Org. Geochem. Vol. 9, No. 3, pp. 117 125, 1986 0146-6380/86 $3.00 + 0.00 Printed in Greal Britain. All rights reserved Copyright (~:;)1986 Pergamon Press Ltd Characterization of melanoidins by IR spectroscopyml. Galactose-glycine melanoidins Y. RUBINSZTAIN, j S. YARIV,2 P. IOSELIS, 1 Z. AIZENSHTAT j and R. IKAN I ~Department of Organic Chemistry and 2Department of Inorganic Chemistry, Hebrew University of Jerusalem, Givat-Ram, Jerusalem, Israel (Received 18 June 1985; accepted 12 November 1985) 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: l). 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 mela- noidin of type 2; no reaction was observed with glycine. Key words: infrared spectroscopy, melanoidins, galactose, glycine, Maillard reaction INTRODUCTION It is known that humic substances account for much of the organic matter that occurs in soils, recent sediments and natural waters. Several pathways have been proposed for the formation of humie substances in various environments; (Waksman, 1932; Flaig, 1964; Maillard, 1912, 1913; Yariv and Cross, 1979). One of them, namely, the melanoidin theory, suggests that an interaction of reducing sugars and amino acids (formed as by-products of metabolism of micro- organisms) leads to their non-enzymatic poly- merization to brown, nitrogenous products similar to those produced by dehydration of foods (Hodge, 1953). A number of investigators (Hoering, 1973; Nissenbaum and Kaplan, 1972) have suggested that water humus is formed by the Maillard reaction. These condensation products of sugars and amino acids might have served as potential precursors of coals, kerogens and petroleum (Stevenson, 1982). Several chemical, pyrolytic, spectroscopic and chro- matographic techniques were used in the elucidation of the molecular structure of humic substances and melanoidins (Schnitzer and Khan, 1972; Ishiwatari, 1970: Ioselis, 1984; Stevenson, 1982; Rubinsztain et al., 1984). Among the spectroscopic techniques infrared spectroscopy (IR) provided valuable infor- mation on the nature and arrangement of functional groups in humic acids and melanoidins. (Stevenson and Goh, 1971; Hoering, 1973; Hedges, 1978; Bobbio et al., 1981; Ertel and Hedges, 1983). In order to 117 0(59:3 H correlate the structural features of melanoidins ob- tained by various analytical methods (Rubinsztain et al., 1984) an extensive study of the infrared spectra was performed. In the present paper we wish to report the IR spectra of melanoidins prepared from various ratios of galactose and glycine. In the forthcoming parts II and Ill of this series we will present the characteristic features of the infrared spectra of a number of other melanoidins which were prepared by Rubinsztain et al. (1984) (part If) and a comparative infrared study of natural humic sub- stances (loselis, 1984) and melanoidins (part IIl). EXPERIMENTAL Synthesis ~)[ melanoidins and pseudomelanoidins Three different melanoidins (samples II, lII and IV) were prepared by condensation of various molec- ular ratios ofglycine and galactose (1:9; 1: 1; 9: I) in hot alkaline solution (Na2CO 3 or KzCO0 (100 C) for 170 hr. (Hedges, 1978; Ioselis et al., 1983). The crude products were filtered through a Whatman No. 42 filter paper and purified by continuous dialysis fol- lowed by lyophilization. For comparative purposes a "pseudomelanoidin" (sample I) was prepared from galactose only under the above described conditions. Sample I I (100 mg) was dissolved in distilled water (100 ml) and treated with aqueous (1%) solution of cupric chloride. The precipitate was collected on a filter paper and its infrared spectrum was recorded.