Monascorubrin and rubropunctatin: Preparation and reaction characteristics with amines

Monascorubrin and rubropunctatin are the two classical orange Monascus pigments, and serve as indispensable precursors in the formation of food colorants Monascus red pigments by amination with primary amines. However, this chemical transformation has been less investigated in vitro due to the hard preparation and preservation of monascorubrin and rubropunctatin. In this study, a way for the preparation of high-purity monascorubrin and rubropunctatin was developed. The in vitro reaction characteristics with various amino acids in different pH environments indicated that an alkaline environment could greatly promote the amination, while the amination was obviously inhibited in an acidic environment. The kinetic study of monascorubrin and rubropunctatin with ammonia suggested that their amination occurred via a second-order reaction. This study not only provides a better understanding of the chemical properties of monascorubrin and rubropunctatin, but also has significance in guiding the industrial applications of Monascus pigments.