"Thermal Peroxidation" of Dietary Pentapeptides Yields N-Terminal 1,2-Dicarbonyls.

Nikolai Kuhnert,Maria Bikaki

doi:10.3389/fnut.2021.663233

Abstract

In this contribution we investigate the thermal degradation of dietary-relevant pentapeptides. Most unsaturated lipids degrade by the well-known peroxidation mechanism. Here we show a degradation mechanism of peptides analogous to lipid peroxidation, forming a series of novel degradation products with possible toxicological relevance. At elevated temperatures above 180°C, pentapeptides with an N-terminal phenylalanine moiety react via a debenzylation to form 1,2-dicabonyl compounds, replacing the N-terminal primary amine. We propose a radical-based reaction mechanism that leads via a common peroxoaminal intermediate to two distinct types of reaction products with a terminal α-1,2 diamide or an α-amide-aldehyde functionality.

Highlights

The majority of protein rich food is processed by thermal treatment prior to human consumption
In order to understand the chemical behavior during thermal processing of food peptides, fifteen selected custom pentapeptides (Supplementary Table 1) with systematically varying amino acid sequences were subjected to thermal processing and subsequent LC-MS characterization using an ESI-Q-TOF MS instrument
Most pentapeptides selected carried a C-terminal tryptophan moiety and an N-terminal phenylalanine moiety to assist in compound identification by LC-UV analysis due to characteristic chromophores indicating the formation of C- or N-terminal breakdown products

Summary

Introduction

The majority of protein rich food is processed by thermal treatment prior to human consumption. As multifunctional molecules embedded in a complex matrix, composed from a myriad of further constituents, proteins are thermally labile, changing conformation at lower temperatures [2] and undergoing a series of chemical transformations above 150◦C, significantly altering the chemical composition of the food [3, 4]. The most investigated of these transformation is the Maillard reaction of amino acids with sugars producing, via a series of intermediates, among small aroma active molecules, such as Strecker aldehydes or pyrazines, as well as ill-defined pigmented polymers, referred to as melanoidines [5,6,7]. Selected examples of structurally characterized degradation products of proteins include simple inorganic and organic compounds (CO2, H2O, NH3, and CO), with a variety of volatile organic compounds (amines, nitriles, amides, hydrocarbons, etc.) and lots of less volatile organic compounds such as diketopiperazines, lactams, and hydantoins

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Conclusion