Pterins belong to a family of heterocyclic compounds present in a wide range of living systems and participate in relevant biological functions. Under anaerobic conditions, the unstable red intermediate generated by UV-A irradiation of biopterin, neopterin and 6-hydroxymethylpterin was identified by H-1 NMR analysis, in alkaline D2O solutions, as 5,8-dihydro-6-formylpterin. (C) 2009 Elsevier B. V. All rights reserved.