Identification of (6R)-5,6,7,8-Tetrahydro-D-monapterin (=(6R)-2-Amino-5,6,7,8-tetrahydro-6-[(1R,2R)-1,2,3 -trihydroxypropyl]pteridin-4(3H)-one) as the Native Pteridine in Tetrahymena pyriformis

The structure of the native pteridine in Tetrahymena pyriformis was determined as (6R)‐5,6,7,8‐tetrahydro‐D‐monapterin (=(6R)‐2‐amino‐5,6,7,8‐tetrahydro‐6‐[(1R,2R)‐1,2,3‐trihydroxypropyl]pteridin‐4(3H)‐one; 4). First, the configuration of the 1,2,3‐trihydroxypropyl side chain was confirmed as D‐threo by the fluorescence‐detected circular dichroism (FDCD) spectrum of its aromatic pterin derivative 2 obtained by I2 oxidation (Fig. 1). The configuration at the 6‐position of 4 was determined as (R) by comparison of its hexaacetyl derivative 6 with authentic (6R)‐ and (6S)‐hexaacetyl‐5,6,7,8‐tetrahydro‐D‐monapterins 6 and 7, respectively, in the HPLC, LC/MS, and LC‐MS/MS (Figs. 3 – 6). (6R)‐5,6,7,8‐Tetrahydro‐D‐monapterin (4) is a newly discovered natural tetrahydropterin.