The fundamental building blocks of red human hair pheomelanin are isoquinoline-containing dimers

A TPCA-based, three-marker analysis has been used to demonstrate that the dimeric benzothiazolylthiazinoisoquinoline units recently identified in natural and synthetic pheomelanins are dominant structural building blocks of red human hair pheomelanin. This finding would prompt a revisit of pheomelanin build-up in terms of coupling of carboxylated benzothiazine intermediates into a specific BT-TIQ-related dimeric building block preceding further polymerization. Thus, BT-TIQ would reproduce all of the main chemical properties and spectral features of human pheomelanin. In this sense, BTTIQ compares to none of the low molecular weight intermediates so far isolated, including the trichochromes, and should, therefore, be duly considered in future models of pheomelanin structure.