Structural characterization of two new orange pigments with strong yellow fluorescence

- Two new orange pigments with similar yellow fluorescence spectra were found.
- The structures of the pigments were similar to rubropunctatin and monascorubrin.
- The pigments were obtained from the reaction of Monascus pigments with methanol.
- Structures of the pigments were elucidated by HRMS, MS/MS, and NMR spectroscopy.

As a result of HPLC analysis of the methanol extract of red yeast rice, two new orange pigments with similar yellow fluorescence spectra (λex = 472 nm, λem = 535 nm) and UV absorption spectra (λmax = 470 nm) were found. When the rubropunctatin and monascorubrin were dissolved in methanol, two new yellow fluorescent compounds were also observed. The results indicated that the reaction of rubropunctatin and monascorubrin with methanol or d4-methanol occurred. The yellow fluorescence pigments were isolated and purified from the red yeast rice. Structures were elucidated by ESI-MS, ESI-MS/MS, and NMR spectroscopy. High-resolution mass spectrometry indicated the molecular formulas C22H26O6 and C24H30O6. The structures of the two new yellow fluorescence pigments, named monasphilol-methoxy A and monasphilol-methoxy B, were similar to rubropunctatin and monascorubrin, respectively. However, the hydroxyl and methoxyl groups in monasphilol-methoxy A and monasphilol-methoxy B, respectively, were substituted for the conjugated ketone carbonyl in rubropunctatin and monascorubrin.