Degradation patterns and stability predictions of the original reds and amino acid derivatives of monascus pigments

The original red isomers and l-phenylalanine derivative isomers of monascus pigment, which have a side chain of either C₅H₁₁ or C₇H₁₅, were produced via Monascus fermentation, and then purified. The degradation patterns of the pigments relating to incubation conditions were investigated. For all tested compounds, a total of seven major fragments were formed during incubation, five in presence of sunlight irradiation and two with no sunlight. The red isomers R5 and R7 showed the same pattern whereas the L-Phe derivative isomers P5 and P7 exhibited another pattern. All of the pigments appeared to be highly sensitive to sunlight irradiation considering that with sunlight, diverse fragments were produced and the side chains C₅H₁₁ and C₇H₁₅ were degraded. The fragments are thought to be formed via degradation and combination reactions of pigments based on a comparison of the fragment molecular masses. Theoretically stable isomers of an original red pigment and some amino acid derivatives were selected based on a comparison of their Hartree conformational energies, which were estimated using geometric optimization based on ab initio molecular orbital calculations. The greater stability of amino acid derivatives was confirmed based on maximum UV-absorption wavelengths and isomer population rates.