Deciphering key coloured compounds from sunless tanning reactions

Sunless tanning has become an incredibly popular way to achieve a tanned look without the potential skin damage from sun exposure, due to its convenience and safety compared to the conventional solar tanning. Dihydroxyacetone (DHA), as the main active ingredient in commercial sunless tanning products, is widely recognized to react with free amino acids (AAs) in the outer stratum corneum of skin to form brown pigments known as “melanoidins” through the Maillard reaction. However, the exact reaction pathways and chemical structures of the melanoidins formed have not yet been reported. To explore the colour development mechanism of DHA, three typical AAs, i.e. arginine, histidine and lysine, were each made to react with DHA using a simplified model system. For the first time, three key coloured compounds with the same chromophore were successfully isolated and identified from these resultant mixtures. The reaction pathway for the formation of these key coloured compounds was also proposed and the associated energy calculated using the methods based on the density functional theory (DFT). In addition, an in-depth understanding of the colour properties of these key coloured compounds has been gained through the colour dilution analysis for their colour contributions and by using CIELAB colour space to evaluate their colour characteristics, respectively.