products of carbohydrates related structural elements in the melanoidins were found to be more sensitive to the nature of the amino acids and the sugar amino acid ratio. The influence of the pyrolysis temperature was tested on galactose-lysine melanoidins by comparison of pyrolysis-mass spectra obtained at 510, 610 and 770°C using multivariate data analysis techniques. The effect of the end temperature on the spectra of this melanoidin was found to be small.

Characterization of melanoidins by IR spectroscopy—I. Galactose-glycine melanoidins

Abstract A comparative IR spectral study was conducted with synthetic melanoidins which were prepared from various ratios of sugars and amino acids and a pseudomelanoidin which was prepared from sugar only. A significant resemblance was observed between the pseudomelanoidin and the melanoidin prepared from a high (9:1) sugar ratio (type 1). Another series of melanoidins (type 2) consists of those prepared from an “excess” of amino acid (9:1 and 1:1). Type 1 showed the presence of carbonyl and hydroxyl groups as well as aromatic vibrations. The same groups were detected in the spectrum of the pseudomelanoidin. These features were observed in a very low concentration in the type 2 melanoidins. Carboxylic groups are present in pseudomelanoidin and in both types of the melanoidins. However, the acid strength of this group is high in melanoidin of type 2, but low in type 1 and in the pseudomelanoidin. The nitrogen atom which is involved in the melanoidin structure is present as a tertiary amine. Heat treatment of KBr discs containing glycine, galactose and a mixture thereof revealed that galactose was converted into pseudomelanoidin; a mixture of galactose-glycine yielded the corresponding melanoidin of type 2; no reaction was observed with glycine.

Chemical, isotopic, spectroscopic and geochemical aspects of natural and synthetic humic substances

It has been suggested that marine humic and fulvic acids are formed by a condensation of sugars with amino acids or proteins via Maillard reaction. In order to investigate this hypothesis a series of melanoidins were synthesised under laboratory conditions and their structure compared with natural humic acids. An extensive collaborative research programme on the chemical and physical properties of natural and synthetic humic substances has been undertaken using spectroscopic (IR, UV, 13C-CP/MAS NMR, ESR), chromatographic (GC and GC/MS), thermogravimetric (TG, DTG, TG-EGD-MS), pyrolytic (stepwise, Rock Eval, Curie point) oxidative (KMnO4) and isotopic methods. The results indicate that various heterocyclic moieties are the main building blocks of humic substances rather than aromatic benzenoid structures. It seems possible that significant portions of humics of Lake Haruna (Japan) as well as of Lake Hula (Israel) are composed of melanoidins and/or pseudomelanoidins. The extent of this contribution has not yet been determined.

Thermal characterization of natural and synthetic humic substances

Abstract Thermogravimetric technique was used for the characterization of natural (humic) and synthetic (melanoidins) substances. The influence of pH on the thermal stability of humic substances was studied. A similarity in thermal behaviour of natural humic substances and of melanoidins (prepared from an excess of sugar) and the unique thermal properties of melanoidins (prepared from basic amino acids) was observed. Thermal behavior of natural and synthetic substances was compared with model compounds of sugar, peptide and kerogen types.

Characterization of melanoidins by IR spectroscopy—II. Melanoidins of galactose with arginine, isoleucine, lysine and valine

Abstract The infrared spectra of melanoidins which were prepared from galactose and neutral and basic amino acids (at various ratios) support in general the conclusion gained in Part I of this series. A striking resemblance in the IR spectra was obtained in melanoidins which were prepared from a high ratio of sugar to amino acids (Type 1). The imprint of the amino acids was observed only in type 2 melanoidins, which are enriched with amino acids.

Light hydrocarbons and volatile compounds produced during the thermal treatment of melanoidins and humic substances

Abstract Pyrolysis of humic substances and melanoidins in closed systems (autoclave and sealed ampoules) yielded a series of light hydrocarbons and volatile products. The yield of C 1 –C 4 hydrocarbons obtained by pyrolysis of certain melanoidins was considerably higher than those obtained from humic substances. Pyrolysis of both, humic substances and melanoidins, produced sugar-deprived products, mainly furan derivatives. In general, the pyrolysis of melanoidins produced more oxygen-containing compounds than hydrocarbons, and only in the cases where the initial ratio of amino acids used in the preparation of melanoidins was dominant, an appreciable amount of hydrocarbons was formed.

Pyrolysis of natural and synthetic humic substances

Thermogravimetric and Rock-Eval techniques were used for the characterization of natural (humic) and synthetic (melanoidins) substances and their hydrocarbon generation potential. A similarity between the thermal behaviour of humic substances and of melanoidins (prepared from an excess of sugar) and the unique thermal properties of melanoidins (prepared from basic amino acids) was observed. Rock-Eval analysis indicated that most synthetic melanoidins (also clay-complexed) generated more hydrocarbons and related compounds than terrestrial humic substances.ZusammenfassungThermogravimetrische und Rock-Eval-Techniken wurden zur Charakterisierung von Natur-(Humin-) und synthetischen (Melanoidin-) Substanzen und deren Fähigkeit zur Kohlenwasserstoffgenerierung benutzt. Es wurde eine Ähnlichkeit im thermischen Verhalten von Huminsubstanzen und (aus überschüssigem Zucker hergestellten) Melanoidinen und den thermischen Eigenschaften von (aus den entsprechenden Aminosäuren hergestellten) Melanoidinen beobachtet. Eine Rock-Eval Analyse erwies, daß die meisten synthetischen Melanoidine (auch als Tonerde-Komplex) mehr Kohlenwasserstoffe und ähnliche Stoffe generieren als natürliche Huminsubstanzen.

Long-living free radicals study of stepwise pyrolyzed melanoidins and humic substances

Abstract ESR measurements of stepwise-pyrolyzed melanoidins and humic substances (at various temperatures, mesh size, and pH values) furnished the following information: the melanoidin structure stabilizes the long-living free radicals in a manner similar to humic substances; the g and Ng values of melanoidins are similar to those of the humic substances, the cleavage of CC and CX (X = heteroatom) bonds increases the Ng value. Thermogravimetric curves, weight loss by stepwise pyrolysis, and 13C-CP/MAS NMR were found to be in good correlation with ESR data regarding the structural features of melanoidins and humic substances.