Hydrothermal Degradation of Amino Acids

Abstract

Within the past years, hydrothermal processes have gathered much attention as promising conversion technologies for especially wet biomass. Amino acids are an integral component of biomass, zoo biomass in particular. However, what happens to them during hydrothermal treatment? Reviewing the available literature going back to the mid of the 20th century revealed an astonishing, but still fragmentary view. In fact, two universal degradation reactions could be identified (i. e., deamination and decarboxylation), competing with each other.Thereby, small structural differences may obviously have huge impacts on the fate of individual amino acids. Nevertheless, the amount of available experimental data is relatively scarce in many cases. In this work, the available knowledge about the degradation of 20 proteinogenic amino acids under hydrothermal conditions was presented and discussed critically. The hydrothermal conversion of proteinaceus biomass as well as the Maillard reaction, both extensively reviewed elsewhere, were only touched on.

Amino-acid modulated hierarchical In/H-Beta zeolites for selective catalytic reduction of NO with CH4 in the presence of H2O and SO2

“…30 In contrast, proline is stable under hydrothermal conditions, and likely “the most important consumption pathway of proline is not degradation but condensation with other compounds, rendering it a relevant precursor to nitrogen-containing humins”. 31 We suspect that tetraethylammonium prolinate could be formed in situ , 32 which could stabilize unpaired electrons in the synthesis mixture making them observable by EPR. To detect the possible radicals formed in the zeolite reaction system, 5,5-dimethylpyrroline- N -oxide (DMPO) was added into the initial synthesis gel as the spin-trapping agent.…”

“…3. While the reaction mixture of Na-Beta-P exhibits a quartet pattern in a ratio of 1 : 2 : 2 : 1 with a splitting of 1.5 mT, which may be ascribed to the resonance transitions of DMPO-• OH, 31 that of Na-Beta-B shows almost no such EPR singles. Therefore, the crystallization promotion effect by proline is possibly caused by the induced free radicals.…”

Elucidating the Maillard Reaction Mechanism in the Hydrothermal Liquefaction of Binary Model Compound Mixtures and Spirulina

“…The aromatic amino acids, especially phenylalanine, are highly resistant even to supercritical water conditions . The relative linearity of phenyl aromatic concentration over the mass ratio on both the aqueous and renewable biocrude phases may indicate that the aromatics in the aqueous and ethanol-soluble biocrude come primarily from the protein and particularly from the phenylalanine, which is 5.3% approximately in the soy protein (according to the supplier composition information).…”

“…Additionally, over the degradation of amino acids in proteins, the abstraction of the amino group (deamination) competes with the carbonyl functional group (decarboxylation), and as a result, only one of these reactions can occur under hydrothermal conditions. 69 Based on the results obtained from the GC−MS analysis of the aqueous and renewable crude product phases and the structure of the amino acids of the soy protein, the Maillard interactions in the present study may be illustrated in Figure 10. The condensation of two amino acids (cyclo-dimerization) into heterocyclic organic structures, followed by reduction and degradation, appears to be the most significant pathway for the amino acids from the hydrolysis of proteins.…”

“…24 The aromatic amino acids, especially phenylalanine, are highly resistant even to supercritical water conditions. 69 The relative linearity of phenyl aromatic concentration over the mass ratio on both the aqueous and renewable biocrude phases may indicate that the aromatics in the aqueous and ethanolsoluble biocrude come primarily from the protein and particularly from the phenylalanine, which is 5.3% approximately in the soy protein (according to the supplier composition information). The organic heterocycles contain- ing cyclopentane, cyclohexane, and lactone structures presented a relatively similar pattern in both the aqueous and biocrude with higher concentrations at a high carbohydrate mass ratio and in the aqueous phase.…”