Mechanism of Colour Formation of Dihydroxyacetone Tanning Agent via Maillard Reaction

Worldwide, sunless tanning has become incredibly prevalent due to the increasing fashionable demand and the awareness of photodamage risks caused by over exposure to ultraviolet radiation. Dihydroxyacetone (DHA), as the main active ingredient in sunless tanning products, can react with the amino groups derived from α-keratin in the stratum corneum (SC) via the Maillard reaction to form brown pigments known as “melanoidins”. Although DHA has been used and studied in tanning reactions for over 60 years, neither the exact chemical structures of melanoidins formed nor the detailed reaction mechanisms have yet been reported. Hence, to systemically investigate the colour development mechanism of DHA tanning reaction, this project had focused on four themes: (1) Three basic amino acids (AAs) were used to react with DHA in a simplified model system to understand the colour development under various reaction conditions; (2) Based on the optimised reaction conditions, three key coloured compounds with the same chromophore (Colourants A1, H1 and L1) were successfully isolated from these model systems and identified, and their possible reaction pathways then proposed and validated using the density functional theory (DFT); (3) The effects of side chain groups of three Nα-protected AAs were studied in different solvents. The ε-amino group of lysine residue was the main reaction site in SC and five main yellow compounds (Colourants 1 ~ 5) were isolated and determined for the first time, and their colour characteristics quantitatively evaluated. Colourant 4 contributed the most to the colour and was recognised as the most coloured key compound; (4) To better understand the reaction in a larger molecular system, a series of short lysine-containing peptides derived from the α-keratin of SC were employed to react with DHA. Interestingly, only the chromophore of Colourant 4 was formed in these reactions, indicating that it may be the most important coloured structure responsible for DHA tanning on skin. This is the first systematic study to reveal the specific chemical structures responsible for the colour formation of DHA with the main or side chain amino groups of amino acids and peptides. This work not only contributes to important new knowledge on the DHA tanning mechanism, but also provides a new strategy for the study of the advanced glycation end-products (AGEs).