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Proceedings of the 31 st European Peptide SymposiumMichal Lebl, Morten Meldal, Knud J. Jensen, Thomas Hoeg-Jensen (Editors)European Peptide Society, 2010Racemization of Amino Acids on Heating with Sugars or(Hydroxyalkyl)aldehydes or -ketonesHans BrücknerResearch Center for BioSystems, Land Use and Nutrition (IFZ), Department of FoodSciences, University of Giessen, Giessen, 35392, GermanyIntroductionPreviously, we have reported on the racemization (epimerization) of -amino acids (AAs)on heating with saccharides in aq. AcOH [1,2]. It was postulated that it occurs at earlystages of the Maillard (MR) reaction. It starts with the reversible formation of Schiff bases(SBs) and proceeds via formation of 1-amino-1-deoxy-2-ketoses from aldoses (Amadorirearrangement products, ARPs) or 2-amino-2-deoxyaldoses from ketoses (Heynsrearrangement products (HRPs) [3]. Here, it is shown that ( -hydroxalkyl) aldehydes or( -hydroxyalkyl)ketones in general induce racemization. Based on these data, a feasibleracemization mechanism for AAs via formation of cyclic lactones is presented.Results and DiscussionMixtures of L-Ala, D-sugars, and aldehydes/ketones were heated at 130 °C in 1 M AcOH.(Table 1). Racemized Ala was isolated via cation exchanger, and enantiomers werequantified on Chirasil-L-Val [1,2]. From data the following conclusions are drawn: bothaldoses and ketoses cause racemization of L-Ala. Fastest racemization was induced byglyceraldehyde, methylglyoxal monohydrate, and pyridoxal. In comparison to ribose,2-desoxyribose induces less racemization indicating the relevance of the -position of thehydroxyl groups in aldehydes and ketones. The polyalcohol sorbitol does not promoteracemization. From the data, the reversible formation of SBs and lactones thereof that showprototropy (tautomerism) is deduced (Figure 1). In equilibrium release of racemized L-Ala(or other chiral -AA) as the result of keto-enol tautomerism occurs. The mechanism isalso applicable to ARPs and HRPs generated in the course of the MR. From ARPs 7-ring-lactones can be formed (or 6-ring -lactones via previous 1,2-enaminolisation) whereasfrom HRPs formation of 6-ring -lactones is possible (see Figure 1). The aldimine of L-Alaand pyridoxal might form an -lactone with the phenolic group. Release of racemized AAson heating of dry, chemically synthesized and opically pure ARPs is attributed todehydration according to the lactone mechanism or/and direct proton abstraction from theC -atom of the AA conjugate [4,5]. For racemization of AAs in the MR see also [6].Table 1. Relative quantities of D-Ala [%D = 100D/(D+L)] resulting from the reaction ofL-Ala (5 mM) and sugars/aldehydes (500 mM) in 1 M aq. AcOH at 130 °C; n.d, notdetermined; n.m., not measurable owing to destruction of Ala; content of Ala is decreasingby orders of magnitude during the course of the experiments and browning is increasing.ReactandTime (hours)1 3 8 15 24 48 72 96 120Sorbitol 0.15 0.23 0.84 1.50 2.72 4.26 6.53 7.43 n.d.Fructose 0.24 1.54 5.56 17.9 24.6 39.5 36.2 35.0 n.d.Glucose 0.14 0.44 0.96 2.23 3.39 10.4 24.1 36.3 40.8Galactose 0.29 1.02 2.52 3.76 11.0 18.9 33.0 40.5 40.4Xylose 0.31 1.04 3.23 8.18 13.9 34.3 42.8 44.1 41.5Ribose 0.82 2.41 6.61 17.3 27.6 40.8 40.7 35.7 n.d.2-Deoxribose n.d. 1.25 3.01 2.85 4.62 11.3 13.5 12.8 13.0Glyceraldehyde n.d. n.d. 27.7 34.7 32.6 30.0 n.d. n.d. n.d.Methylglyoxal 38.6 36.0 42.1 36.6 38.0 28.9 30.9 31.6 37.9Pyridoxal 36.4 41.8 21.3 n.d. 11.5 n.m. n.m. n.m. n.m.64