Identification of brazilein type B as a key marker for the presence of brazilwood in colored historical objects

Abstract

The identification of brazilwood in historical artefacts is challenging and strongly relates to the chosen extraction method. When strong acid hydrolysis is used, redwood's primary dyeing component, brazilein, is never found, but instead leads to the identification of two related marker compounds. Commonly referred to as type B and type C compounds, they are characterized by specific UV and mass spectra. While type C has recently been recognized as urolithin C, the molecular structure of type B has only been inferred and never elucidated. In this work, a combination of synthesis, UV spectroscopy, high-resolution MS and NMR spectroscopy was used to unambiguously determine the previously unreported structure of the brazilein type B derivative. These results support the long-standing hypothesis that an HCl-based extraction protocol leads to the formation of a dehydro-brazilein product. In addition, studies were conducted to evaluate the effect of the extraction conditions on the presence or absence of the marker compounds brazilein and brazilein type B. It was demonstrated that only brazilein type B is detected under strong acid conditions. Finally, this knowledge was used for the unequivocal identification of brazilwood markers in the study of four Italian red velvets using a milder HCl protocol, providing a historical perspective on the use of redwood dyes in Italy during the Renaissance.