(6R)-5,6,7,8-Tetrahydro-L-Monapterin from Escherichia coli, a Novel Natural Unconjugated Tetrahydropterin

The structure of the major tetrahydropterin in Escherichia coli was determined as (6R)-5,6,7,8-tetrahydro-L-monapterin, i. e. (6R)-2-amino-5,6,7,8-tetrahydro-6-[(1S,2S)-1,2,3-trihydroxypropyl]pteridin-4(3H)-one. Although the stereochemical structure of the trihydroxypropyl side chain has been determined previously by fluorescence detected circular dichroism analysis on its aromatic derivative, the most important configuration at C(6) has not been clarified. The major difficulties for the determination of the chirality were instability toward air oxidation and very low concentration of the tetrahydropterin derivative. In the present study, the C(6)-configuration was determined as R by comparing its stable hexaacetyl derivative with authentic (6R)- and (6S)-hexaacetyl-5,6,7,8-tetrahydro-L-monapterins by high performance liquid chromatography (HPLC) and HPLC-mass spectrometry (LC-MS). (6R)-5,6,7,8-Tetrahydro-L-monapterin is a new unconjugated tetrahydropterin from natural sources.