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Projects for CHEM 499
Prebiotic Significance of the Maillard Reaction
Student Research Assistant: Milica Bajagic
The Maillard reaction was studied from a prebiotic point of view. We have shown that the Maillard reaction between ribose and common amino acids occurs readily in the solid state at 65oC. The C-13 NMR spectra of the solid insoluble Maillard products of ribose and serine, or alanine or isoleucine were compared to the spectrum of the insoluble organic carbon on Murchison. We have also performed the reaction with various meteoritic amino acids and have obtained similar results.
Amino acids are well-recognized prebiotic molecules. They can be made in the laboratory by a spark-discharge of a mixture of reducing gases that were presumably present on the prebiotic Earth. Amino acids are also found in the carbonaceous chondrite meteorites, such as Murchison and others. It is now believed that the presence of amino acids on the prebiotic Earth was ubiquitous.
The prebiotic presence of sugars, on the other hand, was largely disputed until very recently. There is no problem with the prebiotic synthesis of sugars, as they can be formed from formaldehyde in an alkaline solution by the so-called formose reaction, which gives a mixture of many sugars. One problem is that the biologically important sugars, specifically ribose, are not favored in the formose reaction. Another problem lies in the fragility of sugar molecules, as demonstrated by the sugar stability study. Thus, the prebiotic reactions of sugars were not much explored, as it was believed that sugars are not robust enough to survive in the prebiotic environment. This belief was first put in doubt when sugar-related organic compounds were isolated from the carbonaceous meteorites. In 2004 Benner's group showed that ribose, one of the key sugars in the present biotic systems, can complex out of the formose mixture by the borates. Kinrade et al. and Lambert et al. showed that ribose makes also stable silicate complexes. This indicates that sugars perhaps can be selected and protected by complexation under the prebiotic conditions, and thus should be looked at as viable prebiotic molecules, and their prebiotic reactions should be explored in a comprehensive manner.
It does appear then, that both amino acids and sugars were ubiquitous on the early Earth and were present in the prebiotic soup. However, amino acids and sugars react with each other readily, in the so-called Maillard reaction. This reaction occurs in several stages. The first stage of the Maillard reaction consists of the reaction between the carbonyl group of the sugar with the amino group of the amino acid, to form a Shiff base, which then rearranges to an amino ketose, also called the Amadori product. The intermediate stage involves the degradation of the Amadori product, which gives a variety of reactive compounds. These compounds react further in the final stages of the Maillard reaction, which is condensation to give high molecular weight polymers, which are known as melanoidins. At the elevated temperatures the reaction gives volatile compounds associated with food aromas and flavors. Because of this the Maillard reaction is of a great importance in food industry, and has been studied extensively from that perspective. Many of the flavors and aromas are heterocyclic compounds. This triggered our interest in connecting the Maillard reaction to prebiotic chemistry, since numerous heterocyclic compounds are found on the meteorites, but the mechanism of their formation is not elucidated. We decided to test the hypothesis that the Maillard reaction could occur under milder temperature conditions than those that are typically used in the food industry studies (160oC or higher), and in the solid state rather than a solution, which would be more applicable to the meteorite milieu. We were also interested in the melanoidin products. The prebiotic significance of the melanoidins as primitive co-enzymes has been suggested but not fully studied. Our interest in melanoidins is not to assign to it any particular function in the process of chemical evolution, but to perhaps connect it to the insoluble carbon fraction on meteorites. The latter is quite abundant, but the mechanism of its formation is not fully elucidated. Could it be that melanoidins are formed at least in part via the Maillard reaction on meteorites? We have tested these two hypotheses, and report the initial results in this paper. We have carried out the Maillard reaction between ribose and common amino acids at 65oC in the solid state, which is in the temperature range used for the laboratory simulation of the prebiotic reactions, and also at the room temperature. We have also carried out the reaction under these mild temperatures in the aqueous solution, which would be applicable to the prebiotic chemistry on Earth. We have analyzed the initial reaction products via C-13 NMR, the volatile products by the GC-MS, and the melanoidins with the IR and solid state C-13 NMR. The latter were compared with the solid state C-13 NMR of the Murchison meteorite. Substantial similarities were found. We have also studied the meteoritic amino acids and have obtained similar results.
Parts of this project were presented at the conferences or published: