In the biosynthesis of neurotransmitting catecholamines, the hydroxylation of phenylalanine and tyrosine requires(6R)-5, 6, 7, 8_tetrahydrobiopterin (1) as the natural cofactor. 1 During the enzymatic reactions, 1 is oxidized to quinonoid (6R)-dihydrobiopterin(2), which is reduced back to 1 by dihydropterin reductase and NADPH (or NADH). la In the absence of such recycling pathway, 2 will spontaneously isomerize to thermodynamically more stable ...