Mass Spectra of The Metabolites of Monascus

Abstract

Mass spectra have been measured for the metabolites of Monascus purpuleus Wentii and correlations between spectra and structure have been made. The fragmentation processes of monascorubrin (1) which belongs to the group of azaphilones and of which structure was elucidated as a unique extended pyrone, are interpreted largely through the use of meta-stable ions (Table 1). The results (Scheme 2) indicate that a loss of an alkyl side chain with a McLafferty rearrangement on the carbonyl oxygen gives rise to the most abundant ion(M.+-C6H12)from which major fragment ions originate. Thus coexistence of rubropunctatin (III) which differs from monascorubrin only by one alkyl chain length does not affect the interpretation of the spectrum. Attention is drawn to the dramatic change of the mass spectral behavior of monascamine (II) which was prepared by a simple conversion of the pyronoid oxygen atom of monascorubrin into-NH-. In case of II a loss of an entire alkyl side chain including the carbonyl group with a hydrogen rearrangement on the unsaturated system affords the most abundant ion(M.+-C8H10O). The difference of the mass spectral behavior between monascorubrin and monascamine may be accounted for from the concept of localization of radical ion which is thought to be the driving force of McLafferty rearrangement. The mass spectra of the isoquinoline derivatives such as monascaminone (VIII), monascaminol (IX) and the others are also discussed in terms of the fragmentation of isoqui noline group. Application of the mass spectral studies on piperitone (XI) and an octalone (XII) to the interpretation of the spectrum of monascoflavine (Monascine) enables one to deduce the correct structure (IVa) of monascoflavine to which two alternative structures (IVa and IVb) have been proposed. The presence of a series of peaks; M+.m*→m/e162 m*→m/e134, is consitstent only with the case of piperitone's reflecting the presence of the same partial structure in the two compounds.