Chapter 21
Pyridoimidazoles, Histidine-Specific Reaction Products
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 23, 2015 | http://pubs.acs.org Publication Date: November 30, 1993 | doi: 10.1021/bk-1994-0543.ch021
U. S. Gi and W. Baltes Institute for Food Chemistry, Technical University of Berlin, Gustav -Meyer-Allee 25, D-1000 Berlin 65, Germany
By reaction of histidine with glucose under the conditions of roasting (at 220°C) and autoclaving (at 120°C, 150°C and 180°C), 2-acetyl -and2-propionyl-pyrido[3,4-d]imidazole were identified, along with the corresponding tetrahydropyrido derivatives. 2-Acetyl-pyrido[3,4d]-imidazole was also formed by heating glucose with tuna fish. Glucose additionally reacts with histidines containing methyl substituents at the 1-, 2- or 3-position to form the corresponding methylated pyrido-imidazoles.
We recently observed the formation of 2-acetyl-pyrido[3,4-d]imidazole III by reaction of histidine with glucose under conditions of roasting or autoclaving foods. (7) The corresponding tetrahydro derivative II was also identified by GC/MS. The structure of III was elucidated by means of IR, M S and ^ - N M R spectra. This compound class represents a new heterocyclic system which, to our knowledge, was not previously reported until now. Experimental Procedure Materials. Reagent grade L-histidine, D-glucose, and pyruvic aldehyde were obtained from commercial sources. Pyruvic aldehyde, diethyl ether, and dichloromethane were freshly distilled before use. Model System Reactions. The following reaction conditions were utilized: 1. Reaction of D-glucose (0.04 mol) with histidine (0.04 mol) in 60 mL of 1.0 M phosphate buffer, pH 5.8. 2. Reaction of pyruvic aldehyde (4 mL) with histamine dihydrochloride (0.02 mol) in 60 mL of 1.0 M phosphate buffer, pH 5.8. Reaction mixtures were individually heated and stirred at 120°, 150°, or 180°C for 1 h in a laboratory autoclave fitted with a Teflon insert. After cooling to room temperature, the mixtures were extracted with 5 χ 40 mL of diethyl ether. The combined ether fractions were treated with aqueous sodium bicarbonate, and the residual water in the ether layer was frozen-out at -20°C. After filtration through a plug of cotton, the ether extracts were carefully concentrated to 2 mL using a Vigreux column.
0097-6156/94/0543-0263$06.00/0 © 1994 American Chemical Society
In Thermally Generated Flavors; Parliment, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 23, 2015 | http://pubs.acs.org Publication Date: November 30, 1993 | doi: 10.1021/bk-1994-0543.ch021
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T H E R M A L L Y G E N E R A T E D FLAVORS
3. Reaction of tuna muscle with D-glucose. Fresh tuna muscle (18 g) was ground and mixed with 1 g of D-glucose. The mixture was heated at 150°C in 60 mL of 1.0 M phosphate buffer, extracted, and concentrated as described above. In order to remove residual fat, the concentrated ether extract (2 mL) was purified using gel permeation chromatography on a 450 χ 25 mm SR 45 column, filled with 200-400 mesh S X 3 Bio Beads (Biorad, 2000 D), and equilibrated with dichloromethane. The fat was eluted with the first 100 mL of dichloromethane, and the components of interest were eluted in the subsequent 100 mL dichloromethane fraction, which was concentrated to a 2-mL volume using a Vigreux column. The composition of this fraction was investigated by GC/MS. 4. Roasting of D-glucose (0.004 mol) with histidine (0.004 mol). The reactants were ground in a mortar with 10% of 1.0 M phosphate buffer and 10% sand, and the resulting mixture was layered in a reaction tube with glass wool. After the tube was wrapped with heating tape and a contact thermometer was attached, it was connected to three cooling traps in series (20°C, 0°C, -196°C). Roasting was carried out by heating the tube at 20°C/min to a final temperature of 220°C, which was maintained for 10 min. The volatiles were purged into the cooling traps by nitrogen at a flow of 50 mL/min. The condensates were extracted with ether, and the combined ether extracts were treated as described above. Preparation of Compound III. A concentrated ether extract from model system Reaction 2 (see above) was fractionated on a 30 χ 2 cm column equipped with a cooling jacket and filled with 20 g of acidic alumina (ICN Biochemicals, deactivated with 10% water). A gradient elution was performed using pentane-diethyl ether, and the following fractions were collected which corresponded to the mobile phase ratios: 1) 90:10, 2) 80:20, 3) 70:30, 4) 60:40, 5) 50:50. Fraction 5 yielded about 1 mg of colorless crystals, identified as 2-acetyl-pyrido[3,4-
